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KV-Cache multi-tier offloading async batched lookup - #44193

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orozery merged 32 commits into
vllm-project:mainfrom
effi-ofer:lookup
Jun 10, 2026
Merged

KV-Cache multi-tier offloading async batched lookup#44193
orozery merged 32 commits into
vllm-project:mainfrom
effi-ofer:lookup

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@effi-ofer

@effi-ofer effi-ofer commented Jun 1, 2026

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Purpose

Add async and batch lookup to multi-tier offloading. This significantly speeds up the secondary tier overall performance.

Test Plan

  • Run the kv_offloading/tiering tests concentrating on fs secondary tier as well as object store secondary tier (obj store PR has yet to be merged).
  • Run inference perf benchmark using llm-d-benchmark.

Test Result

.venv/bin/python -m pytest tests/v1/kv_offload/test_fs_tier.py

=================================================================== test session starts ===================================================================
platform linux -- Python 3.12.13, pytest-9.0.3, pluggy-1.6.0
rootdir: /workspace/vllm
configfile: pyproject.toml
plugins: anyio-4.13.0
collected 9 items

tests/v1/kv_offload/test_fs_tier.py ......... [100%]

==================================================================== warnings summary =====================================================================
:488
:488: DeprecationWarning: builtin type SwigPyPacked has no module attribute

:488
:488: DeprecationWarning: builtin type SwigPyObject has no module attribute

tests/v1/kv_offload/test_fs_tier.py: 14 warnings
/workspace/vllm/.venv/lib/python3.12/site-packages/torch/jit/_script.py:365: DeprecationWarning: torch.jit.script_method is deprecated. Please switch to torch.compile or torch.export.
warnings.warn(

-- Docs: https://docs.pytest.org/en/stable/how-to/capture-warnings.html
============================================================= 9 passed, 16 warnings in 4.10s ==============================================================


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  • The purpose of the PR, such as "Fix some issue (link existing issues this PR will resolve)".
  • The test plan, such as providing test command.
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  • (Optional) The necessary documentation update, such as updating supported_models.md and examples for a new model.

@effi-ofer
effi-ofer requested review from ApostaC and orozery as code owners June 1, 2026 10:53
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@effi-ofer

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@orozery @rshavitt @ronensc

@mergify mergify Bot added the v1 label Jun 1, 2026
@mergify

mergify Bot commented Jun 3, 2026

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This pull request has merge conflicts that must be resolved before it can be
merged. Please rebase the PR, @effi-ofer.

https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/working-with-forks/syncing-a-fork

@mergify mergify Bot added the needs-rebase label Jun 3, 2026
effi-ofer added 7 commits June 5, 2026 21:51
Signed-off-by: Effi Ofer <effi.ofer@gmail.com>
Signed-off-by: Effi Ofer <effi.ofer@gmail.com>
Signed-off-by: Effi Ofer <effi.ofer@gmail.com>
Signed-off-by: Effi Ofer <effi.ofer@gmail.com>
Signed-off-by: Effi Ofer <effi.ofer@gmail.com>
Signed-off-by: Effi Ofer <effi.ofer@gmail.com>
Signed-off-by: Effi Ofer <effi.ofer@gmail.com>
Signed-off-by: Effi Ofer <effi.ofer@gmail.com>
@mergify mergify Bot removed the needs-rebase label Jun 5, 2026
Comment thread vllm/v1/kv_offload/tiering/async_lookup.py Outdated
Signed-off-by: Effi Ofer <effi.ofer@gmail.com>
)
for k in keys
]
results = self._tier._agent.query_memory(descriptors, "OBJ", "OBJ")

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Severity: LOW

batch_lookup() runs on the background worker thread and calls self._tier._agent.query_memory() without synchronization, while the scheduler thread concurrently calls _agent.register_memory(), _agent.transfer(), _agent.check_xfer_state(), etc. The nixl agent is a C/C++ extension with no documented thread-safety guarantees — concurrent access may corrupt internal state, leading to memory safety violations or undefined behavior.
Helpful? Add 👍 / 👎

💡 Fix Suggestion

Suggestion: Add a threading.Lock to ObjectStoreSecondaryTierManager to serialize all access to self._agent. Specifically:

  1. In ObjectStoreSecondaryTierManager.__init__() (around line 103), after creating self._agent, create a lock:

    self._agent_lock = threading.Lock()
  2. In ObjAsyncLookupWorker.batch_lookup() (line 81), wrap the query_memory call with the lock:

    with self._tier._agent_lock:
        results = self._tier._agent.query_memory(descriptors, "OBJ", "OBJ")
  3. Wrap every other self._agent call in the scheduler-thread methods (_submit_transfer, get_finished_jobs, lookup/_exists, shutdown, and _register_primary_kv) with the same self._agent_lock. This includes calls to register_memory(), prep_xfer_dlist(), transfer(), check_xfer_state(), release_xfer_handle(), release_dlist_handle(), and deregister_memory().

This mirrors the pattern used in vllm/distributed/kv_transfer/kv_connector/v1/nixl/worker.py where NIXL thread-safety is explicitly addressed by limiting concurrent access.

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From Claude:

The bot's concern is effectively a non-issue for the OBJ backend. Here's why:

1. queryMem doesn't touch devIdToObjKey_ — the only shared mutable state in the engine. It reads object keys directly from the descriptor's metaInfo field (the key string passed in from Python).
2. postXfer only reads devIdToObjKey_, and registerMem/deregisterMem are the only writers. These registration calls happen at startup (in __init__), long before the background thread starts issuing query_memory calls.
3. The S3 client itself (checkObjectExistsAsync) uses an asio thread pool executor internally — it's designed for concurrent async I/O. Multiple concurrent HTTP requests to S3 are the normal operating mode.
4. No shared mutable state is touched concurrently between queryMem (background thread) and postXfer/check_xfer_state (scheduler thread). They both just issue independent HTTP requests through the same S3 client, which handles concurrency internally via its executor.

Bottom line: The depthfirst bot flagged this based on the general principle "shared object, no lock = danger." But in practice, queryMem and the transfer methods are effectively stateless operations dispatching independent HTTP requests through an internally-concurrent S3 client. No lock needed. You could add a one-line comment in the PR noting this for future readers, but no code change is required.

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Thanks @effi-ofer !
Can you please change the PR title to be more descriptive?

Comment thread vllm/v1/kv_offload/tiering/base.py Outdated
max_results: Capacity of the lookup state cache.
"""
...

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Let's avoid changing base.py and manager.py.
The tiering manager should not be aware of AsyncLookup.
The secondary tier wanting async lookup (e.g. fs tier) would delegate lookup, on_request_finished, and on_schedule_end to the the async lookup component.

return int.from_bytes(key[:8], "big")


class AsyncLookupWorker(ABC):

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Let's call this AsyncLookupManager instead.
The manager manages lookups and uses a background thread to execute them.


def __init__(
self,
tier_idx: int,

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Let's drop tier_idx as it is not propagated by the tiering manager.
Instead, use tier_type: str.

def __init__(
self,
tier_idx: int,
max_results: int = 1_000_000,

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Let's drop max_results to simplify things.
i.e. don't support LRU eviction.

FOUND: int = 1 # present in this tier


def _slot(key: OffloadKey) -> int:

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I don't see a reason for this operation.
We can use OffloadKey as dict keys directly, without going through _slot.

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It was saving us a bit of memory (30MB on 1M entries) but at an increased risk of collision. Removed.

if self._lookup_state.get(slot, _IN_FLIGHT) == _IN_FLIGHT:
self._lookup_state[slot] = status

def update_cached_exists(self, keys: Collection[OffloadKey]) -> None:

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Assuming we drop the LRU, let's change this function to invalidate.
It should simply remove keys from self._lookup_state.

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created cleanup()

# Only apply if the slot is still _IN_FLIGHT. A FOUND written
# by update_cached_exists() during the worker's lookup takes
# precedence and must not be overwritten.
if self._lookup_state.get(slot, _IN_FLIGHT) == _IN_FLIGHT:

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Assuming we drop LRU, we can remove this if.

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no longer relevant

def shutdown(self) -> None:
"""Stop the worker thread."""
self._shutdown_event.set()
self._thread.join(timeout=5.0)

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Can we drop the timeout?

while not self._shutdown_event.is_set():
# Block until flush() posts a batch.
try:
pending = self._lookup_queue.get(timeout=1.0)

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If we change _lookup_queue to a SimpleQueue we can drop the timeout and the try-catch.
On shutdown, just use a "poisen pillow", e.g. pushing of a dummy value to _lookup_queue to unblock the worker thread and allow it to read the shutdown event.
We can actually switch to a shutdown boolean instead of an event.

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That's already done - switched to SimpleQueue, removed the timeout and try-catch, and use None as the poison pill. And I removed _shutdown_event entirely since the None handles the exit cleanly - no boolean or event needed.

def batch_lookup(
self, keys: list[OffloadKey], req_context: ReqContext
) -> list[bool | None]:
return [os.path.exists(self._tier.file_mapper.get_file_name(k)) for k in keys]

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We can avoid list allocation, changing batch_lookup to return Iterable[bool | None]:

Suggested change
return [os.path.exists(self._tier.file_mapper.get_file_name(k)) for k in keys]
return (os.path.exists(self._tier.file_mapper.get_file_name(k)) for k in keys)

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no longer relevant.

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Why not?

@effi-ofer effi-ofer changed the title Lookup KV-Cache multi-tier offloading async batched lookup Jun 7, 2026
effi-ofer added 6 commits June 7, 2026 22:54
Signed-off-by: Effi Ofer <effi.ofer@gmail.com>
Signed-off-by: Effi Ofer <effi.ofer@gmail.com>
Signed-off-by: Effi Ofer <effi.ofer@gmail.com>
Signed-off-by: Effi Ofer <effi.ofer@gmail.com>
Signed-off-by: Effi Ofer <effi.ofer@gmail.com>
Signed-off-by: Effi Ofer <effi.ofer@gmail.com>

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Thanks @effi-ofer !
Can you please also add a test_async_lookup.py file with unit testing?

Comment on lines +171 to +173
self._lookup_state[key] = LookupState(
result=result, request_ids=state.request_ids
)

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Let's switch LookupState type from NamedTuple to @dataclass(slots=True).
Then, we can simply set state.result = result.


class LookupState(NamedTuple):
result: bool | None
request_ids: set[str]

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Let's add a small comment explaining what does this field hold.

def batch_lookup(
self, keys: list[OffloadKey], req_context: ReqContext
) -> list[bool | None]:
return [os.path.exists(self._tier.file_mapper.get_file_name(k)) for k in keys]

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Why not?

Comment on lines -154 to -156
"""
Collect completed jobs from the finished-jobs queue.
"""

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revert deletion

Comment on lines -167 to -168
Shuts down the thread pool, clearing pending tasks and waiting for
active threads to complete.

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Merge into the existing docstring instead of overriding.

len(keys),
exc,
)
hits = [False] * len(keys)

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Assuming we change batch_lookup to return an Iterable:

Suggested change
hits = [False] * len(keys)
hits = (False for _ in range(keys))

)
for k in keys
]
results = self._tier._agent.query_memory(descriptors, "OBJ", "OBJ")

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From Claude:

The bot's concern is effectively a non-issue for the OBJ backend. Here's why:

1. queryMem doesn't touch devIdToObjKey_ — the only shared mutable state in the engine. It reads object keys directly from the descriptor's metaInfo field (the key string passed in from Python).
2. postXfer only reads devIdToObjKey_, and registerMem/deregisterMem are the only writers. These registration calls happen at startup (in __init__), long before the background thread starts issuing query_memory calls.
3. The S3 client itself (checkObjectExistsAsync) uses an asio thread pool executor internally — it's designed for concurrent async I/O. Multiple concurrent HTTP requests to S3 are the normal operating mode.
4. No shared mutable state is touched concurrently between queryMem (background thread) and postXfer/check_xfer_state (scheduler thread). They both just issue independent HTTP requests through the same S3 client, which handles concurrency internally via its executor.

Bottom line: The depthfirst bot flagged this based on the general principle "shared object, no lock = danger." But in practice, queryMem and the transfer methods are effectively stateless operations dispatching independent HTTP requests through an internally-concurrent S3 client. No lock needed. You could add a one-line comment in the PR noting this for future readers, but no code change is required.

self, key: OffloadKey, req_context: ReqContext | None = None
) -> bool | None:
return os.path.exists(self.file_mapper.get_file_name(key))
def lookup(self, key: OffloadKey, req_context: ReqContext) -> bool | None:

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Need to verify fs tier unit tests don't pass req_context = None.

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Claude claims all fs tier test calls pass _CTX (which is ReqContext(req_id="test")), never None. No issue there.
This is also true for the obj test unit.

request_ids.
"""
keys_to_remove: list[OffloadKey] = []
for key, state in self._lookup_state.items():

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We should keep a req_id->set(keys) dictionary to avoid iterating over all _lookup_state items here.

Signed-off-by: Effi Ofer <effi.ofer@gmail.com>
effi-ofer added 5 commits June 8, 2026 22:06
Signed-off-by: Effi Ofer <effi.ofer@gmail.com>
Signed-off-by: Effi Ofer <effi.ofer@gmail.com>
Signed-off-by: Effi Ofer <effi.ofer@gmail.com>
Signed-off-by: Effi Ofer <effi.ofer@gmail.com>
Signed-off-by: Effi Ofer <effi.ofer@gmail.com>
"""
if self._need_to_drain:
self.drain_results()
self._need_to_drain = True # TODO: results might not be ready yet

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  1. Switch this to False here
  2. Set it to True in flush, unconditionally.

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scheduler flush 1 sets to True
scheduler flush 2 sets to True
worker picks up the two batches, process the first.
scheduler drains results and sets need to drain to False.

Comment on lines +228 to +232
if hit is True:
results.append((key, True))
elif hit is False:
results.append((key, False))
# hit is None → stays in-flight (not added to results)

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None is no longer possible:

Suggested change
if hit is True:
results.append((key, True))
elif hit is False:
results.append((key, False))
# hit is None → stays in-flight (not added to results)
results.append((key, hit))

touch the primary tier or scheduler state.

Returns a list parallel to keys: True if present, False if not
found, None if the tier is busy (retry later).

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Update docstring, None is not possible.

effi-ofer added 2 commits June 9, 2026 10:54
Signed-off-by: Effi Ofer <effi.ofer@gmail.com>
Signed-off-by: Effi Ofer <effi.ofer@gmail.com>
@orozery orozery added the ready ONLY add when PR is ready to merge/full CI is needed label Jun 9, 2026
effi-ofer added 2 commits June 9, 2026 12:37
Signed-off-by: Effi Ofer <effi.ofer@gmail.com>
Signed-off-by: Effi Ofer <effi.ofer@gmail.com>
Comment on lines +37 to +40
while time.monotonic() < deadline:
if not mgr._pending_results.empty():
return
time.sleep(0.01)

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Let's avoid polling and timeout:

import threading

class InMemoryLookupManager(AsyncLookupManager):
    """Test subclass backed by an in-memory set."""

    def __init__(self, existing_keys: set[OffloadKey] | None = None):
        super().__init__(tier_type="test")
        self._existing = existing_keys or set()
        self._results_ready = threading.Event()

    def batch_lookup(
        self, keys: list[OffloadKey], req_context: ReqContext
    ) -> Iterable[bool]:
        results = [k in self._existing for k in keys]
        self._results_ready.set()
        return results

Then replace all self._wait_for_drain(mgr) with:

mgr._results_ready.wait()
mgr._results_ready.clear()

return results


def async_lookup(

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Let's rename this to lookup_and_wait

Comment thread tests/v1/kv_offload/tiering/test_fs_tier.py
return results


def async_lookup(

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Same comment as test_fs_tier.py:

  1. Rename async_lookup to lookup_and_wait
  2. Switch to using an event instead of polling with timeout.

effi-ofer added 2 commits June 9, 2026 19:36
Signed-off-by: Effi Ofer <effi.ofer@gmail.com>
Signed-off-by: Effi Ofer <effi.ofer@gmail.com>

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Thanks @effi-ofer !

@orozery
orozery enabled auto-merge (squash) June 10, 2026 08:58
@orozery
orozery merged commit af65e08 into vllm-project:main Jun 10, 2026
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wcynb1023 pushed a commit to wcynb1023/vllm that referenced this pull request Jun 11, 2026
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Co-authored-by: Or Ozeri <oro@il.ibm.com>
Saddss pushed a commit to Saddss/vllm that referenced this pull request Jun 14, 2026
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Co-authored-by: Or Ozeri <oro@il.ibm.com>
vivek8123 pushed a commit to odh-on-pz/vllm-upstream that referenced this pull request Jun 18, 2026
Signed-off-by: Effi Ofer <effi.ofer@gmail.com>
Co-authored-by: Or Ozeri <oro@il.ibm.com>
divineearthly pushed a commit to divineearthly/vllm that referenced this pull request Jun 19, 2026
Signed-off-by: Effi Ofer <effi.ofer@gmail.com>
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nkzhenhua pushed a commit to nkzhenhua/vllm that referenced this pull request Jun 24, 2026
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Dao007forever pushed a commit to Dao007forever/vllm that referenced this pull request Jul 18, 2026
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philippesic pushed a commit to philippesic/vllm-semantic-cache that referenced this pull request Jul 19, 2026
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plasticchris pushed a commit to plasticchris/vllm that referenced this pull request Jul 20, 2026
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