KV-Cache multi-tier offloading async batched lookup - #44193
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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>
| ) | ||
| 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.
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💡 Fix Suggestion
Suggestion: Add a threading.Lock to ObjectStoreSecondaryTierManager to serialize all access to self._agent. Specifically:
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In
ObjectStoreSecondaryTierManager.__init__()(around line 103), after creatingself._agent, create a lock:self._agent_lock = threading.Lock()
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In
ObjAsyncLookupWorker.batch_lookup()(line 81), wrap thequery_memorycall with the lock:with self._tier._agent_lock: results = self._tier._agent.query_memory(descriptors, "OBJ", "OBJ")
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Wrap every other
self._agentcall in the scheduler-thread methods (_submit_transfer,get_finished_jobs,lookup/_exists,shutdown, and_register_primary_kv) with the sameself._agent_lock. This includes calls toregister_memory(),prep_xfer_dlist(),transfer(),check_xfer_state(),release_xfer_handle(),release_dlist_handle(), andderegister_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.
orozery
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Thanks @effi-ofer !
Can you please change the PR title to be more descriptive?
| 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") | ||
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| 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.
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| 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 | ||
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| 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 | ||
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| 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.
| # 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) |
| 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]:
| 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.
Signed-off-by: Effi Ofer <effi.ofer@gmail.com>
orozery
left a comment
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Thanks @effi-ofer !
Can you please also add a test_async_lookup.py file with unit testing?
| 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.
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| 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] |
| """ | ||
| Collect completed jobs from the finished-jobs queue. | ||
| """ |
| 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:
| hits = [False] * len(keys) | |
| hits = (False for _ in range(keys)) |
| ) | ||
| for k in keys | ||
| ] | ||
| results = self._tier._agent.query_memory(descriptors, "OBJ", "OBJ") |
There was a problem hiding this comment.
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>
| """ | ||
| if self._need_to_drain: | ||
| self.drain_results() | ||
| self._need_to_drain = True # TODO: results might not be ready yet |
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- Switch this to
Falsehere - Set it to
Trueinflush, 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.
| 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:
| 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. | ||
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| 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.
Signed-off-by: Effi Ofer <effi.ofer@gmail.com>
| 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 | ||
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| def async_lookup( |
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Let's rename this to lookup_and_wait
| return results | ||
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| def async_lookup( |
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Same comment as test_fs_tier.py:
- Rename
async_lookuptolookup_and_wait - Switch to using an event instead of polling with timeout.
Signed-off-by: Effi Ofer <effi.ofer@gmail.com> Co-authored-by: Or Ozeri <oro@il.ibm.com>
Signed-off-by: Effi Ofer <effi.ofer@gmail.com> Co-authored-by: Or Ozeri <oro@il.ibm.com>
Signed-off-by: Effi Ofer <effi.ofer@gmail.com> Co-authored-by: Or Ozeri <oro@il.ibm.com>
Signed-off-by: Effi Ofer <effi.ofer@gmail.com> Co-authored-by: Or Ozeri <oro@il.ibm.com> Signed-off-by: divineearthly <divineearthly@gmail.com>
Signed-off-by: Effi Ofer <effi.ofer@gmail.com> Co-authored-by: Or Ozeri <oro@il.ibm.com>
Signed-off-by: Effi Ofer <effi.ofer@gmail.com> Co-authored-by: Or Ozeri <oro@il.ibm.com>
Signed-off-by: Effi Ofer <effi.ofer@gmail.com> Co-authored-by: Or Ozeri <oro@il.ibm.com>
Signed-off-by: Effi Ofer <effi.ofer@gmail.com> Co-authored-by: Or Ozeri <oro@il.ibm.com>
Purpose
Add async and batch lookup to multi-tier offloading. This significantly speeds up the secondary tier overall performance.
Test Plan
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_methodis deprecated. Please switch totorch.compileortorch.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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