From 90ed9db7b353d47ee2ed0c768e663c1804d65a71 Mon Sep 17 00:00:00 2001
From: Pravein Govindan Kannan <pravein.govindan.kannan@ibm.com>
Date: Wed, 21 Jan 2026 14:11:27 +0530
Subject: [PATCH 01/28] Add support to prepare Fifo

Signed-off-by: Pravein Govindan Kannan <pravein.govindan.kannan@ibm.com>
---
 src/plugins/uccl/uccl_backend.cpp | 120 +++++++++++++++---------------
 src/plugins/uccl/uccl_backend.h   |   5 +-
 2 files changed, 62 insertions(+), 63 deletions(-)

diff --git a/src/plugins/uccl/uccl_backend.cpp b/src/plugins/uccl/uccl_backend.cpp
index ba558d8472..bc18f086a6 100644
--- a/src/plugins/uccl/uccl_backend.cpp
+++ b/src/plugins/uccl/uccl_backend.cpp
@@ -177,7 +177,16 @@ nixlUcclEngine::getSupportedMems() const {
 nixl_status_t
 nixlUcclEngine::getPublicData(const nixlBackendMD *meta, std::string &str) const {
     nixlUcclBackendMD *priv = (nixlUcclBackendMD *)meta;
-    str = std::to_string(priv->mr_id);
+
+    // Export fifo_item as hex string
+    str.clear();
+    str.reserve(FIFO_ITEM_SIZE * 2);
+    for (int i = 0; i < FIFO_ITEM_SIZE; i++) {
+        char hex[3];
+        snprintf(hex, sizeof(hex), "%02x", static_cast<unsigned char>(priv->fifo_item[i]));
+        str += hex;
+    }
+    NIXL_DEBUG << "Exporting Meta Info =" << hex_str << sts::endl;
 
     return NIXL_SUCCESS;
 }
@@ -292,6 +301,17 @@ nixlUcclEngine::registerMem(const nixlBlobDesc &mem,
     priv->length = mem.len;
     priv->ref_cnt = 1;
     priv->mr_id = reinterpret_cast<uint64_t>(mr); // Store the memory region handle
+
+    // Pre-compute fifo_item for one-sided RDMA operations
+    int result = uccl_engine_prepare_fifo(engine_, mr, (void *)mem.addr,
+                                                 mem.len, priv->fifo_item);
+    if (result != 0) {
+        NIXL_ERROR << "Failed to prepare fifo_item for memory region";
+        uccl_engine_mr_destroy(mr);
+        delete priv;
+        return NIXL_ERR_BACKEND;
+    }
+
     out = priv;
     mem_reg_info_[mem.addr] = priv;
     NIXL_DEBUG << "Registering memory: " << mem.addr << "Device: " << mem.devId
@@ -349,7 +369,23 @@ nixlUcclEngine::loadRemoteMD(const nixlBlobDesc &input,
     output_md->addr = (void *)input.addr;
     output_md->length = input.len;
     output_md->ref_cnt = 1;
-    output_md->mr_id = strtoul(input.metaInfo.c_str(), NULL, 10);
+
+    // Decode fifo_item from hex string
+    const std::string &hex_str = input.metaInfo;
+    NIXL_DEBUG << "Meta Info =" << hex_str << sts::endl;
+    if (hex_str.length() == FIFO_ITEM_SIZE * 2) {
+        for (int i = 0; i < FIFO_ITEM_SIZE; i++) {
+            std::string byte_str = hex_str.substr(i * 2, 2);
+            output_md->fifo_item[i] = static_cast<char>(strtoul(byte_str.c_str(), NULL, 16));
+        }
+        NIXL_DEBUG << "Parsed fifo_item from remote metadata";
+    } else {
+        NIXL_ERROR << "Invalid fifo_item hex string length: " << hex_str.length()
+                   << " (expected " << FIFO_ITEM_SIZE * 2 << ")";
+        delete output_md;
+        output = nullptr;
+        return NIXL_ERR_INVALID_PARAM;
+    }
 
     return NIXL_SUCCESS;
 }
@@ -369,7 +405,6 @@ nixlUcclEngine::prepXfer(const nixl_xfer_op_t &operation,
                          const std::string &remote_agent,
                          nixlBackendReqH *&handle,
                          const nixl_opt_b_args_t *opt_args) const {
-    int result = 0;
     nixlUcclBackendMD *lmd;
     nixlUcclBackendMD *rmd;
     bool rcmode = false;
@@ -411,9 +446,11 @@ nixlUcclEngine::prepXfer(const nixl_xfer_op_t &operation,
             return NIXL_ERR_INVALID_PARAM;
         }
     }
-    // Collect all tx_data into vectors for batch sending
-    std::vector<md_t> md_vector;
-    std::vector<nixlUcclBackendMD *> local_priv_vector;
+    handle = new nixlUcclReqH(conn);
+    nixlUcclReqH *uccl_handle = static_cast<nixlUcclReqH *>(handle);
+
+    uccl_handle->fifo_items.clear();
+    uccl_handle->fifo_items.resize(lcnt);
 
     std::lock_guard<std::mutex> lock(mem_mutex_);
     for (size_t i = 0; i < lcnt; i++) {
@@ -441,62 +478,21 @@ nixlUcclEngine::prepXfer(const nixl_xfer_op_t &operation,
             NIXL_ERROR << "Local memory region not properly registered";
             return NIXL_ERR_BACKEND;
         }
-
-        // Prepare the memory region metadata for batch sending
-        md_t md;
-        tx_msg_t tx_data;
-        tx_data.data_ptr = remote_addr;
-        tx_data.data_size = rsize;
-
-        // RC mode is supported for both READ/WRITE operations
-        // UC mode is supported only for WRITE operations
-        md.op = rcmode ? UCCL_RW_RC : UCCL_WRITE;
-        md.data.tx_data = tx_data;
-
-        // Add to vectors for batch processing
-        md_vector.push_back(md);
-        local_priv_vector.push_back(local_priv);
-    }
-
-    // Send all tx_data as a vector
-    result = uccl_engine_send_tx_md_vector(conn, md_vector.data(), md_vector.size());
-    if (result < 0) {
-        NIXL_ERROR << "Failed to send transfer metadata vector";
-        return NIXL_ERR_BACKEND;
-    }
-
-    if (rcmode) {
-        if (!handle) {
-            handle = new nixlUcclReqH(conn);
-        }
-        nixlUcclReqH *uccl_handle = static_cast<nixlUcclReqH *>(handle);
-
-        uccl_handle->fifo_items.clear();
-        uccl_handle->fifo_items.resize(local_priv_vector.size());
-
-        for (size_t i = 0; i < local_priv_vector.size(); i++) {
-            char fifo_item[FIFO_ITEM_SIZE];
-            int retry_count = 0;
-            const int max_retries = 50;
-            do {
-                result = uccl_engine_get_fifo_item(conn, i, &fifo_item);
-                if (result == 0) {
-                    memcpy(uccl_handle->fifo_items[i].data(), fifo_item, FIFO_ITEM_SIZE);
-                    break;
-                }
-                retry_count++;
-                if (retry_count < max_retries) {
-                    NIXL_DEBUG << "Failed to get FIFO item, retry " << retry_count << "/"
-                               << max_retries << " for item " << i;
-                    std::this_thread::sleep_for(std::chrono::microseconds(10));
-                }
-            } while (retry_count < max_retries);
-
-            if (result != 0) {
-                NIXL_ERROR << "Failed to get FIFO item after " << max_retries
-                           << " retries for item " << i;
-                return NIXL_ERR_BACKEND;
-            }
+        if (rcmode) {
+            memcpy(uccl_handle->fifo_items[i].data(), rmd->fifo_item, FIFO_ITEM_SIZE);
+
+            // Update the address and size in the fifo_item for the actual transfer
+            // FifoItem layout: addr (8 bytes at offset 0), size (4 bytes at offset 8)
+            uint64_t *fifo_addr = reinterpret_cast<uint64_t *>(uccl_handle->fifo_items[i].data());
+            uint32_t *fifo_size = reinterpret_cast<uint32_t *>(uccl_handle->fifo_items[i].data() + 8);
+            *fifo_addr = remote_addr;
+            *fifo_size = static_cast<uint32_t>(rsize);
+
+            NIXL_DEBUG << "Using pre-shared fifo_item[" << i << "]: addr=" << std::hex
+                       << remote_addr << ", size=" << std::dec << rsize;
+        } else {
+            // TODO : Suppport UC one-sided
+            return NIXL_ERR_BACKEND;
         }
     }
 
diff --git a/src/plugins/uccl/uccl_backend.h b/src/plugins/uccl/uccl_backend.h
index 3348add3d2..4cb3340393 100644
--- a/src/plugins/uccl/uccl_backend.h
+++ b/src/plugins/uccl/uccl_backend.h
@@ -139,7 +139,9 @@ class nixlUcclEngine : public nixlBackendEngine {
 // UCCL Backend Memory Descriptor
 class nixlUcclBackendMD : public nixlBackendMD {
 public:
-    nixlUcclBackendMD(bool isPrivate) : nixlBackendMD(isPrivate) {}
+    nixlUcclBackendMD(bool isPrivate) : nixlBackendMD(isPrivate) {
+        memset(fifo_item, 0, FIFO_ITEM_SIZE);
+    }
 
     virtual ~nixlUcclBackendMD() {}
 
@@ -147,6 +149,7 @@ class nixlUcclBackendMD : public nixlBackendMD {
     size_t length;
     int ref_cnt;
     uint64_t mr_id; // UCCL memory region id
+    char fifo_item[FIFO_ITEM_SIZE];
 };
 
 // UCCL Backend Request Handle

From 8a354ea45dd11caf8100582ddbbff309f823fa9f Mon Sep 17 00:00:00 2001
From: Pravein Govindan Kannan <pravein.govindan.kannan@ibm.com>
Date: Wed, 21 Jan 2026 14:20:22 +0530
Subject: [PATCH 02/28] Fix errors

Signed-off-by: Pravein Govindan Kannan <pravein.govindan.kannan@ibm.com>
---
 src/plugins/uccl/uccl_backend.cpp | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/src/plugins/uccl/uccl_backend.cpp b/src/plugins/uccl/uccl_backend.cpp
index bc18f086a6..05b2bf5f95 100644
--- a/src/plugins/uccl/uccl_backend.cpp
+++ b/src/plugins/uccl/uccl_backend.cpp
@@ -186,7 +186,7 @@ nixlUcclEngine::getPublicData(const nixlBackendMD *meta, std::string &str) const
         snprintf(hex, sizeof(hex), "%02x", static_cast<unsigned char>(priv->fifo_item[i]));
         str += hex;
     }
-    NIXL_DEBUG << "Exporting Meta Info =" << hex_str << sts::endl;
+    NIXL_DEBUG << "Exporting Meta Info =" << str << std::endl;
 
     return NIXL_SUCCESS;
 }
@@ -372,7 +372,7 @@ nixlUcclEngine::loadRemoteMD(const nixlBlobDesc &input,
 
     // Decode fifo_item from hex string
     const std::string &hex_str = input.metaInfo;
-    NIXL_DEBUG << "Meta Info =" << hex_str << sts::endl;
+    NIXL_DEBUG << "Meta Info =" << hex_str << std::endl;
     if (hex_str.length() == FIFO_ITEM_SIZE * 2) {
         for (int i = 0; i < FIFO_ITEM_SIZE; i++) {
             std::string byte_str = hex_str.substr(i * 2, 2);

From a9bcd2bafbe18e1b6a6ed74a0bd718fe265ffd67 Mon Sep 17 00:00:00 2001
From: Pravein Govindan Kannan <pravein.govindan.kannan@ibm.com>
Date: Thu, 22 Jan 2026 15:08:34 +0530
Subject: [PATCH 03/28] Add vector read/write support

Signed-off-by: Pravein Govindan Kannan <pravein.govindan.kannan@ibm.com>
---
 src/plugins/uccl/uccl_backend.cpp | 156 ++++++++++++++++--------------
 src/plugins/uccl/uccl_backend.h   |   2 +-
 2 files changed, 85 insertions(+), 73 deletions(-)

diff --git a/src/plugins/uccl/uccl_backend.cpp b/src/plugins/uccl/uccl_backend.cpp
index 05b2bf5f95..9aa943d970 100644
--- a/src/plugins/uccl/uccl_backend.cpp
+++ b/src/plugins/uccl/uccl_backend.cpp
@@ -107,10 +107,7 @@ nixlUcclEngine::~nixlUcclEngine() {
         std::lock_guard<std::mutex> lock(mem_mutex_);
         for (auto &[addr, priv] : mem_reg_info_) {
             if (priv && priv->mr_id != 0) {
-                uccl_mr_t *mr = reinterpret_cast<uccl_mr_t *>(priv->mr_id);
-                if (mr) {
-                    uccl_engine_mr_destroy(mr);
-                }
+                uccl_engine_mr_destroy(engine_, priv->mr_id);
             }
             delete priv;
         }
@@ -290,8 +287,10 @@ nixlUcclEngine::registerMem(const nixlBlobDesc &mem,
     }
 
     // Register memory with UCCL engine
-    uccl_mr_t *mr = uccl_engine_reg(engine_, mem.addr, mem.len);
-    if (!mr) {
+    uccl_mr_t mr;
+    // Register memory with UCCL engine
+    int result = uccl_engine_reg(engine_, mem.addr, mem.len, mr);
+    if (result != 0) {
         NIXL_ERROR << "Failed to register memory with UCCL engine";
         return NIXL_ERR_BACKEND;
     }
@@ -300,7 +299,7 @@ nixlUcclEngine::registerMem(const nixlBlobDesc &mem,
     priv->addr = (void *)mem.addr;
     priv->length = mem.len;
     priv->ref_cnt = 1;
-    priv->mr_id = reinterpret_cast<uint64_t>(mr); // Store the memory region handle
+    priv->mr_id = mr;
 
     // Pre-compute fifo_item for one-sided RDMA operations
     int result = uccl_engine_prepare_fifo(engine_, mr, (void *)mem.addr,
@@ -328,14 +327,8 @@ nixlUcclEngine::deregisterMem(nixlBackendMD *meta) {
     if (priv->ref_cnt > 0) return NIXL_SUCCESS;
 
     // Deregister memory from UCCL engine
-    if (priv->mr_id != 0) {
-        uccl_mr_t *mr = reinterpret_cast<uccl_mr_t *>(priv->mr_id);
-        if (mr) {
-            uccl_engine_mr_destroy(mr);
-            NIXL_DEBUG << "Deregistered memory: " << priv->addr << " mr_id: " << priv->mr_id;
-        }
-        priv->mr_id = 0;
-    }
+    uccl_engine_mr_destroy(engine_, priv->mr_id);
+    NIXL_DEBUG << "Deregistered memory: " << priv->addr << " mr_id: " << priv->mr_id;
 
     mem_reg_info_.erase((uint64_t)priv->addr);
     delete priv;
@@ -481,15 +474,27 @@ nixlUcclEngine::prepXfer(const nixl_xfer_op_t &operation,
         if (rcmode) {
             memcpy(uccl_handle->fifo_items[i].data(), rmd->fifo_item, FIFO_ITEM_SIZE);
 
+            // Log the original fifo_item address before overwriting
+            uint64_t original_fifo_addr = *reinterpret_cast<uint64_t *>(rmd->fifo_item);
+            uint32_t *rkey_ptr = reinterpret_cast<uint32_t *>(rmd->fifo_item + 32); // padding offset
+            NIXL_ERROR << "DEBUG: Original fifo_item addr=" << std::hex << original_fifo_addr
+                       << ", rmd->addr=" << std::hex << (uintptr_t)rmd->addr
+                       << ", remote_addr=" << std::hex << remote_addr
+                       << ", first_rkey=" << rkey_ptr[0];
+
             // Update the address and size in the fifo_item for the actual transfer
-            // FifoItem layout: addr (8 bytes at offset 0), size (4 bytes at offset 8)
-            uint64_t *fifo_addr = reinterpret_cast<uint64_t *>(uccl_handle->fifo_items[i].data());
-            uint32_t *fifo_size = reinterpret_cast<uint32_t *>(uccl_handle->fifo_items[i].data() + 8);
-            *fifo_addr = remote_addr;
-            *fifo_size = static_cast<uint32_t>(rsize);
-
-            NIXL_DEBUG << "Using pre-shared fifo_item[" << i << "]: addr=" << std::hex
-                       << remote_addr << ", size=" << std::dec << rsize;
+            int result = uccl_engine_update_fifo(
+                uccl_handle->fifo_items[i].data(),
+                remote_addr,
+                static_cast<uint32_t>(rsize)
+            );
+            if (result != 0) {
+                NIXL_ERROR << "Failed to update FIFO item";
+                return NIXL_ERR_BACKEND;
+            }
+
+            NIXL_ERROR << "DEBUG: After update fifo_item addr=" << std::hex
+                       << *fifo_addr << ", size=" << std::dec << *fifo_size;
         } else {
             // TODO : Suppport UC one-sided
             return NIXL_ERR_BACKEND;
@@ -548,10 +553,11 @@ nixlUcclEngine::postXfer(const nixl_xfer_op_t &operation,
             return NIXL_ERR_INVALID_PARAM;
         }
     }
-
-    // Process each descriptor pair
-    // TODO: Use a vector send async API to send all the transfers at once
-    std::lock_guard<std::mutex> lock(mem_mutex_); // Lock once for the entire loop
+    std::vector<uccl_mr_t> mr_ids;
+    std::vector<void*> addr_v;
+    std::vector<size_t> size_v;
+    
+    std::lock_guard<std::mutex> lock(mem_mutex_); // Lock once for the entire operation
     for (size_t i = 0; i < lcnt; i++) {
         lmd = (nixlUcclBackendMD *)local[i].metadataP;
         rmd = (nixlUcclBackendMD *)remote[i].metadataP;
@@ -579,61 +585,60 @@ nixlUcclEngine::postXfer(const nixl_xfer_op_t &operation,
         }
 
         auto local_priv = local_mem_iter->second;
-        if (local_priv->mr_id == 0) {
-            NIXL_ERROR << "Local memory region not properly registered";
-            return NIXL_ERR_BACKEND;
-        }
 
-        uccl_mr_t *local_mr = reinterpret_cast<uccl_mr_t *>(local_priv->mr_id);
+        mr_ids.push_back(local_priv->mr_id);
+        addr_v.push_back((void *)local_addr);
+        size_v.push_back(lsize);
+    }
 
-        int result = 0;
-        uint64_t transfer_id = 0;
+    // Perform the vector operation (single call for all transfers)
+    int result = 0;
+    uint64_t transfer_id = 0;
+    nixlUcclReqH *uccl_handle = static_cast<nixlUcclReqH *>(handle);
 
-        char *fifo_item_data = nullptr;
-        if (rcmode && handle) {
-            nixlUcclReqH *uccl_handle = static_cast<nixlUcclReqH *>(handle);
-            if (i < uccl_handle->fifo_items.size()) {
-                fifo_item_data = uccl_handle->fifo_items[i].data();
-            } else {
-                NIXL_ERROR << "No FIFO item found for item: " << i
-                           << ", fifo_items.size()=" << uccl_handle->fifo_items.size();
-                return NIXL_ERR_BACKEND;
-            }
-        }
+    auto fifo_items = uccl_handle->fifo_items;
 
-        switch (operation) {
-        case NIXL_READ: {
-            result = uccl_engine_read(
-                conn, local_mr, (void *)local_addr, lsize, fifo_item_data, &transfer_id);
-            break;
-        }
-        case NIXL_WRITE:
-            if (rcmode) {
-                result = uccl_engine_write_rc(
-                    conn, local_mr, (void *)local_addr, lsize, fifo_item_data, &transfer_id);
-            } else {
-                result = uccl_engine_write(conn, local_mr, (void *)local_addr, lsize, &transfer_id);
-            }
-            break;
-        default:
-            NIXL_ERROR << "Unsupported operation type: " << operation;
+    switch (operation) {
+    case NIXL_READ: {
+        if (!rcmode) {
+            NIXL_ERROR << "NIXL_READ operations require UCCL_RCMODE=1";
             return NIXL_ERR_INVALID_PARAM;
         }
-
-        if (result != 0) {
-            NIXL_ERROR << "UCCL operation failed with result: " << result;
-            return NIXL_ERR_BACKEND;
+        result = uccl_engine_read_vector(
+            conn, mr_ids, addr_v, size_v, fifo_items, lcnt, &transfer_id);
+        break;
+    }
+    case NIXL_WRITE: {
+        if (rcmode) {
+            // RC write_vector takes non-const void*
+            result = uccl_engine_write_vector_rc(
+                conn, mr_ids, addr_v, size_v, fifo_items, lcnt, &transfer_id);
+        } else {
+            // TODO: Non-RC write
+            // std::vector<void const*> const_addr_v(addr_v.begin(), addr_v.end());
+            // result = uccl_engine_write_vector(
+            //     conn, mr_ids, const_addr_v, size_v, lcnt, &transfer_id);
         }
+        break;
+    }
+    default:
+        NIXL_ERROR << "Unsupported operation type: " << operation;
+        return NIXL_ERR_INVALID_PARAM;
+    }
 
-        if (!handle) {
-            handle = new nixlUcclReqH(conn);
-        }
-        uccl_handle = static_cast<nixlUcclReqH *>(handle);
-        uccl_handle->pending_transfer_ids.insert(transfer_id);
+    if (result != 0) {
+        NIXL_ERROR << "UCCL operation failed with result: " << result;
+        return NIXL_ERR_BACKEND;
+    }
 
-        NIXL_DEBUG << "Successfully posted " << (operation == NIXL_READ ? "READ" : "WRITE")
-                   << " operation: " << lsize << " bytes with transfer_id: " << transfer_id;
+    if (!handle) {
+        handle = new nixlUcclReqH(conn);
     }
+    uccl_handle->pending_transfer_ids.insert(transfer_id);
+
+    NIXL_DEBUG << "Successfully posted vector " << (operation == NIXL_READ ? "READ" : "WRITE")
+               << " operation with " << lcnt << " iovecs, transfer_id: " << transfer_id;
+    
     if (opt_args && opt_args->hasNotif) {
         uccl_handle->notif_msg = opt_args->notifMsg;
     }
@@ -665,12 +670,16 @@ nixlUcclEngine::checkXfer(nixlBackendReqH *handle) const {
         uint64_t transfer_id = *it;
         int is_done = uccl_engine_xfer_status(conn, transfer_id);
         if (is_done) {
+            NIXL_ERROR << "DEBUG: Transfer " << transfer_id << " completed";
             it = uccl_handle->pending_transfer_ids.erase(it);
         } else {
             ++it;
         }
     }
     bool all_done = uccl_handle->pending_transfer_ids.empty();
+    if (all_done) {
+        NIXL_ERROR << "DEBUG: All transfers complete";
+    }
     if (all_done && !uccl_handle->notif_msg.empty()) {
         nixlSerDes ser_des;
         ser_des.addStr("msg", uccl_handle->notif_msg);
@@ -694,6 +703,7 @@ nixlUcclEngine::checkXfer(nixlBackendReqH *handle) const {
         }
     }
 
+    NIXL_ERROR << "DEBUG checkXfer: returning " << (all_done ? "SUCCESS" : "IN_PROG");
     return (all_done) ? NIXL_SUCCESS : NIXL_IN_PROG;
 }
 
@@ -715,7 +725,9 @@ nixl_status_t
 nixlUcclEngine::getNotifs(notif_list_t &notif_list) {
     if (notif_list.size() != 0) return NIXL_ERR_INVALID_PARAM;
 
+    NIXL_ERROR << "DEBUG getNotifs: polling for notifications";
     std::vector<notify_msg_t> notify_msgs = uccl_engine_get_notifs();
+    NIXL_ERROR << "DEBUG getNotifs: got " << notify_msgs.size() << " notifications";
     for (size_t i = 0; i < notify_msgs.size(); i++) {
         size_t msg_len = sizeof(notify_msgs[i].msg);
         std::string serialized_str(notify_msgs[i].msg, msg_len);
diff --git a/src/plugins/uccl/uccl_backend.h b/src/plugins/uccl/uccl_backend.h
index 4cb3340393..57cccae16d 100644
--- a/src/plugins/uccl/uccl_backend.h
+++ b/src/plugins/uccl/uccl_backend.h
@@ -148,7 +148,7 @@ class nixlUcclBackendMD : public nixlBackendMD {
     void *addr;
     size_t length;
     int ref_cnt;
-    uint64_t mr_id; // UCCL memory region id
+    uccl_mr_t mr_id; // UCCL memory region id
     char fifo_item[FIFO_ITEM_SIZE];
 };
 

From 8fd6a223c620f5fdea115d11e4931998cdf064e1 Mon Sep 17 00:00:00 2001
From: Pravein Govindan Kannan <pravein.govindan.kannan@ibm.com>
Date: Fri, 23 Jan 2026 13:07:34 +0530
Subject: [PATCH 04/28] Use prepare fifo APIs

Signed-off-by: Pravein Govindan Kannan <pravein.govindan.kannan@ibm.com>
---
 src/plugins/uccl/uccl_backend.cpp | 57 +++++++------------------------
 src/plugins/uccl/uccl_backend.h   |  3 +-
 2 files changed, 15 insertions(+), 45 deletions(-)

diff --git a/src/plugins/uccl/uccl_backend.cpp b/src/plugins/uccl/uccl_backend.cpp
index 9aa943d970..0e63006e85 100644
--- a/src/plugins/uccl/uccl_backend.cpp
+++ b/src/plugins/uccl/uccl_backend.cpp
@@ -288,7 +288,6 @@ nixlUcclEngine::registerMem(const nixlBlobDesc &mem,
 
     // Register memory with UCCL engine
     uccl_mr_t mr;
-    // Register memory with UCCL engine
     int result = uccl_engine_reg(engine_, mem.addr, mem.len, mr);
     if (result != 0) {
         NIXL_ERROR << "Failed to register memory with UCCL engine";
@@ -302,11 +301,11 @@ nixlUcclEngine::registerMem(const nixlBlobDesc &mem,
     priv->mr_id = mr;
 
     // Pre-compute fifo_item for one-sided RDMA operations
-    int result = uccl_engine_prepare_fifo(engine_, mr, (void *)mem.addr,
+    result = uccl_engine_prepare_fifo(engine_, mr, (void *)mem.addr,
                                                  mem.len, priv->fifo_item);
     if (result != 0) {
         NIXL_ERROR << "Failed to prepare fifo_item for memory region";
-        uccl_engine_mr_destroy(mr);
+        uccl_engine_mr_destroy(engine_, mr);
         delete priv;
         return NIXL_ERR_BACKEND;
     }
@@ -442,7 +441,6 @@ nixlUcclEngine::prepXfer(const nixl_xfer_op_t &operation,
     handle = new nixlUcclReqH(conn);
     nixlUcclReqH *uccl_handle = static_cast<nixlUcclReqH *>(handle);
 
-    uccl_handle->fifo_items.clear();
     uccl_handle->fifo_items.resize(lcnt);
 
     std::lock_guard<std::mutex> lock(mem_mutex_);
@@ -466,35 +464,14 @@ nixlUcclEngine::prepXfer(const nixl_xfer_op_t &operation,
             return NIXL_ERR_BACKEND;
         }
 
-        auto local_priv = local_mem_iter->second;
-        if (local_priv->mr_id == 0) {
-            NIXL_ERROR << "Local memory region not properly registered";
-            return NIXL_ERR_BACKEND;
-        }
         if (rcmode) {
-            memcpy(uccl_handle->fifo_items[i].data(), rmd->fifo_item, FIFO_ITEM_SIZE);
-
-            // Log the original fifo_item address before overwriting
-            uint64_t original_fifo_addr = *reinterpret_cast<uint64_t *>(rmd->fifo_item);
-            uint32_t *rkey_ptr = reinterpret_cast<uint32_t *>(rmd->fifo_item + 32); // padding offset
-            NIXL_ERROR << "DEBUG: Original fifo_item addr=" << std::hex << original_fifo_addr
-                       << ", rmd->addr=" << std::hex << (uintptr_t)rmd->addr
-                       << ", remote_addr=" << std::hex << remote_addr
-                       << ", first_rkey=" << rkey_ptr[0];
-
-            // Update the address and size in the fifo_item for the actual transfer
-            int result = uccl_engine_update_fifo(
-                uccl_handle->fifo_items[i].data(),
-                remote_addr,
-                static_cast<uint32_t>(rsize)
-            );
-            if (result != 0) {
-                NIXL_ERROR << "Failed to update FIFO item";
-                return NIXL_ERR_BACKEND;
-            }
+            // Deserialize fifo_item from char[] into FifoItem struct
+            deserialize_fifo_item(rmd->fifo_item, &uccl_handle->fifo_items[i]);
 
-            NIXL_ERROR << "DEBUG: After update fifo_item addr=" << std::hex
-                       << *fifo_addr << ", size=" << std::dec << *fifo_size;
+            uccl_engine_update_fifo(uccl_handle->fifo_items[i], remote_addr, rsize);
+
+            NIXL_DEBUG << "Using pre-shared fifo_item[" << i << "]: addr=" << std::hex
+                       << remote_addr << ", size=" << std::dec << rsize;
         } else {
             // TODO : Suppport UC one-sided
             return NIXL_ERR_BACKEND;
@@ -594,9 +571,7 @@ nixlUcclEngine::postXfer(const nixl_xfer_op_t &operation,
     // Perform the vector operation (single call for all transfers)
     int result = 0;
     uint64_t transfer_id = 0;
-    nixlUcclReqH *uccl_handle = static_cast<nixlUcclReqH *>(handle);
-
-    auto fifo_items = uccl_handle->fifo_items;
+    uccl_handle = static_cast<nixlUcclReqH *>(handle);
 
     switch (operation) {
     case NIXL_READ: {
@@ -605,14 +580,13 @@ nixlUcclEngine::postXfer(const nixl_xfer_op_t &operation,
             return NIXL_ERR_INVALID_PARAM;
         }
         result = uccl_engine_read_vector(
-            conn, mr_ids, addr_v, size_v, fifo_items, lcnt, &transfer_id);
+            conn, mr_ids, addr_v, size_v, uccl_handle->fifo_items, lcnt, &transfer_id);
         break;
     }
     case NIXL_WRITE: {
         if (rcmode) {
-            // RC write_vector takes non-const void*
-            result = uccl_engine_write_vector_rc(
-                conn, mr_ids, addr_v, size_v, fifo_items, lcnt, &transfer_id);
+            result = uccl_engine_write_rc_vector(
+                conn, mr_ids, addr_v, size_v, uccl_handle->fifo_items, lcnt, &transfer_id);
         } else {
             // TODO: Non-RC write
             // std::vector<void const*> const_addr_v(addr_v.begin(), addr_v.end());
@@ -677,9 +651,7 @@ nixlUcclEngine::checkXfer(nixlBackendReqH *handle) const {
         }
     }
     bool all_done = uccl_handle->pending_transfer_ids.empty();
-    if (all_done) {
-        NIXL_ERROR << "DEBUG: All transfers complete";
-    }
+
     if (all_done && !uccl_handle->notif_msg.empty()) {
         nixlSerDes ser_des;
         ser_des.addStr("msg", uccl_handle->notif_msg);
@@ -703,7 +675,6 @@ nixlUcclEngine::checkXfer(nixlBackendReqH *handle) const {
         }
     }
 
-    NIXL_ERROR << "DEBUG checkXfer: returning " << (all_done ? "SUCCESS" : "IN_PROG");
     return (all_done) ? NIXL_SUCCESS : NIXL_IN_PROG;
 }
 
@@ -725,9 +696,7 @@ nixl_status_t
 nixlUcclEngine::getNotifs(notif_list_t &notif_list) {
     if (notif_list.size() != 0) return NIXL_ERR_INVALID_PARAM;
 
-    NIXL_ERROR << "DEBUG getNotifs: polling for notifications";
     std::vector<notify_msg_t> notify_msgs = uccl_engine_get_notifs();
-    NIXL_ERROR << "DEBUG getNotifs: got " << notify_msgs.size() << " notifications";
     for (size_t i = 0; i < notify_msgs.size(); i++) {
         size_t msg_len = sizeof(notify_msgs[i].msg);
         std::string serialized_str(notify_msgs[i].msg, msg_len);
diff --git a/src/plugins/uccl/uccl_backend.h b/src/plugins/uccl/uccl_backend.h
index 57cccae16d..94e3115c9d 100644
--- a/src/plugins/uccl/uccl_backend.h
+++ b/src/plugins/uccl/uccl_backend.h
@@ -35,6 +35,7 @@
 #include "uccl_engine.h"
 
 #define FIFO_ITEM_SIZE 64
+// FifoItem and deserialize_fifo_item are now provided by uccl_engine.h
 
 class nixlUcclBackendMD;
 class nixlUcclReqH;
@@ -162,7 +163,7 @@ class nixlUcclReqH : public nixlBackendReqH {
     uccl_conn_t *conn;
     std::unordered_set<uint64_t> pending_transfer_ids;
     nixl_blob_t notif_msg;
-    std::vector<std::array<char, FIFO_ITEM_SIZE>> fifo_items;
+    std::vector<FifoItem> fifo_items;
 };
 
 #endif

From 897f4374adf66c6cb06a6c1b25178c72ee0f5469 Mon Sep 17 00:00:00 2001
From: Pravein Govindan Kannan <pravein.govindan.kannan@ibm.com>
Date: Fri, 23 Jan 2026 14:06:49 +0530
Subject: [PATCH 05/28] Remove RCMODE

Signed-off-by: Pravein Govindan Kannan <pravein.govindan.kannan@ibm.com>
---
 src/plugins/uccl/uccl_backend.cpp     | 34 +--------------------------
 test/gtest/plugins/uccl/uccl_test.cpp |  3 ---
 2 files changed, 1 insertion(+), 36 deletions(-)

diff --git a/src/plugins/uccl/uccl_backend.cpp b/src/plugins/uccl/uccl_backend.cpp
index 0e63006e85..c2c9b11868 100644
--- a/src/plugins/uccl/uccl_backend.cpp
+++ b/src/plugins/uccl/uccl_backend.cpp
@@ -428,16 +428,6 @@ nixlUcclEngine::prepXfer(const nixl_xfer_op_t &operation,
         return NIXL_ERR_INVALID_PARAM;
     }
 
-    const char *uccl_rcmode = std::getenv("UCCL_RCMODE");
-    rcmode = (uccl_rcmode && std::strcmp(uccl_rcmode, "1") == 0);
-
-    if (operation == NIXL_READ) {
-        if (!rcmode) {
-            NIXL_ERROR
-                << "UCCL_RCMODE environment variable must be set to 1 for NIXL_READ operations";
-            return NIXL_ERR_INVALID_PARAM;
-        }
-    }
     handle = new nixlUcclReqH(conn);
     nixlUcclReqH *uccl_handle = static_cast<nixlUcclReqH *>(handle);
 
@@ -520,16 +510,6 @@ nixlUcclEngine::postXfer(const nixl_xfer_op_t &operation,
         return NIXL_ERR_INVALID_PARAM;
     }
 
-    const char *uccl_rcmode = std::getenv("UCCL_RCMODE");
-    rcmode = (uccl_rcmode && std::strcmp(uccl_rcmode, "1") == 0);
-
-    if (operation == NIXL_READ) {
-        if (!rcmode) {
-            NIXL_ERROR
-                << "UCCL_RCMODE environment variable must be set to 1 for NIXL_READ operations";
-            return NIXL_ERR_INVALID_PARAM;
-        }
-    }
     std::vector<uccl_mr_t> mr_ids;
     std::vector<void*> addr_v;
     std::vector<size_t> size_v;
@@ -575,24 +555,13 @@ nixlUcclEngine::postXfer(const nixl_xfer_op_t &operation,
 
     switch (operation) {
     case NIXL_READ: {
-        if (!rcmode) {
-            NIXL_ERROR << "NIXL_READ operations require UCCL_RCMODE=1";
-            return NIXL_ERR_INVALID_PARAM;
-        }
         result = uccl_engine_read_vector(
             conn, mr_ids, addr_v, size_v, uccl_handle->fifo_items, lcnt, &transfer_id);
         break;
     }
     case NIXL_WRITE: {
-        if (rcmode) {
-            result = uccl_engine_write_rc_vector(
+            result = uccl_engine_write_vector(
                 conn, mr_ids, addr_v, size_v, uccl_handle->fifo_items, lcnt, &transfer_id);
-        } else {
-            // TODO: Non-RC write
-            // std::vector<void const*> const_addr_v(addr_v.begin(), addr_v.end());
-            // result = uccl_engine_write_vector(
-            //     conn, mr_ids, const_addr_v, size_v, lcnt, &transfer_id);
-        }
         break;
     }
     default:
@@ -644,7 +613,6 @@ nixlUcclEngine::checkXfer(nixlBackendReqH *handle) const {
         uint64_t transfer_id = *it;
         int is_done = uccl_engine_xfer_status(conn, transfer_id);
         if (is_done) {
-            NIXL_ERROR << "DEBUG: Transfer " << transfer_id << " completed";
             it = uccl_handle->pending_transfer_ids.erase(it);
         } else {
             ++it;
diff --git a/test/gtest/plugins/uccl/uccl_test.cpp b/test/gtest/plugins/uccl/uccl_test.cpp
index 50e9042063..30f96466c6 100644
--- a/test/gtest/plugins/uccl/uccl_test.cpp
+++ b/test/gtest/plugins/uccl/uccl_test.cpp
@@ -255,9 +255,6 @@ TestUcclBackend::TestUcclBackend() {
 template<TestUcclBackend::TestType test_type, enum nixl_xfer_op_t op>
 void
 TestUcclBackend::testXfer() {
-    if (op == NIXL_READ) {
-        m_env.addVar("UCCL_RCMODE", "1");
-    }
     const std::string initiator_name = "initiator";
     const std::string target_name = "target";
 

From 5921a683b41d26f12b773fb3c5ab6d7f94fd08ec Mon Sep 17 00:00:00 2001
From: Pravein Govindan Kannan <pravein.govindan.kannan@ibm.com>
Date: Fri, 23 Jan 2026 14:20:58 +0530
Subject: [PATCH 06/28] Remove rcmode in prepXfer

Signed-off-by: Pravein Govindan Kannan <pravein.govindan.kannan@ibm.com>
---
 src/plugins/uccl/uccl_backend.cpp | 16 +++++-----------
 1 file changed, 5 insertions(+), 11 deletions(-)

diff --git a/src/plugins/uccl/uccl_backend.cpp b/src/plugins/uccl/uccl_backend.cpp
index c2c9b11868..90d8370025 100644
--- a/src/plugins/uccl/uccl_backend.cpp
+++ b/src/plugins/uccl/uccl_backend.cpp
@@ -399,7 +399,6 @@ nixlUcclEngine::prepXfer(const nixl_xfer_op_t &operation,
                          const nixl_opt_b_args_t *opt_args) const {
     nixlUcclBackendMD *lmd;
     nixlUcclBackendMD *rmd;
-    bool rcmode = false;
     handle = nullptr;
 
     NIXL_DEBUG << "UCCL PrepXfer: " << operation << " remote_agent: " << remote_agent;
@@ -454,18 +453,13 @@ nixlUcclEngine::prepXfer(const nixl_xfer_op_t &operation,
             return NIXL_ERR_BACKEND;
         }
 
-        if (rcmode) {
-            // Deserialize fifo_item from char[] into FifoItem struct
-            deserialize_fifo_item(rmd->fifo_item, &uccl_handle->fifo_items[i]);
+        // Deserialize fifo_item from char[] into FifoItem struct
+        deserialize_fifo_item(rmd->fifo_item, &uccl_handle->fifo_items[i]);
 
-            uccl_engine_update_fifo(uccl_handle->fifo_items[i], remote_addr, rsize);
+        uccl_engine_update_fifo(uccl_handle->fifo_items[i], remote_addr, rsize);
 
-            NIXL_DEBUG << "Using pre-shared fifo_item[" << i << "]: addr=" << std::hex
-                       << remote_addr << ", size=" << std::dec << rsize;
-        } else {
-            // TODO : Suppport UC one-sided
-            return NIXL_ERR_BACKEND;
-        }
+        NIXL_DEBUG << "Using pre-shared fifo_item[" << i << "]: addr=" << std::hex
+                   << remote_addr << ", size=" << std::dec << rsize;
     }
 
     return NIXL_SUCCESS;

From f03ee88f274fdaa43e017a50d0540a611b4511d0 Mon Sep 17 00:00:00 2001
From: Pravein Govindan Kannan <pravein.govindan.kannan@ibm.com>
Date: Fri, 23 Jan 2026 14:26:01 +0530
Subject: [PATCH 07/28] Remove rcmode from CI

Signed-off-by: Pravein Govindan Kannan <pravein.govindan.kannan@ibm.com>
---
 .gitlab/test_nixlbench.sh         | 2 +-
 src/plugins/uccl/uccl_backend.cpp | 1 -
 2 files changed, 1 insertion(+), 2 deletions(-)

diff --git a/.gitlab/test_nixlbench.sh b/.gitlab/test_nixlbench.sh
index 1d5aa2f376..b7a769825c 100755
--- a/.gitlab/test_nixlbench.sh
+++ b/.gitlab/test_nixlbench.sh
@@ -111,7 +111,7 @@ if $HAS_GPU ; then
     for op_type in READ WRITE; do
         for initiator in $seg_types; do
             for target in $seg_types; do
-                UCCL_RCMODE=1 run_nixlbench_two_workers --backend UCCL --op_type $op_type --initiator_seg_type $initiator --target_seg_type $target --check_consistency
+                run_nixlbench_two_workers --backend UCCL --op_type $op_type --initiator_seg_type $initiator --target_seg_type $target --check_consistency
             done
         done
     done
diff --git a/src/plugins/uccl/uccl_backend.cpp b/src/plugins/uccl/uccl_backend.cpp
index 90d8370025..d6522a6043 100644
--- a/src/plugins/uccl/uccl_backend.cpp
+++ b/src/plugins/uccl/uccl_backend.cpp
@@ -475,7 +475,6 @@ nixlUcclEngine::postXfer(const nixl_xfer_op_t &operation,
     nixlUcclReqH *uccl_handle;
     nixlUcclBackendMD *lmd;
     nixlUcclBackendMD *rmd;
-    bool rcmode = false;
 
     NIXL_DEBUG << "UCCL PostXfer: " << operation << " remote_agent: " << remote_agent;
 

From 76f8c81b0a8c6cc83151f62df14ac22727d35f90 Mon Sep 17 00:00:00 2001
From: Pravein Govindan Kannan <pravein.govindan.kannan@ibm.com>
Date: Fri, 23 Jan 2026 15:19:56 +0530
Subject: [PATCH 08/28] Fix cleanup

Signed-off-by: Pravein Govindan Kannan <pravein.govindan.kannan@ibm.com>
---
 src/plugins/uccl/uccl_backend.cpp | 30 ++++++++++++++++++++++--------
 src/plugins/uccl/uccl_backend.h   |  1 +
 2 files changed, 23 insertions(+), 8 deletions(-)

diff --git a/src/plugins/uccl/uccl_backend.cpp b/src/plugins/uccl/uccl_backend.cpp
index d6522a6043..5963cbf737 100644
--- a/src/plugins/uccl/uccl_backend.cpp
+++ b/src/plugins/uccl/uccl_backend.cpp
@@ -89,7 +89,7 @@ getNixlParam(const nixl_b_params_t *custom_params, const std::string &key, int d
 }
 
 nixlUcclEngine::nixlUcclEngine(const nixlBackendInitParams *init_params)
-    : nixlBackendEngine(init_params) {
+    : nixlBackendEngine(init_params), stop_listener_(false) {
 
     local_agent_name_ = init_params->localAgent;
     nixl_b_params_t *custom_params = init_params->customParams;
@@ -103,6 +103,8 @@ nixlUcclEngine::nixlUcclEngine(const nixlBackendInitParams *init_params)
 }
 
 nixlUcclEngine::~nixlUcclEngine() {
+    stop_listener_ = true;
+
     {
         std::lock_guard<std::mutex> lock(mem_mutex_);
         for (auto &[addr, priv] : mem_reg_info_) {
@@ -128,27 +130,38 @@ nixlUcclEngine::~nixlUcclEngine() {
         connected_agents_.clear();
     }
 
-    if (listener_thread_.joinable()) {
-        listener_thread_.detach();
-    }
-
     if (engine_) {
-        // Add a small delay to allow UCCL internal cleanup to complete
-        std::this_thread::sleep_for(std::chrono::milliseconds(1));
         uccl_engine_destroy(engine_);
         engine_ = nullptr;
     }
+
+    std::this_thread::sleep_for(std::chrono::milliseconds(100));
+
+    if (listener_thread_.joinable()) {
+        listener_thread_.detach();
+    }
 }
 
 void
 nixlUcclEngine::startListener() {
     // The listener waits for connections from remote agents
     NIXL_DEBUG << "UCCL accepting connections";
-    while (true) {
+    while (!stop_listener_) {
+        // Check if engine is still valid before using it
+        if (!engine_) {
+            NIXL_DEBUG << "Engine destroyed, listener thread exiting";
+            break;
+        }
+
         char ip_buf[256];
         int remote_gpu_idx;
         uccl_conn_t *conn = uccl_engine_accept(engine_, ip_buf, sizeof(ip_buf), &remote_gpu_idx);
         if (!conn) {
+            // Check if we should stop (engine destroyed or shutdown requested)
+            if (stop_listener_ || !engine_) {
+                NIXL_DEBUG << "Listener thread stopping";
+                break;
+            }
             NIXL_ERROR << "Failed to accept connection from remote agent";
             continue;
         }
@@ -160,6 +173,7 @@ nixlUcclEngine::startListener() {
             connected_agents_[ip_buf] = reinterpret_cast<uint64_t>(conn);
         }
     }
+    NIXL_DEBUG << "UCCL listener thread exiting";
 }
 
 nixl_mem_list_t
diff --git a/src/plugins/uccl/uccl_backend.h b/src/plugins/uccl/uccl_backend.h
index 94e3115c9d..c6643137fe 100644
--- a/src/plugins/uccl/uccl_backend.h
+++ b/src/plugins/uccl/uccl_backend.h
@@ -135,6 +135,7 @@ class nixlUcclEngine : public nixlBackendEngine {
     std::unordered_map<uint64_t, nixlUcclBackendMD *> mem_reg_info_;
     std::unordered_map<std::string, uint64_t> connected_agents_; // agent name -> conn_id
     std::thread listener_thread_;
+    std::atomic<bool> stop_listener_;
 };
 
 // UCCL Backend Memory Descriptor

From e80c85b56139451aef1613b4a3ff7698cb549993 Mon Sep 17 00:00:00 2001
From: Pravein Govindan Kannan <pravein.govindan.kannan@ibm.com>
Date: Wed, 28 Jan 2026 17:33:25 +0530
Subject: [PATCH 09/28] Maintain a single transfer ID

Signed-off-by: Pravein Govindan Kannan <pravein.govindan.kannan@ibm.com>
---
 src/plugins/uccl/uccl_backend.cpp | 20 +++++---------------
 src/plugins/uccl/uccl_backend.h   |  2 +-
 2 files changed, 6 insertions(+), 16 deletions(-)

diff --git a/src/plugins/uccl/uccl_backend.cpp b/src/plugins/uccl/uccl_backend.cpp
index 5963cbf737..cb1e7f4647 100644
--- a/src/plugins/uccl/uccl_backend.cpp
+++ b/src/plugins/uccl/uccl_backend.cpp
@@ -584,7 +584,7 @@ nixlUcclEngine::postXfer(const nixl_xfer_op_t &operation,
     if (!handle) {
         handle = new nixlUcclReqH(conn);
     }
-    uccl_handle->pending_transfer_ids.insert(transfer_id);
+    uccl_handle->transfer_id = transfer_id;
 
     NIXL_DEBUG << "Successfully posted vector " << (operation == NIXL_READ ? "READ" : "WRITE")
                << " operation with " << lcnt << " iovecs, transfer_id: " << transfer_id;
@@ -615,19 +615,8 @@ nixlUcclEngine::checkXfer(nixlBackendReqH *handle) const {
         return NIXL_ERR_BACKEND;
     }
 
-    auto it = uccl_handle->pending_transfer_ids.begin();
-    while (it != uccl_handle->pending_transfer_ids.end()) {
-        uint64_t transfer_id = *it;
-        int is_done = uccl_engine_xfer_status(conn, transfer_id);
-        if (is_done) {
-            it = uccl_handle->pending_transfer_ids.erase(it);
-        } else {
-            ++it;
-        }
-    }
-    bool all_done = uccl_handle->pending_transfer_ids.empty();
-
-    if (all_done && !uccl_handle->notif_msg.empty()) {
+    int is_done = uccl_engine_xfer_status(conn, uccl_handle->transfer_id);
+    if (is_done) {
         nixlSerDes ser_des;
         ser_des.addStr("msg", uccl_handle->notif_msg);
         std::string serialized = ser_des.exportStr();
@@ -648,9 +637,10 @@ nixlUcclEngine::checkXfer(nixlBackendReqH *handle) const {
             NIXL_DEBUG << "All transfers in handle completed, sent notification: "
                        << uccl_handle->notif_msg;
         }
+        return NIXL_SUCCESS;
     }
 
-    return (all_done) ? NIXL_SUCCESS : NIXL_IN_PROG;
+    return NIXL_IN_PROG;
 }
 
 nixl_status_t
diff --git a/src/plugins/uccl/uccl_backend.h b/src/plugins/uccl/uccl_backend.h
index c6643137fe..ecfa609d59 100644
--- a/src/plugins/uccl/uccl_backend.h
+++ b/src/plugins/uccl/uccl_backend.h
@@ -162,7 +162,7 @@ class nixlUcclReqH : public nixlBackendReqH {
     virtual ~nixlUcclReqH() {}
 
     uccl_conn_t *conn;
-    std::unordered_set<uint64_t> pending_transfer_ids;
+    uint64_t transfer_id;
     nixl_blob_t notif_msg;
     std::vector<FifoItem> fifo_items;
 };

From 047fd3cb55fdf7b765c451703a60b9c1f341eede Mon Sep 17 00:00:00 2001
From: Pravein Govindan Kannan <pravein.govindan.kannan@ibm.com>
Date: Thu, 29 Jan 2026 10:09:47 +0530
Subject: [PATCH 10/28] Fix formatting

Signed-off-by: Pravein Govindan Kannan <pravein.govindan.kannan@ibm.com>
---
 src/plugins/uccl/uccl_backend.cpp | 37 +++++++++++++++----------------
 1 file changed, 18 insertions(+), 19 deletions(-)

diff --git a/src/plugins/uccl/uccl_backend.cpp b/src/plugins/uccl/uccl_backend.cpp
index cb1e7f4647..215aa25fb3 100644
--- a/src/plugins/uccl/uccl_backend.cpp
+++ b/src/plugins/uccl/uccl_backend.cpp
@@ -89,7 +89,8 @@ getNixlParam(const nixl_b_params_t *custom_params, const std::string &key, int d
 }
 
 nixlUcclEngine::nixlUcclEngine(const nixlBackendInitParams *init_params)
-    : nixlBackendEngine(init_params), stop_listener_(false) {
+    : nixlBackendEngine(init_params),
+      stop_listener_(false) {
 
     local_agent_name_ = init_params->localAgent;
     nixl_b_params_t *custom_params = init_params->customParams;
@@ -197,7 +198,6 @@ nixlUcclEngine::getPublicData(const nixlBackendMD *meta, std::string &str) const
         snprintf(hex, sizeof(hex), "%02x", static_cast<unsigned char>(priv->fifo_item[i]));
         str += hex;
     }
-    NIXL_DEBUG << "Exporting Meta Info =" << str << std::endl;
 
     return NIXL_SUCCESS;
 }
@@ -315,8 +315,7 @@ nixlUcclEngine::registerMem(const nixlBlobDesc &mem,
     priv->mr_id = mr;
 
     // Pre-compute fifo_item for one-sided RDMA operations
-    result = uccl_engine_prepare_fifo(engine_, mr, (void *)mem.addr,
-                                                 mem.len, priv->fifo_item);
+    result = uccl_engine_prepare_fifo(engine_, mr, (void *)mem.addr, mem.len, priv->fifo_item);
     if (result != 0) {
         NIXL_ERROR << "Failed to prepare fifo_item for memory region";
         uccl_engine_mr_destroy(engine_, mr);
@@ -326,7 +325,7 @@ nixlUcclEngine::registerMem(const nixlBlobDesc &mem,
 
     out = priv;
     mem_reg_info_[mem.addr] = priv;
-    NIXL_DEBUG << "Registering memory: " << mem.addr << "Device: " << mem.devId
+    NIXL_DEBUG << "Registering memory: " << std::hex << mem.addr << " Device: " << mem.devId
                << " ref_cnt: " << priv->ref_cnt << " mr_id: " << priv->mr_id;
 
     return NIXL_SUCCESS;
@@ -341,7 +340,7 @@ nixlUcclEngine::deregisterMem(nixlBackendMD *meta) {
 
     // Deregister memory from UCCL engine
     uccl_engine_mr_destroy(engine_, priv->mr_id);
-    NIXL_DEBUG << "Deregistered memory: " << priv->addr << " mr_id: " << priv->mr_id;
+    NIXL_DEBUG << "Deregistered memory: " << std::hex << priv->addr << " mr_id: " << priv->mr_id;
 
     mem_reg_info_.erase((uint64_t)priv->addr);
     delete priv;
@@ -351,7 +350,7 @@ nixlUcclEngine::deregisterMem(nixlBackendMD *meta) {
 nixl_status_t
 nixlUcclEngine::loadLocalMD(nixlBackendMD *input, nixlBackendMD *&output) {
     nixlUcclBackendMD *input_md = (nixlUcclBackendMD *)input;
-    NIXL_DEBUG << "UCCL Load Local MD: " << input_md->addr << "Meta Info:" << input_md->mr_id;
+    NIXL_DEBUG << "UCCL Load Local MD: " << std::hex << input_md->addr << "Meta Info:" << input_md->mr_id;
 
     nixlUcclBackendMD *output_md = (nixlUcclBackendMD *)output;
     output_md->addr = (void *)input_md->addr;
@@ -367,7 +366,7 @@ nixlUcclEngine::loadRemoteMD(const nixlBlobDesc &input,
                              const nixl_mem_t &nixl_mem,
                              const std::string &remote_agent,
                              nixlBackendMD *&output) {
-    NIXL_DEBUG << "UCCL Load Remote MD: " << input.addr << "Meta Info:" << input.metaInfo
+    NIXL_DEBUG << "UCCL Load Remote MD: " << std::hex <<  input.addr << " Meta Info:" << input.metaInfo
                << " remote_agent: " << remote_agent;
 
     output = new nixlUcclBackendMD(true);
@@ -378,7 +377,7 @@ nixlUcclEngine::loadRemoteMD(const nixlBlobDesc &input,
 
     // Decode fifo_item from hex string
     const std::string &hex_str = input.metaInfo;
-    NIXL_DEBUG << "Meta Info =" << hex_str << std::endl;
+
     if (hex_str.length() == FIFO_ITEM_SIZE * 2) {
         for (int i = 0; i < FIFO_ITEM_SIZE; i++) {
             std::string byte_str = hex_str.substr(i * 2, 2);
@@ -386,8 +385,8 @@ nixlUcclEngine::loadRemoteMD(const nixlBlobDesc &input,
         }
         NIXL_DEBUG << "Parsed fifo_item from remote metadata";
     } else {
-        NIXL_ERROR << "Invalid fifo_item hex string length: " << hex_str.length()
-                   << " (expected " << FIFO_ITEM_SIZE * 2 << ")";
+        NIXL_ERROR << "Invalid fifo_item hex string length: " << hex_str.length() << " (expected "
+                   << FIFO_ITEM_SIZE * 2 << ")";
         delete output_md;
         output = nullptr;
         return NIXL_ERR_INVALID_PARAM;
@@ -472,8 +471,8 @@ nixlUcclEngine::prepXfer(const nixl_xfer_op_t &operation,
 
         uccl_engine_update_fifo(uccl_handle->fifo_items[i], remote_addr, rsize);
 
-        NIXL_DEBUG << "Using pre-shared fifo_item[" << i << "]: addr=" << std::hex
-                   << remote_addr << ", size=" << std::dec << rsize;
+        NIXL_DEBUG << "Using pre-shared fifo_item[" << i << "]: addr=" << std::hex << remote_addr
+                   << ", size=" << std::dec << rsize;
     }
 
     return NIXL_SUCCESS;
@@ -518,9 +517,9 @@ nixlUcclEngine::postXfer(const nixl_xfer_op_t &operation,
     }
 
     std::vector<uccl_mr_t> mr_ids;
-    std::vector<void*> addr_v;
+    std::vector<void *> addr_v;
     std::vector<size_t> size_v;
-    
+
     std::lock_guard<std::mutex> lock(mem_mutex_); // Lock once for the entire operation
     for (size_t i = 0; i < lcnt; i++) {
         lmd = (nixlUcclBackendMD *)local[i].metadataP;
@@ -555,7 +554,7 @@ nixlUcclEngine::postXfer(const nixl_xfer_op_t &operation,
         size_v.push_back(lsize);
     }
 
-    // Perform the vector operation (single call for all transfers)
+    // Perform a vector operation
     int result = 0;
     uint64_t transfer_id = 0;
     uccl_handle = static_cast<nixlUcclReqH *>(handle);
@@ -567,8 +566,8 @@ nixlUcclEngine::postXfer(const nixl_xfer_op_t &operation,
         break;
     }
     case NIXL_WRITE: {
-            result = uccl_engine_write_vector(
-                conn, mr_ids, addr_v, size_v, uccl_handle->fifo_items, lcnt, &transfer_id);
+        result = uccl_engine_write_vector(
+            conn, mr_ids, addr_v, size_v, uccl_handle->fifo_items, lcnt, &transfer_id);
         break;
     }
     default:
@@ -588,7 +587,7 @@ nixlUcclEngine::postXfer(const nixl_xfer_op_t &operation,
 
     NIXL_DEBUG << "Successfully posted vector " << (operation == NIXL_READ ? "READ" : "WRITE")
                << " operation with " << lcnt << " iovecs, transfer_id: " << transfer_id;
-    
+
     if (opt_args && opt_args->hasNotif) {
         uccl_handle->notif_msg = opt_args->notifMsg;
     }

From 04384003d7740b475fbf4988ab4a553f62e1c1df Mon Sep 17 00:00:00 2001
From: Pravein Govindan Kannan <pravein.govindan.kannan@ibm.com>
Date: Thu, 29 Jan 2026 10:10:08 +0530
Subject: [PATCH 11/28] Fix formatting after changes

Signed-off-by: Pravein Govindan Kannan <pravein.govindan.kannan@ibm.com>
---
 src/plugins/uccl/uccl_backend.cpp | 7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/src/plugins/uccl/uccl_backend.cpp b/src/plugins/uccl/uccl_backend.cpp
index 215aa25fb3..da60a1dfde 100644
--- a/src/plugins/uccl/uccl_backend.cpp
+++ b/src/plugins/uccl/uccl_backend.cpp
@@ -350,7 +350,8 @@ nixlUcclEngine::deregisterMem(nixlBackendMD *meta) {
 nixl_status_t
 nixlUcclEngine::loadLocalMD(nixlBackendMD *input, nixlBackendMD *&output) {
     nixlUcclBackendMD *input_md = (nixlUcclBackendMD *)input;
-    NIXL_DEBUG << "UCCL Load Local MD: " << std::hex << input_md->addr << "Meta Info:" << input_md->mr_id;
+    NIXL_DEBUG << "UCCL Load Local MD: " << std::hex << input_md->addr
+               << "Meta Info:" << input_md->mr_id;
 
     nixlUcclBackendMD *output_md = (nixlUcclBackendMD *)output;
     output_md->addr = (void *)input_md->addr;
@@ -366,8 +367,8 @@ nixlUcclEngine::loadRemoteMD(const nixlBlobDesc &input,
                              const nixl_mem_t &nixl_mem,
                              const std::string &remote_agent,
                              nixlBackendMD *&output) {
-    NIXL_DEBUG << "UCCL Load Remote MD: " << std::hex <<  input.addr << " Meta Info:" << input.metaInfo
-               << " remote_agent: " << remote_agent;
+    NIXL_DEBUG << "UCCL Load Remote MD: " << std::hex << input.addr
+               << " Meta Info:" << input.metaInfo << " remote_agent: " << remote_agent;
 
     output = new nixlUcclBackendMD(true);
     nixlUcclBackendMD *output_md = static_cast<nixlUcclBackendMD *>(output);

From 83a8eeaa2c65595ca242abcc3e0339e710a4bd5d Mon Sep 17 00:00:00 2001
From: Pravein Govindan Kannan <pravein.govindan.kannan@ibm.com>
Date: Tue, 3 Feb 2026 14:07:02 +0530
Subject: [PATCH 12/28] Remove redundant FIFO_ITEM_SIZE

Signed-off-by: Pravein Govindan Kannan <pravein.govindan.kannan@ibm.com>
---
 src/plugins/uccl/uccl_backend.cpp | 26 ++++++++------------------
 src/plugins/uccl/uccl_backend.h   |  7 ++-----
 2 files changed, 10 insertions(+), 23 deletions(-)

diff --git a/src/plugins/uccl/uccl_backend.cpp b/src/plugins/uccl/uccl_backend.cpp
index da60a1dfde..26bc2533a7 100644
--- a/src/plugins/uccl/uccl_backend.cpp
+++ b/src/plugins/uccl/uccl_backend.cpp
@@ -148,7 +148,6 @@ nixlUcclEngine::startListener() {
     // The listener waits for connections from remote agents
     NIXL_DEBUG << "UCCL accepting connections";
     while (!stop_listener_) {
-        // Check if engine is still valid before using it
         if (!engine_) {
             NIXL_DEBUG << "Engine destroyed, listener thread exiting";
             break;
@@ -174,7 +173,6 @@ nixlUcclEngine::startListener() {
             connected_agents_[ip_buf] = reinterpret_cast<uint64_t>(conn);
         }
     }
-    NIXL_DEBUG << "UCCL listener thread exiting";
 }
 
 nixl_mem_list_t
@@ -190,10 +188,11 @@ nixl_status_t
 nixlUcclEngine::getPublicData(const nixlBackendMD *meta, std::string &str) const {
     nixlUcclBackendMD *priv = (nixlUcclBackendMD *)meta;
 
-    // Export fifo_item as hex string
+    // Export fifo_item as hex string.
+    // The fifo_item is used to perform one-sided operation
     str.clear();
-    str.reserve(FIFO_ITEM_SIZE * 2);
-    for (int i = 0; i < FIFO_ITEM_SIZE; i++) {
+    str.reserve(FIFO_SIZE * 2);
+    for (int i = 0; i < FIFO_SIZE; i++) {
         char hex[3];
         snprintf(hex, sizeof(hex), "%02x", static_cast<unsigned char>(priv->fifo_item[i]));
         str += hex;
@@ -379,15 +378,14 @@ nixlUcclEngine::loadRemoteMD(const nixlBlobDesc &input,
     // Decode fifo_item from hex string
     const std::string &hex_str = input.metaInfo;
 
-    if (hex_str.length() == FIFO_ITEM_SIZE * 2) {
-        for (int i = 0; i < FIFO_ITEM_SIZE; i++) {
+    if (hex_str.length() == FIFO_SIZE * 2) {
+        for (int i = 0; i < FIFO_SIZE; i++) {
             std::string byte_str = hex_str.substr(i * 2, 2);
             output_md->fifo_item[i] = static_cast<char>(strtoul(byte_str.c_str(), NULL, 16));
         }
-        NIXL_DEBUG << "Parsed fifo_item from remote metadata";
     } else {
         NIXL_ERROR << "Invalid fifo_item hex string length: " << hex_str.length() << " (expected "
-                   << FIFO_ITEM_SIZE * 2 << ")";
+                   << FIFO_SIZE * 2 << ")";
         delete output_md;
         output = nullptr;
         return NIXL_ERR_INVALID_PARAM;
@@ -454,12 +452,6 @@ nixlUcclEngine::prepXfer(const nixl_xfer_op_t &operation,
         uintptr_t local_addr = local[i].addr;
         uintptr_t remote_addr = remote[i].addr;
 
-        NIXL_DEBUG << "prepXfer iovec[" << i << "]: local[i].addr=" << std::hex << local_addr
-                   << ", lmd->addr=" << std::hex << lmd->addr << ", lmd->mr_id=" << std::dec
-                   << lmd->mr_id << ", remote[i].addr=" << std::hex << remote_addr
-                   << ", rmd->addr=" << std::hex << rmd->addr << ", rmd->mr_id=" << std::dec
-                   << rmd->mr_id;
-
         // Validate the local address is registered
         auto local_mem_iter = mem_reg_info_.find((uint64_t)lmd->addr);
         if (local_mem_iter == mem_reg_info_.end()) {
@@ -472,8 +464,6 @@ nixlUcclEngine::prepXfer(const nixl_xfer_op_t &operation,
 
         uccl_engine_update_fifo(uccl_handle->fifo_items[i], remote_addr, rsize);
 
-        NIXL_DEBUG << "Using pre-shared fifo_item[" << i << "]: addr=" << std::hex << remote_addr
-                   << ", size=" << std::dec << rsize;
     }
 
     return NIXL_SUCCESS;
@@ -634,7 +624,7 @@ nixlUcclEngine::checkXfer(nixlBackendReqH *handle) const {
                 NIXL_ERROR << "Failed to send notify message";
                 return NIXL_ERR_BACKEND;
             }
-            NIXL_DEBUG << "All transfers in handle completed, sent notification: "
+            NIXL_DEBUG << "Transfer complete, sent notification: "
                        << uccl_handle->notif_msg;
         }
         return NIXL_SUCCESS;
diff --git a/src/plugins/uccl/uccl_backend.h b/src/plugins/uccl/uccl_backend.h
index ecfa609d59..97d1fd1416 100644
--- a/src/plugins/uccl/uccl_backend.h
+++ b/src/plugins/uccl/uccl_backend.h
@@ -34,9 +34,6 @@
 
 #include "uccl_engine.h"
 
-#define FIFO_ITEM_SIZE 64
-// FifoItem and deserialize_fifo_item are now provided by uccl_engine.h
-
 class nixlUcclBackendMD;
 class nixlUcclReqH;
 
@@ -142,7 +139,7 @@ class nixlUcclEngine : public nixlBackendEngine {
 class nixlUcclBackendMD : public nixlBackendMD {
 public:
     nixlUcclBackendMD(bool isPrivate) : nixlBackendMD(isPrivate) {
-        memset(fifo_item, 0, FIFO_ITEM_SIZE);
+        memset(fifo_item, 0, FIFO_SIZE);
     }
 
     virtual ~nixlUcclBackendMD() {}
@@ -151,7 +148,7 @@ class nixlUcclBackendMD : public nixlBackendMD {
     size_t length;
     int ref_cnt;
     uccl_mr_t mr_id; // UCCL memory region id
-    char fifo_item[FIFO_ITEM_SIZE];
+    char fifo_item[FIFO_SIZE];
 };
 
 // UCCL Backend Request Handle

From d70747d316c9ead246180d86c046427d65618b98 Mon Sep 17 00:00:00 2001
From: Pravein Govindan Kannan <pravein.govindan.kannan@ibm.com>
Date: Tue, 3 Feb 2026 14:11:22 +0530
Subject: [PATCH 13/28] Update commit SHA of UCCL

Signed-off-by: Pravein Govindan Kannan <pravein.govindan.kannan@ibm.com>
---
 .gitlab/build.sh | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/.gitlab/build.sh b/.gitlab/build.sh
index 819209dcab..ae280bfe1c 100755
--- a/.gitlab/build.sh
+++ b/.gitlab/build.sh
@@ -33,7 +33,7 @@ LIBFABRIC_VERSION=${LIBFABRIC_VERSION:-v1.21.0}
 # LIBFABRIC_INSTALL_DIR can be set via environment variable, defaults to INSTALL_DIR
 LIBFABRIC_INSTALL_DIR=${LIBFABRIC_INSTALL_DIR:-$INSTALL_DIR}
 # UCCL_COMMIT_SHA is the commit SHA of UCCL.
-UCCL_COMMIT_SHA="a962f611021afc2e3c9358f6da4ae96539cbca0f"
+UCCL_COMMIT_SHA="3af0d38dd6ba5b1070a718ab631826999fef695f"
 
 if [ -z "$INSTALL_DIR" ]; then
     echo "Usage: $0 <install_dir> <ucx_install_dir>"

From 92f0466c36f560725efdeea40e7670a904c44307 Mon Sep 17 00:00:00 2001
From: Pravein Govindan Kannan <pravein.govindan.kannan@ibm.com>
Date: Tue, 3 Feb 2026 14:16:43 +0530
Subject: [PATCH 14/28] Remove unused variable

Signed-off-by: Pravein Govindan Kannan <pravein.govindan.kannan@ibm.com>
---
 src/plugins/uccl/uccl_backend.cpp | 8 +-------
 1 file changed, 1 insertion(+), 7 deletions(-)

diff --git a/src/plugins/uccl/uccl_backend.cpp b/src/plugins/uccl/uccl_backend.cpp
index 26bc2533a7..cf4c35034b 100644
--- a/src/plugins/uccl/uccl_backend.cpp
+++ b/src/plugins/uccl/uccl_backend.cpp
@@ -449,7 +449,6 @@ nixlUcclEngine::prepXfer(const nixl_xfer_op_t &operation,
         lmd = (nixlUcclBackendMD *)local[i].metadataP;
         rmd = (nixlUcclBackendMD *)remote[i].metadataP;
         size_t rsize = remote[i].len;
-        uintptr_t local_addr = local[i].addr;
         uintptr_t remote_addr = remote[i].addr;
 
         // Validate the local address is registered
@@ -521,11 +520,6 @@ nixlUcclEngine::postXfer(const nixl_xfer_op_t &operation,
         uintptr_t local_addr = local[i].addr;
         uintptr_t remote_addr = remote[i].addr;
 
-        NIXL_DEBUG << "postXfer iovec[" << i << "]: local[i].addr=" << std::hex << local_addr
-                   << ", lsize=" << std::dec << lsize << ", remote[i].addr=" << std::hex
-                   << remote_addr << ", rsize=" << std::dec << rsize << ", lmd->addr=" << std::hex
-                   << lmd->addr << ", rmd->addr=" << std::hex << rmd->addr;
-
         if (lsize != rsize) {
             NIXL_ERROR << "Local and remote sizes don't match: " << lsize << " != " << rsize;
             return NIXL_ERR_INVALID_PARAM;
@@ -545,7 +539,7 @@ nixlUcclEngine::postXfer(const nixl_xfer_op_t &operation,
         size_v.push_back(lsize);
     }
 
-    // Perform a vector operation
+    // Perform a vector read/write operation
     int result = 0;
     uint64_t transfer_id = 0;
     uccl_handle = static_cast<nixlUcclReqH *>(handle);

From d276d725d32c272c9325a45a29433136e178c296 Mon Sep 17 00:00:00 2001
From: Pravein Govindan Kannan <pravein.govindan.kannan@ibm.com>
Date: Tue, 3 Feb 2026 14:19:58 +0530
Subject: [PATCH 15/28] Remove unused vars

Signed-off-by: Pravein Govindan Kannan <pravein.govindan.kannan@ibm.com>
---
 src/plugins/uccl/uccl_backend.cpp | 4 +---
 1 file changed, 1 insertion(+), 3 deletions(-)

diff --git a/src/plugins/uccl/uccl_backend.cpp b/src/plugins/uccl/uccl_backend.cpp
index cf4c35034b..a05f9b32cc 100644
--- a/src/plugins/uccl/uccl_backend.cpp
+++ b/src/plugins/uccl/uccl_backend.cpp
@@ -510,15 +510,13 @@ nixlUcclEngine::postXfer(const nixl_xfer_op_t &operation,
     std::vector<void *> addr_v;
     std::vector<size_t> size_v;
 
-    std::lock_guard<std::mutex> lock(mem_mutex_); // Lock once for the entire operation
+    std::lock_guard<std::mutex> lock(mem_mutex_);
     for (size_t i = 0; i < lcnt; i++) {
         lmd = (nixlUcclBackendMD *)local[i].metadataP;
-        rmd = (nixlUcclBackendMD *)remote[i].metadataP;
         size_t lsize = local[i].len;
         size_t rsize = remote[i].len;
         // Use local[i].addr for the actual iovec address, not lmd->addr (which is base address)
         uintptr_t local_addr = local[i].addr;
-        uintptr_t remote_addr = remote[i].addr;
 
         if (lsize != rsize) {
             NIXL_ERROR << "Local and remote sizes don't match: " << lsize << " != " << rsize;

From 800179bcf9961929a1ea1dbdad042f1b5e3299f5 Mon Sep 17 00:00:00 2001
From: Pravein Govindan Kannan <pravein.govindan.kannan@ibm.com>
Date: Tue, 3 Feb 2026 14:20:59 +0530
Subject: [PATCH 16/28] Fix error

Signed-off-by: Pravein Govindan Kannan <pravein.govindan.kannan@ibm.com>
---
 src/plugins/uccl/uccl_backend.cpp | 1 -
 1 file changed, 1 deletion(-)

diff --git a/src/plugins/uccl/uccl_backend.cpp b/src/plugins/uccl/uccl_backend.cpp
index a05f9b32cc..c13c3933e1 100644
--- a/src/plugins/uccl/uccl_backend.cpp
+++ b/src/plugins/uccl/uccl_backend.cpp
@@ -477,7 +477,6 @@ nixlUcclEngine::postXfer(const nixl_xfer_op_t &operation,
                          const nixl_opt_b_args_t *opt_args) const {
     nixlUcclReqH *uccl_handle;
     nixlUcclBackendMD *lmd;
-    nixlUcclBackendMD *rmd;
 
     NIXL_DEBUG << "UCCL PostXfer: " << operation << " remote_agent: " << remote_agent;
 

From be118d192f77e45db174cb111431d0f23d28823b Mon Sep 17 00:00:00 2001
From: Pravein Govindan Kannan <pravein.govindan.kannan@ibm.com>
Date: Tue, 3 Feb 2026 15:27:46 +0530
Subject: [PATCH 17/28] Add recent updates to README

Signed-off-by: Pravein Govindan Kannan <pravein.govindan.kannan@ibm.com>
---
 src/plugins/uccl/README.md | 12 +++++-------
 1 file changed, 5 insertions(+), 7 deletions(-)

diff --git a/src/plugins/uccl/README.md b/src/plugins/uccl/README.md
index 7078a6b1f2..27db3584b8 100644
--- a/src/plugins/uccl/README.md
+++ b/src/plugins/uccl/README.md
@@ -34,20 +34,18 @@ UCCL engine would auto discover the right NIC to be used for the GPU based on th
 
 Refer to [README](https://github.com/uccl-project/uccl/tree/main/collective/rdma#environment-variables-in-uccl) for the complete list of environment variables that can be set to customize UCCL.
 
-**Important**: For `NIXL_READ` operations set `UCCL_RCMODE=1`. By default, UCCL uses RDMA UC (Unreliable Connection). However, `READ` operations need to operate on RDMA RC (Reliable Connection). `WRITE` operations can work on both RDMA UC and RC.
-
 ### Usage References
 
 1) [NIXL Benchmark](https://github.com/uccl-project/uccl/blob/main/p2p/benchmarks/benchmark_nixl.py) in UCCL P2P: Refer to  this [README](https://github.com/uccl-project/uccl/tree/main/p2p) on how to run the script.
 
-2) [NIXL connector](https://github.com/vllm-project/vllm/commit/e731733d30d0aed3252dc60427927768bfc0ca73) in vLLM. vLLM's NIXL connector uses `NIXL_READ` operations, hence set env `UCCL_RCMODE` to 1.
+2) [NIXL connector](https://github.com/vllm-project/vllm/commit/e731733d30d0aed3252dc60427927768bfc0ca73) in vLLM.
 
 ### Road Map
 
-- [ ] Add Intra-node communication support
+- ✅ Add asynchronous posting of reads over multiple workers to mitigate latency increase upon fragmentation
 
-- [ ] Add Progress Thread support
+- 🚧 Add Intra-node communication support
 
-- [ ] Add asynchronous posting of reads over multiple workers to mitigate latency increase upon fragmentation
+- 🚧 Add Progress Thread support
 
-- [ ] Add support for other transport (TCP, TCP-X, etc.)
\ No newline at end of file
+- 🚧 Add support for other transport (TCP, TCP-X, etc.)
\ No newline at end of file

From dc3d822ede5172e19a401b4d1bef1ec690454178 Mon Sep 17 00:00:00 2001
From: Pravein Govindan Kannan <pravein.govindan.kannan@ibm.com>
Date: Tue, 3 Feb 2026 15:51:44 +0530
Subject: [PATCH 18/28] Fix copyright and format

Signed-off-by: Pravein Govindan Kannan <pravein.govindan.kannan@ibm.com>
---
 src/plugins/uccl/uccl_backend.cpp     | 4 +---
 test/gtest/plugins/uccl/uccl_test.cpp | 2 +-
 2 files changed, 2 insertions(+), 4 deletions(-)

diff --git a/src/plugins/uccl/uccl_backend.cpp b/src/plugins/uccl/uccl_backend.cpp
index c13c3933e1..2a8650f06c 100644
--- a/src/plugins/uccl/uccl_backend.cpp
+++ b/src/plugins/uccl/uccl_backend.cpp
@@ -462,7 +462,6 @@ nixlUcclEngine::prepXfer(const nixl_xfer_op_t &operation,
         deserialize_fifo_item(rmd->fifo_item, &uccl_handle->fifo_items[i]);
 
         uccl_engine_update_fifo(uccl_handle->fifo_items[i], remote_addr, rsize);
-
     }
 
     return NIXL_SUCCESS;
@@ -615,8 +614,7 @@ nixlUcclEngine::checkXfer(nixlBackendReqH *handle) const {
                 NIXL_ERROR << "Failed to send notify message";
                 return NIXL_ERR_BACKEND;
             }
-            NIXL_DEBUG << "Transfer complete, sent notification: "
-                       << uccl_handle->notif_msg;
+            NIXL_DEBUG << "Transfer complete, sent notification: " << uccl_handle->notif_msg;
         }
         return NIXL_SUCCESS;
     }
diff --git a/test/gtest/plugins/uccl/uccl_test.cpp b/test/gtest/plugins/uccl/uccl_test.cpp
index 30f96466c6..5ac46e0f79 100644
--- a/test/gtest/plugins/uccl/uccl_test.cpp
+++ b/test/gtest/plugins/uccl/uccl_test.cpp
@@ -1,5 +1,5 @@
 /*
- * SPDX-FileCopyrightText: Copyright (c) 2025 NVIDIA CORPORATION & AFFILIATES. All rights reserved.
+ * SPDX-FileCopyrightText: Copyright (c) 2025-2026 NVIDIA CORPORATION & AFFILIATES. All rights reserved.
  * SPDX-License-Identifier: Apache-2.0
  *
  * Licensed under the Apache License, Version 2.0 (the "License");

From 90d27cb57804e058a3781d72d545102c0565de4b Mon Sep 17 00:00:00 2001
From: Pravein Govindan Kannan <pravein.govindan.kannan@ibm.com>
Date: Tue, 3 Feb 2026 19:20:16 +0530
Subject: [PATCH 19/28] Increase CI image tag

Signed-off-by: Pravein Govindan Kannan <pravein.govindan.kannan@ibm.com>
---
 .ci/jenkins/lib/build-matrix.yaml | 2 +-
 .ci/jenkins/lib/test-matrix.yaml  | 2 +-
 2 files changed, 2 insertions(+), 2 deletions(-)

diff --git a/.ci/jenkins/lib/build-matrix.yaml b/.ci/jenkins/lib/build-matrix.yaml
index a1e7636621..c24dac1b96 100644
--- a/.ci/jenkins/lib/build-matrix.yaml
+++ b/.ci/jenkins/lib/build-matrix.yaml
@@ -42,7 +42,7 @@ env:
   TEST_TIMEOUT: 30
   UCX_TLS: "^shm"
   STORAGE_DRIVER: 'overlay'
-  CI_IMAGE_TAG: "20260129-1"
+  CI_IMAGE_TAG: "20260203-1"
 
 
 runs_on_dockers:
diff --git a/.ci/jenkins/lib/test-matrix.yaml b/.ci/jenkins/lib/test-matrix.yaml
index 79c516e58e..feeba48701 100644
--- a/.ci/jenkins/lib/test-matrix.yaml
+++ b/.ci/jenkins/lib/test-matrix.yaml
@@ -30,7 +30,7 @@ env:
   NIXL_INSTALL_DIR: /opt/nixl
   TEST_TIMEOUT: 30
   NPROC: 32
-  CI_IMAGE_TAG: "20260129-1"
+  CI_IMAGE_TAG: "20260203-1"
 
 docker_opt: "-e NPROC -e EXECUTOR_NUMBER --security-opt seccomp=unconfined --security-opt apparmor=unconfined --shm-size=1G --ulimit nofile=65535:65535 --ulimit stack=67108864 --ulimit memlock=-1:-1 --network=host --ipc=host --cap-add=SYS_PTRACE --cap-add=SYS_ADMIN --privileged --gpus all --device=/dev/infiniband --device=/dev/gdrdrv --entrypoint=''"
 

From facdff67f6b6e56be8a9a661ce563eb81060a25d Mon Sep 17 00:00:00 2001
From: Pravein Govindan Kannan <pravein.govindan.kannan@ibm.com>
Date: Wed, 4 Feb 2026 18:42:22 +0530
Subject: [PATCH 20/28] Use latest UCCL commit SHA

Signed-off-by: Pravein Govindan Kannan <pravein.govindan.kannan@ibm.com>
---
 .gitlab/build.sh | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/.gitlab/build.sh b/.gitlab/build.sh
index 78342c7878..4e0a36e07e 100755
--- a/.gitlab/build.sh
+++ b/.gitlab/build.sh
@@ -35,7 +35,7 @@ LIBFABRIC_VERSION=${LIBFABRIC_VERSION:-v1.21.0}
 # LIBFABRIC_INSTALL_DIR can be set via environment variable, defaults to INSTALL_DIR
 LIBFABRIC_INSTALL_DIR=${LIBFABRIC_INSTALL_DIR:-$INSTALL_DIR}
 # UCCL_COMMIT_SHA is the commit SHA of UCCL.
-UCCL_COMMIT_SHA="3af0d38dd6ba5b1070a718ab631826999fef695f"
+UCCL_COMMIT_SHA="91f92325dced4e1313f6f35ac94c63d745f68df9"
 AZURITE_VER="3.35.0"
 TMPDIR=$(mktemp -d)
 

From 99df3d5a55081083f701655f6863b25618878ff8 Mon Sep 17 00:00:00 2001
From: Pravein Govindan Kannan <pravein.govindan.kannan@ibm.com>
Date: Thu, 19 Feb 2026 12:31:35 +0530
Subject: [PATCH 21/28] Add recent UCCL Commit - Fixing build issue on ARM
 platform - Add TCP support

Signed-off-by: Pravein Govindan Kannan <pravein.govindan.kannan@ibm.com>
---
 .gitlab/build.sh | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/.gitlab/build.sh b/.gitlab/build.sh
index 4e0a36e07e..80af1d4aae 100755
--- a/.gitlab/build.sh
+++ b/.gitlab/build.sh
@@ -35,7 +35,7 @@ LIBFABRIC_VERSION=${LIBFABRIC_VERSION:-v1.21.0}
 # LIBFABRIC_INSTALL_DIR can be set via environment variable, defaults to INSTALL_DIR
 LIBFABRIC_INSTALL_DIR=${LIBFABRIC_INSTALL_DIR:-$INSTALL_DIR}
 # UCCL_COMMIT_SHA is the commit SHA of UCCL.
-UCCL_COMMIT_SHA="91f92325dced4e1313f6f35ac94c63d745f68df9"
+UCCL_COMMIT_SHA="a550d613f9a61e4b3b08ddf577a232891fb75ec7"
 AZURITE_VER="3.35.0"
 TMPDIR=$(mktemp -d)
 

From d1e653662d591d23ba37121bafebfffe618dbec8 Mon Sep 17 00:00:00 2001
From: Pravein Govindan Kannan <pravein.govindan.kannan@ibm.com>
Date: Tue, 24 Feb 2026 19:32:18 +0530
Subject: [PATCH 22/28] Remove sleep during cleanup

Signed-off-by: Pravein Govindan Kannan <pravein.govindan.kannan@ibm.com>
---
 src/plugins/uccl/uccl_backend.cpp | 2 --
 1 file changed, 2 deletions(-)

diff --git a/src/plugins/uccl/uccl_backend.cpp b/src/plugins/uccl/uccl_backend.cpp
index 2a8650f06c..42faf662b0 100644
--- a/src/plugins/uccl/uccl_backend.cpp
+++ b/src/plugins/uccl/uccl_backend.cpp
@@ -136,8 +136,6 @@ nixlUcclEngine::~nixlUcclEngine() {
         engine_ = nullptr;
     }
 
-    std::this_thread::sleep_for(std::chrono::milliseconds(100));
-
     if (listener_thread_.joinable()) {
         listener_thread_.detach();
     }

From 3144cf975e3fa9efa497665642ababb4f7d18e0b Mon Sep 17 00:00:00 2001
From: Pravein Govindan Kannan <pravein.govindan.kannan@ibm.com>
Date: Wed, 25 Feb 2026 19:51:46 +0530
Subject: [PATCH 23/28] Add stop_accept API for graceful cleanup

Signed-off-by: Pravein Govindan Kannan <pravein.govindan.kannan@ibm.com>
---
 .gitlab/build.sh                  |  2 +-
 src/plugins/uccl/uccl_backend.cpp | 12 ++++++++----
 2 files changed, 9 insertions(+), 5 deletions(-)

diff --git a/.gitlab/build.sh b/.gitlab/build.sh
index 631bef6681..708c5f3070 100755
--- a/.gitlab/build.sh
+++ b/.gitlab/build.sh
@@ -35,7 +35,7 @@ LIBFABRIC_VERSION=${LIBFABRIC_VERSION:-v1.21.0}
 # LIBFABRIC_INSTALL_DIR can be set via environment variable, defaults to INSTALL_DIR
 LIBFABRIC_INSTALL_DIR=${LIBFABRIC_INSTALL_DIR:-$INSTALL_DIR}
 # UCCL_COMMIT_SHA is the commit SHA of UCCL.
-UCCL_COMMIT_SHA="a550d613f9a61e4b3b08ddf577a232891fb75ec7"
+UCCL_COMMIT_SHA="2de728f1a27ea3f3b66059baf838f940e243ebc6"
 AZURITE_VER="3.35.0"
 TMPDIR=$(mktemp -d)
 
diff --git a/src/plugins/uccl/uccl_backend.cpp b/src/plugins/uccl/uccl_backend.cpp
index 42faf662b0..466ab56d1b 100644
--- a/src/plugins/uccl/uccl_backend.cpp
+++ b/src/plugins/uccl/uccl_backend.cpp
@@ -106,6 +106,14 @@ nixlUcclEngine::nixlUcclEngine(const nixlBackendInitParams *init_params)
 nixlUcclEngine::~nixlUcclEngine() {
     stop_listener_ = true;
 
+    if (engine_) {
+        uccl_engine_stop_accept(engine_);
+    }
+
+    if (listener_thread_.joinable()) {
+        listener_thread_.join();
+    }
+
     {
         std::lock_guard<std::mutex> lock(mem_mutex_);
         for (auto &[addr, priv] : mem_reg_info_) {
@@ -135,10 +143,6 @@ nixlUcclEngine::~nixlUcclEngine() {
         uccl_engine_destroy(engine_);
         engine_ = nullptr;
     }
-
-    if (listener_thread_.joinable()) {
-        listener_thread_.detach();
-    }
 }
 
 void

From 1bc2d51ce77a7321f3b1a6872ed8588acfee4e5d Mon Sep 17 00:00:00 2001
From: Pravein Govindan Kannan <pravein.govindan.kannan@ibm.com>
Date: Thu, 26 Feb 2026 10:37:05 +0530
Subject: [PATCH 24/28] Use CodeRabbit suggestions

Signed-off-by: Pravein Govindan Kannan <pravein.govindan.kannan@ibm.com>
---
 src/plugins/uccl/uccl_backend.cpp | 10 +++-------
 1 file changed, 3 insertions(+), 7 deletions(-)

diff --git a/src/plugins/uccl/uccl_backend.cpp b/src/plugins/uccl/uccl_backend.cpp
index 466ab56d1b..7cc42ede77 100644
--- a/src/plugins/uccl/uccl_backend.cpp
+++ b/src/plugins/uccl/uccl_backend.cpp
@@ -150,17 +150,13 @@ nixlUcclEngine::startListener() {
     // The listener waits for connections from remote agents
     NIXL_DEBUG << "UCCL accepting connections";
     while (!stop_listener_) {
-        if (!engine_) {
-            NIXL_DEBUG << "Engine destroyed, listener thread exiting";
-            break;
-        }
 
         char ip_buf[256];
         int remote_gpu_idx;
         uccl_conn_t *conn = uccl_engine_accept(engine_, ip_buf, sizeof(ip_buf), &remote_gpu_idx);
         if (!conn) {
-            // Check if we should stop (engine destroyed or shutdown requested)
-            if (stop_listener_ || !engine_) {
+            // Check if we should stop
+            if (stop_listener_) {
                 NIXL_DEBUG << "Listener thread stopping";
                 break;
             }
@@ -597,7 +593,7 @@ nixlUcclEngine::checkXfer(nixlBackendReqH *handle) const {
         return NIXL_ERR_BACKEND;
     }
 
-    int is_done = uccl_engine_xfer_status(conn, uccl_handle->transfer_id);
+    bool is_done = uccl_engine_xfer_status(conn, uccl_handle->transfer_id);
     if (is_done) {
         nixlSerDes ser_des;
         ser_des.addStr("msg", uccl_handle->notif_msg);

From ca40dee35d7c624f80f080a163d03511af1abfb2 Mon Sep 17 00:00:00 2001
From: Pravein Govindan Kannan <pravein.govindan.kannan@ibm.com>
Date: Thu, 26 Feb 2026 19:10:00 +0530
Subject: [PATCH 25/28] Add cleanup change to address crash

Signed-off-by: Pravein Govindan Kannan <pravein.govindan.kannan@ibm.com>
---
 benchmark/nixlbench/src/worker/nixl/nixl_worker.cpp | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/benchmark/nixlbench/src/worker/nixl/nixl_worker.cpp b/benchmark/nixlbench/src/worker/nixl/nixl_worker.cpp
index fbd95a544a..8cdaed4123 100644
--- a/benchmark/nixlbench/src/worker/nixl/nixl_worker.cpp
+++ b/benchmark/nixlbench/src/worker/nixl/nixl_worker.cpp
@@ -310,6 +310,9 @@ xferBenchNixlWorker::xferBenchNixlWorker(int *argc, char ***argv, std::vector<st
 }
 
 xferBenchNixlWorker::~xferBenchNixlWorker() {
+    delete rt;
+    rt = nullptr;
+
     if (agent) {
         delete agent;
         agent = nullptr;

From 9cff793db882d8773750119ef382b4888c5d3f34 Mon Sep 17 00:00:00 2001
From: Pravein Govindan Kannan <pravein.govindan.kannan@ibm.com>
Date: Thu, 26 Feb 2026 20:05:44 +0530
Subject: [PATCH 26/28] Bump up the IMAGE_TAG

Signed-off-by: Pravein Govindan Kannan <pravein.govindan.kannan@ibm.com>
---
 .ci/jenkins/lib/build-matrix.yaml | 2 +-
 .ci/jenkins/lib/test-matrix.yaml  | 2 +-
 2 files changed, 2 insertions(+), 2 deletions(-)

diff --git a/.ci/jenkins/lib/build-matrix.yaml b/.ci/jenkins/lib/build-matrix.yaml
index 0177fbb7db..6b5585fbf9 100644
--- a/.ci/jenkins/lib/build-matrix.yaml
+++ b/.ci/jenkins/lib/build-matrix.yaml
@@ -43,7 +43,7 @@ env:
   TEST_TIMEOUT: 30
   UCX_TLS: "^shm"
   STORAGE_DRIVER: 'overlay'
-  CI_IMAGE_TAG: "20260219-1"
+  CI_IMAGE_TAG: "20260226-1"
 
 
 runs_on_dockers:
diff --git a/.ci/jenkins/lib/test-matrix.yaml b/.ci/jenkins/lib/test-matrix.yaml
index 1fe42a0bdc..5f627c111f 100644
--- a/.ci/jenkins/lib/test-matrix.yaml
+++ b/.ci/jenkins/lib/test-matrix.yaml
@@ -49,7 +49,7 @@ env:
   SLURM_JOB_TIMEOUT: '02:20:00'
   SLURM_IMMEDIATE_TIMEOUT: "3600"
   STORAGE_DRIVER: overlay
-  CI_IMAGE_TAG: "20260219-1"
+  CI_IMAGE_TAG: "20260226-1"
 
 empty_volumes:
   - {mountPath: /var/lib/containers/storage, memory: false}

From bb7b0a7fac86b681a1adb044fe369446f2720d73 Mon Sep 17 00:00:00 2001
From: Pravein Govindan Kannan <pravein.govindan.kannan@ibm.com>
Date: Thu, 26 Feb 2026 21:58:48 +0530
Subject: [PATCH 27/28] Revert CI_IMAGE_TAG

Signed-off-by: Pravein Govindan Kannan <pravein.govindan.kannan@ibm.com>
---
 .ci/jenkins/lib/build-matrix.yaml | 2 +-
 .ci/jenkins/lib/test-matrix.yaml  | 2 +-
 2 files changed, 2 insertions(+), 2 deletions(-)

diff --git a/.ci/jenkins/lib/build-matrix.yaml b/.ci/jenkins/lib/build-matrix.yaml
index 6b5585fbf9..0177fbb7db 100644
--- a/.ci/jenkins/lib/build-matrix.yaml
+++ b/.ci/jenkins/lib/build-matrix.yaml
@@ -43,7 +43,7 @@ env:
   TEST_TIMEOUT: 30
   UCX_TLS: "^shm"
   STORAGE_DRIVER: 'overlay'
-  CI_IMAGE_TAG: "20260226-1"
+  CI_IMAGE_TAG: "20260219-1"
 
 
 runs_on_dockers:
diff --git a/.ci/jenkins/lib/test-matrix.yaml b/.ci/jenkins/lib/test-matrix.yaml
index 5f627c111f..1fe42a0bdc 100644
--- a/.ci/jenkins/lib/test-matrix.yaml
+++ b/.ci/jenkins/lib/test-matrix.yaml
@@ -49,7 +49,7 @@ env:
   SLURM_JOB_TIMEOUT: '02:20:00'
   SLURM_IMMEDIATE_TIMEOUT: "3600"
   STORAGE_DRIVER: overlay
-  CI_IMAGE_TAG: "20260226-1"
+  CI_IMAGE_TAG: "20260219-1"
 
 empty_volumes:
   - {mountPath: /var/lib/containers/storage, memory: false}

From 872e33a4b3b80e4eb3cc5221c29a13399204420b Mon Sep 17 00:00:00 2001
From: Pravein Govindan Kannan <pravein.govindan.kannan@ibm.com>
Date: Fri, 27 Feb 2026 06:42:49 +0530
Subject: [PATCH 28/28] Update IMAGE_TAG

Signed-off-by: Pravein Govindan Kannan <pravein.govindan.kannan@ibm.com>
---
 .ci/jenkins/lib/build-matrix.yaml | 2 +-
 .ci/jenkins/lib/test-matrix.yaml  | 2 +-
 2 files changed, 2 insertions(+), 2 deletions(-)

diff --git a/.ci/jenkins/lib/build-matrix.yaml b/.ci/jenkins/lib/build-matrix.yaml
index 0177fbb7db..6b5585fbf9 100644
--- a/.ci/jenkins/lib/build-matrix.yaml
+++ b/.ci/jenkins/lib/build-matrix.yaml
@@ -43,7 +43,7 @@ env:
   TEST_TIMEOUT: 30
   UCX_TLS: "^shm"
   STORAGE_DRIVER: 'overlay'
-  CI_IMAGE_TAG: "20260219-1"
+  CI_IMAGE_TAG: "20260226-1"
 
 
 runs_on_dockers:
diff --git a/.ci/jenkins/lib/test-matrix.yaml b/.ci/jenkins/lib/test-matrix.yaml
index 1fe42a0bdc..5f627c111f 100644
--- a/.ci/jenkins/lib/test-matrix.yaml
+++ b/.ci/jenkins/lib/test-matrix.yaml
@@ -49,7 +49,7 @@ env:
   SLURM_JOB_TIMEOUT: '02:20:00'
   SLURM_IMMEDIATE_TIMEOUT: "3600"
   STORAGE_DRIVER: overlay
-  CI_IMAGE_TAG: "20260219-1"
+  CI_IMAGE_TAG: "20260226-1"
 
 empty_volumes:
   - {mountPath: /var/lib/containers/storage, memory: false}