[Bugfix][KV Offload] Bound the HIT_PENDING wait so a stalled write cannot defer requests indefinitely - #49850
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Documentation preview: https://vllm--49850.org.readthedocs.build/en/49850/ |
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I validated the configuration and deadline mechanics on the exact nightly we deploy, Two things are worth fixing before merge.
The safety argument for the 60-second default is P2P-specific, but the configuration is not ( Outside those points, the safety properties look correct: expired blocks are excluded before |
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Both accepted, thanks — will post the diff and validation once the branch is updated. Timer identity. Agreed, and it undercuts the default I argued for: if a newly-pending key inherits an older key's clock, the 40s-plus-margin ceiling doesn't hold for it. Going with reset-on-identity-change, in the narrower form: _LookupScan reports first_pending_key: bytes | None instead of saw_hit_pending (the bool becomes redundant), and the timer becomes (first_pending_key, start_time) — same key accumulates, different key re-arms, None disarms. Stays at two scalars rather than a per-key map, which matters at the call volume the issue documents. One tradeoff rather than burying it: if the newly-blocking key sits earlier in the prefix than the one currently timed, the accumulated time is discarded and the deadline restarts. Conservative direction — can delay a release, never expire early — but it makes the bound per-blocking-key rather than strictly per-key. Deadline scope. Taking the documentation branch. Metric description becomes your wording, and the usage doc will state that backends without an equivalent to _LOAD_TIMEOUT_S + _ABORT_ACK_TIMEOUT_S can cross the deadline on a healthy slow write — recomputation, not incorrectness — with 0 to disable. Happy to add a per-spec default instead if reviewers prefer. And thanks for running the configuration and transition tests on nightly-0ba2aa35a — stock-fails/patched-passes on your own deployment target is a stronger signal than anything I can produce CPU-only. |
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This pull request has merge conflicts that must be resolved before it can be |
TieringOffloadingManager.lookup() returns HIT_PENDING for a primary-tier block whose write is still in flight, and the connector scheduler counts that block as a hit while setting defer_lookup. get_num_new_matched_tokens() then returns None, which the core scheduler treats as "ask again next step" with no bound on the number of steps. If the owning write leaks, the key stays HIT_PENDING forever and every request whose candidate prefix reaches it is deferred until the client times out. Driving the real get_num_new_matched_tokens() against a permanently pending manager, an unfixed tree defers 1000/1000 scheduling steps and burns 3003 lookup() calls without ever resolving. Bound the wait. Once a request has been continuously deferred on HIT_PENDING for longer than hit_pending_deadline_s, pending blocks stop counting as hits for that request: the prefix is truncated at the first HIT_PENDING key and the request recomputes locally instead. The fix lives in the connector scheduler rather than in TieringOffloadingManager, because the two scan helpers call manager.lookup() polymorphically -- one change covers CPUOffloadingSpec, TieringOffloadingSpec and any future manager. The knob lives on the base OffloadingSpec, which both concrete specs inherit. Truncation breaks before incrementing hit_count (prefix scan) and resets the streak exactly as MISS does (sliding-window scan). Counting a pending block would hand it to prepare_load, which asserts block.is_ready, turning the hang into an assertion failure. Only HIT_PENDING is bounded. deferred_lookup_start_time is armed by any deferral including RETRY and feeds the LOOKUP_ASYNC_DELAY histogram, so this adds a separate timer armed only when a scan observed a live HIT_PENDING. To arm it correctly the scan reports why it deferred, which the old int | None return swallowed; both helpers now return a _LookupScan carrying saw_hit_pending. The signal comes out of the scan that already ran, not from a second lookup sweep. A pass that defers without observing HIT_PENDING disarms the timer, as do the paths that skip the lookup and any pass that resolves. _LookupScan is a NamedTuple return rather than an out-parameter on the existing int | None signature. Both designs need the downgrade_hit_pending input, so both break a stale stub; the difference is how. A stub that has not been updated fails loudly under the NamedTuple (AttributeError on num_hit_chunks) but silently reports saw_hit_pending=False under an out-parameter, which is the failure mode this signal exists to detect. The deadline is per-request sticky rather than per-(req_id, key) as the issue suggests. Downgrading only costs a cache hit and forces recomputation, never correctness, and this keeps the state at two scalars on a path that runs millions of times. The default of 60s clears the worst case for a live self-initiated promotion (P2P _LOAD_TIMEOUT_S 30s + _ABORT_ACK_TIMEOUT_S 10s), after which the finished-job poll resolves the block either way. A request blocked on an unrelated leaked write has no such bound and is released at any value. Set 0 to disable. hit_pending_deadline_s is validated with `not (value >= 0)` rather than `value < 0` so NaN is rejected. NaN passes a bare `< 0` guard, and every later `now - start < nan` is then also False, which would expire the request on its second deferred pass instead of honouring any deadline. Expiries are counted by vllm:kv_offload_hit_pending_deadline_expired and logged, so recomputation makes the symptom survivable without hiding the leak. This bounds the request-level impact; it does not repair the stalled transfer, which still needs independent cleanup. Signed-off-by: Jason Yao <wsyjh8@gmail.com>
The deadline added in the previous commit was per-request and sticky: one clock, armed the first time any scan reported a live HIT_PENDING and left running until no scan reported one. But a scan does not stop at the first pending key, so a single request can cross several pending blocks, and those blocks belong to different writes. Under that timer a request 59s into waiting on key 1 carried the whole 59s over to key 2 the moment key 1's write landed, and expired on key 2 one second later even though key 2's own write had only just started and was perfectly healthy. Any key could inherit an almost-spent clock, which left the 60s default no real margin. Key the timer on the blocking block instead. RequestOffloadState now tracks the pair (hit_pending_key, hit_pending_start_time): the same key on a later pass keeps accruing, a different key re-arms from scratch, and a pass that observes no pending key at all disarms both halves together. They are always cleared as one, because a key left behind without its start time would read as a *different* blocker on the next pass and silently re-arm. Carrying the identity out of the scan means _LookupScan now reports first_pending_key: OffloadKey | None rather than saw_hit_pending: bool, and RequestOffloadState.saw_hit_pending becomes observed_pending_key. The signal still comes out of the scan that already ran, not from a second manager.lookup() sweep. It is still two scalars rather than a per-(req_id, key) map: the issue reports a single wedged request accumulating 1,270,000 lookup() calls, so request state has to stay O(1) in the prefix length. The prefix scan runs forward, so its first pending key is the lowest-index one, which is the block that bounds how far that prefix can ever resolve. The sliding-window scan runs backwards, so its first is the highest-index one. Either end works: the choice only has to be deterministic, and groups are walked in the fixed self._lookup_groups order, so an unchanged set of stalled writes selects the same key on every pass and the timer accrues instead of resetting. The bound this yields is per-blocking-key, not per-request. A request that keeps acquiring new blockers can wait longer than one deadline in aggregate, up to N x deadline for N consecutive slow-but-unleaked writes. That is the conservative direction -- each change of identity means the previous write actually landed -- and the case this deadline exists for, a leaked write that pins one key forever, still expires on time. Also repairs a direct call site the previous commit's return-type change left behind: test_scheduler.py asserted `_maximal_prefix_lookup(...) == 1`, which a _LookupScan never satisfies, so that test has been failing since the return type changed. It now reads .num_hit_chunks. Signed-off-by: Jason Yao <wsyjh8@gmail.com>
…ound The 60s default is derived from the P2P secondary tier's own ceiling on a live promotion: _LOAD_TIMEOUT_S (30s) + _ABORT_ACK_TIMEOUT_S (10s) = 40s, after which the finished-job poll resolves the block either way. That ceiling is specific to P2P. The CPU, filesystem and object-store backends publish no equivalent bound on how long a write may take, so on those a healthy but slow write can cross any fixed deadline. The wording shipped in the first commit did not say so, and drew the stronger conclusion that a non-zero vllm:kv_offload_hit_pending_deadline_expired "means offload writes are leaking". On a slow filesystem or object store it may mean nothing of the kind. State the scope consistently in all three places the knob is described -- the metric documentation, the comment on the config knob in base.py, and the docs table. A leak is one explanation for an expiry, and the only one on P2P; elsewhere a slow write is another, and the cost when that happens is a cache miss and local recomputation, never incorrect output. Deployments on a slower backend may need to raise the default. Signed-off-by: Jason Yao <wsyjh8@gmail.com>
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Thanks @thegoldenflow !
This adds relatively a lot of code, while I feel it's necessity as doubtful, given that we can solve things on lower tiers (e.g. the P2P tier).
I suggest we postpone this for now.
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Thanks @orozery , Understood, thanks for taking a look — converting to draft and parking this. Agreed the right first move is fixing leaks at their source; will look at the remaining P2P-tier cleanup path instead. Happy to revisit this only if a stall surfaces on a path without a source-level fix. |
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Purpose
Fixes #49829.
OffloadingConnectorScheduler.get_num_new_matched_tokens()returnsNoneto tell the core scheduler "I can't determine the hit count yet, ask again next step". The core scheduler pops the request and re-prepends it to the skipped-waiting queue — with no bound on how many times that can repeat.LookupResult.HIT_PENDINGis one of the things that produces thatNone: a block present in the primary tier whose write (a GPU→CPU save, or a secondary→primary promotion) is still in flight. Both_maximal_prefix_lookupand_sliding_window_lookupcount a pending block as a hit and setdefer_lookup, so the request waits for the write rather than recomputing. If that write leaks, the key staysHIT_PENDINGforever and every request whose candidate prefix reaches it is deferred on every scheduling pass until the client gives up — the issue reports requests dying at the 300 s aiohttp timeout, and one wedged request accumulating 1,270,000lookup()calls.The fix. Once a request has been continuously deferred on a given
HIT_PENDINGblock for longer thanhit_pending_deadline_s, that block stops counting as a hit for that request: the candidate prefix is truncated there and the request proceeds to local recomputation instead of waiting on a write that is not coming.Design
Placed in the connector scheduler, not in
TieringOffloadingManagerThe issue points at
TieringOffloadingManager.lookup(), but the two scan helpers callself.manager.lookup()polymorphically, so one scheduler-side fix covers every backend without duplicating deadline logic per manager.This matters because the defect is not tiering-specific.
CPUOffloadingSpecis the default spec (factory.py:45) and its manager returnsHIT_PENDINGfromcpu/manager.py:133with the same absence of a deadline; the scheduler calls thatlookup()directly, so a stalled GPU→CPU store defers requests identically on the default, non-tiering path. A manager-side fix would have left the common case unfixed. The knob lives on the baseOffloadingSpecandTieringOffloadingSpec→CPUOffloadingSpec→OffloadingSpec, so both paths inherit it structurally (asserted intests/v1/kv_offload/test_spec_config.py).The pending block is also non-evictable — it is only made evictable once its write completes — so a leaked write leaves it permanently pinned and permanently
HIT_PENDING. There is no recovery path today.Truncate, never count
On expiry
_maximal_prefix_lookupbreaks before incrementinghit_count, and_sliding_window_lookupresets the streak exactly asMISSdoes. This is load-bearing: counting a pending block would hand it toprepare_load, which assertsblock.is_ready(cpu/manager.py:146) — trading a hang for an assertion crash. Downgrading also cannot trigger a competing promotion, sinceTieringOffloadingManager.lookup()short-circuits on a primary-tierHIT_PENDINGbefore consulting any secondary tier.A dedicated timer, keyed on the blocking block
deferred_lookup_start_timeis armed by any deferral —RETRY,HIT_PENDING, and the self-inflicted_chunks_being_loadedpath alike — and feeds theLOOKUP_ASYNC_DELAYhistogram. Reusing it would let a longRETRYstall arm aHIT_PENDINGdowngrade with zero grace and would change that histogram's meaning. This adds a separate timer, armed only when a scan actually observed a liveHIT_PENDING.RETRYsemantics are untouched (test_retry_deferral_never_expires).To arm it the scan has to report what blocked it, which the old
int | Nonereturn swallowed. Both helpers now return a_LookupScanNamedTuple carryingfirst_pending_key: OffloadKey | Nonealongside the count — a NamedTuple rather than an out-parameter so that a stale test stub fails loudly under mypy instead of silently reporting "nothing pending". The signal comes out of the scan that already ran, deliberately not from a secondmanager.lookup()sweep, which would compound the call-volume problem the issue documents.Request state is
(hit_pending_key, start_time): the same blocking key accrues, a different key re-arms, andNonedisarms — as do the paths that skip the lookup entirely (in-flight transfers,skip_reading_prefix_cache) and any pass that resolves.reset_cache()clears both fields on surviving requests.The bound is therefore per blocking key, not per request (#49850 review). Two scalars, not a per-key map, on a path the issue documents at 1,270,000
lookup()calls for a single request. The consequence, stated rather than buried: a request that successively acquires new earliest-pending keys can wait up to N × deadline in aggregate. Reaching a client timeout that way requires N sequential slow-but-unleaked writes; the mechanism #49829 reports still resolves cleanly, since a leaked write holds one key and the identity stays stable. If reviewers want a hard per-request ceiling, a never-re-armed first-arm timestamp is one more scalar — I left it out rather than introduce a second constant I cannot ground the way 60 s grounds below.Choosing the default (60 s,
0disables)The check cannot distinguish two situations:
_LOAD_TIMEOUT_S(30 s) +_ABORT_ACK_TIMEOUT_S(10 s) = 40 s (tiering/p2p/session/client.py:34-35). This is the only case a too-short deadline can harm, by discarding a hit that was about to land.HIT_PENDINGhas no deadline; a stalled write can defer requests until the client timeout #49829 reports, and it is resolved at any deadline value.60 s clears case A's ceiling with margin, since case B is insensitive to the value. That ceiling is P2P-specific: CPU, fs, and obj backends have no equivalent transfer bound, so a healthy-but-slow write can cross the deadline there. The cost is a cache miss and local recomputation, never incorrectness, and
0restores the previous behaviour. The issue mentions 8 s working in a 64×16K pool run; that run established that 8 s eliminates the hang but did not measure whether it pre-empts live promotions and costs hit rate, so 8 s is offered as an operator-side value, not a validated default.Observability
New counter
vllm:kv_offload_hit_pending_deadline_expired, registered exactly like the existingALLOCATION_FAILURE, plus alogger.warningnaming the request. A non-zero value means a write remained pending beyond the configured deadline — on P2P that implies a leak; on backends without a transfer bound it can also mean a healthy slow write. Recomputation makes the symptom survivable without making the underlying leak invisible. This bounds the request-level impact; it does not repair the stalled transfer, which still needs independent cleanup or cancellation.Not addressed here
Backoff for deferred-lookup retries (the
RETRYpath) is deliberately out of scope — it belongs with #49176, and this change leavesRETRYsemantics byte-for-byte identical.Test Plan
Ubuntu 24.04 (WSL2), Python 3.12.3, torch 2.13.0+cpu, no accelerator.
request_runnerbuilds, so the full offloading suite runs here: 632 of 668 items, ~3.5 min. Baseline and branch are run in the same tree, switching only the source under test.Equal failure counts and equal node-ID sets can hide a changed failure reason, so the two runs are diffed reason-aware: for every
FAILURES/ERRORSblock the terminalE ...line is reduced to its exception class and compared on(section, test, exception class).A standalone reproducer drives the real
get_num_new_matched_tokens()against a manager whose block is permanentlyHIT_PENDING, in two scenarios — the original hang, and a second key becoming pending while the first is still outstanding. Timers are backdated rather than slept, matching the_LOAD_TIMEOUT_Sidiom intests/v1/kv_offload/tiering/p2p/test_sessions.py.New coverage:
TestHitPendingDeadline(12 cases through the realget_num_new_matched_tokens()— wedged request released, partial prefix preserved, identity re-arm, same-key accrual, disarm,RETRYnever expires,0disables,reset_cacheclears);TestLookupScanSignalplus cases inTestMaximalPrefixLookup/TestSlidingWindowLookup;tests/v1/kv_offload/test_spec_config.py(new — both specs surface the knob); and aHIT_PENDINGcase intest_tiering_offloading.pypinning the two properties the scheduler-side placement relies on.Test Result
1. Reproducer, three arms. Deadline 60 s, timers backdated:
Scenario B is the case raised in review: the second key inherited the first key's clock and died 2 s into its own healthy write. It fails on the first commit and passes here, so the new coverage is behavioural rather than API-shaped.
2. Full offloading suite:
bf4f633b4b139909e3Reason-aware diff: IDENTICAL — same FAILED node-ID sets, same exception classes, +36 passing. All 32 baseline failures are in
test_gpu_worker.pyonassert gpu_tensor.is_cuda or gpu_tensor.is_xpu(cpu/gpu_worker.py:202), i.e. the missing accelerator; they reproduce unchanged onmain.3. Targeted selection: 63 passed, 88 deselected. The 15 call sites touched in
test_scheduler.pylive in 13 test functions, all of which execute in this environment (23 passed).4. Lint and type checking, all 7 changed files:
pre-commit run --filesexits 0 with every hook Passed, includingUpdate Dockerfile dependency graph(its earlier failure was a Windows-only/bin/bash not found). mypy 3.11/3.12/3.13 manual-stage all Passed. Because "mypy passed" is only meaningful if mypy analysed these files, a deliberate-error canary was used:The file was restored and verified byte-identical afterwards.
CI state on this PR
pre-run-checkis red, and it is not a test failure. Its log:pre-commit— the required check — is skipped as a consequence. Real CI needs a maintainer label; everything above is what can be produced without it.Interaction with in-flight work
Files touched:
offloading/scheduler.py,offloading/metrics.py,v1/kv_offload/base.py,docs/features/kv_offloading_usage.md, and three test files. Verified withgit merge-tree(a real three-way merge), not by eyeballing path lists.tiering/p2p/. I also confirmed_LOAD_TIMEOUT_S = 30.0and_ABORT_ACK_TIMEOUT_S = 10.0are identical atorigin/main, at [Bugfix][KV Offload][P2P] Scope serve state to fetch rounds #49877's head, and here — this PR's 60 s default is grounded in those two constants.tiering/async_lookup.py,tiering/manager.py,tiering/obj/manager.py,tiering/fs/*,csrc/fs_io.cpp, and four tiering tests. The only file it shares with this PR istests/v1/kv_offload/tiering/test_tiering_offloading.py, where its cases and mine land in different parts of the same class; that file auto-merges cleanly. Its trial merge does conflict intiering/fs/io.py, a file this PR never touches — the same conflict reproduces from plainmain, so it is pre-existing rebase debt, neither caused nor worsened here. Behaviourally the two stay disjoint: [KV Offload] Fix failed-load livelock by marking the lookup verdict as a miss #49328 owns theRETRY/async-lookup invalidation path, which this PR leaves unchanged.AI assistance
This PR includes AI-assisted code (Claude). I reviewed every changed line, reproduced the original hang and verified the fix myself, and I take responsibility for this change.
Worth flagging in both directions. Line-by-line review of the diff is what caught 14 stale monkeypatch stubs — thirteen
_maximal_prefix_lookupstubs returning a bareintand one_sliding_window_lookupstub taking three positional parameters, none matching the_LookupScansignature. But review also missed a 15th site:test_scheduler.py:321calls_maximal_prefix_lookupdirectly and compared its result to1, which the NamedTuple return silently broke. A test run finds that one immediately; reading the diff did not. All 15 sites are now corrected and all 15 execute under the suite above.Model evaluation: N/A. The change only decides, for a block whose write is still in flight past the deadline, whether the request reuses a cached KV block or recomputes it locally. Recomputation yields the same values, so generated tokens are unchanged; the only observable effects are cache hit rate and the new counter. At the default, behaviour is unchanged unless a write has been stalled for a full minute — the bug being fixed.
Verification status
Two items remain, both requiring hardware I don't have:
get_num_new_matched_tokens()directly. A serving run against a real offloading deployment — confirming a stalled write now yields recomputation and an incrementedvllm:kv_offload_hit_pending_deadline_expiredrather than a client timeout — is the next step.HIT_PENDINGhas no deadline; a stalled write can defer requests until the client timeout #49829 was obtained with an 8 s configuration. A pool run at the shipped default is needed to confirm it clears the hang without pre-empting live promotions in a way that costs hit rate.Essential Elements of an Effective PR Description Checklist
supported_models.mdandexamplesfor a new model.