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[https://nvbugs/6701568][fix] Recompute-pause a started request only when no other generation work progresses - #21144
Fixes https://nvbugs/6701568: qwen3.5_27b bf16 (2x H100, maxbs 512, maxnt 2048, ISL/OSL 1000/2000) Total Token Throughput dropped from 9333.1 to 7987.3 tokens/sec (-14.4%) between the bug's good commit 22d1def and bad commit b024d1d (3 reps each, measured alongside the first table below).
Bisecting the first-parent range lands on #15252 ("Add KV cache V2 recompute pause path"). When suspending other requests does not free enough KV pages for a generation request, KVCacheV2Scheduler now frees a started request's KV cache and re-prefills it later. It does this even when other generation requests were already scheduled in the same pass, which release pages as they finish. In this benchmark the KV pool fills up (kv_cache_util reaches 0.999) and the run prefills 995,002 context tokens instead of 516,000, while generation-only steps cost the same (26.0-26.8 ms mean host step time in every build). The extra prefill matches started requests being discarded and recomputed.
This change withholds the destructive recompute pause once a generation request has been scheduled earlier in the same pass; the blocked request self-suspends and waits instead, as it did before #15252. A generation request that suspension cannot make room for still recompute-pauses when no generation request has been scheduled earlier in the pass, so a pass in which nothing else runs still resolves the stall #15252 addresses. Requests in flight from an earlier pass (another pipeline-parallel micro-batch) do not withhold it: they keep their pages, and without the teardown that micro-batch would schedule no generation request.
The wait is configurable with TLLM_V2_RECOMPUTE_PAUSE_MAX_WAIT_ITERS. With k > 0, a blocked request waits at most k consecutive scheduling iterations and then recompute-pauses as on main. 0 recompute-pauses right away, as on main. The default (unset or negative) waits for as long as other generation requests are scheduled. The default stays unbounded because no finite bound kept this fix on the bug's workload (see "Bounding the wait" below).
Measurements
Root cause, perf/test_perf.py::test_perf[qwen3.5_27b-bench-pytorch-bfloat16-maxbs:512-maxnt:2048-input_output_len:1000,2000-gpus:2], 2x H100 80GB HBM3, all arms on one node, 3 interleaved rounds, wheels built from each commit. Total Token Throughput (tokens/sec):
On current main, with the CI wheel of L0_PostMerge #3042 (main 49b3197) and only scheduler_v2.py swapped per arm, all arms interleaved on one node (this harness version counts 1,024,000 prefill tokens when nothing is re-prefilled):
Case
main
this PR
Prefill tokens (main / this PR)
the bug's case above (TP2), 2x H100
7965.3 / 7970.9
9358.3 / 9358.5 (+17.4%)
2,006,732 / 1,024,000
the same with pp:2, 2x H100
2068.4 / 2077.8
2296.9 / 2296.2 (+10.8%)
1,880,352 / 1,024,000
Both cases fill the KV pool. Every rep completed 512/512 requests with no deadlock warning.
The GB200 attention-DP case whose deadlock #15252 fixed (NVBug 6602927), perf/test_perf_sanity.py::test_e2e[aggr-deepseek_r1_fp4_v2_grace_blackwell-r1_fp4_v2_dep4_mtp1_1k8k] (DEP4, MTP1, con2048 x 5, 1k/8k; in no CI list since #20677), same wheel and swap, four same-node pairs in alternating order: every run completed 10,240/10,240 requests with no stall or deadlock. Output throughput is lower with this PR in all four pairs: main 21,199.7 / 24,115.5 / 19,655.5 / 24,906.6, this PR 19,798.7 / 21,178.6 / 19,012.7 / 21,192.9 tok/s (means 22,469 vs 20,296, -9.7%). There main's teardown re-prefills about three times the prompt tokens (12.0-12.9M vs about 3.1M per rank) but keeps a larger decode batch (207-213 vs 187 scheduled requests per iteration), which pays off in that case.
Bounding the wait
Same harness, one rep per arm on a shared H100 node. Scheduling in these cases is deterministic run to run, so the prefill counts are exact, and main and the unbounded default reproduce the numbers above. Total Token Throughput (tok/s) / prefill tokens:
TLLM_V2_RECOMPUTE_PAUSE_MAX_WAIT_ITERS
the bug's case (TP2)
the same with pp:2
main
7942.8 / 2,006,732
2067.4 / 1,880,352
0
7957.3 / 2,006,732
1, 2, 4
7946.6, 7945.1, 7946.2 / 2,006,732
k=2: 2041.8 / 1,880,352; k=4: 2066.1 / 1,876,634
8
7943.7 / 2,001,030
12, 16, 32
9352.3, 9344.0, 9355.2 / 1,024,000
k=32: 2069.2 / 1,834,398
unset (default)
9311.2 / 1,024,000
2295.9 / 1,024,000
With the variable set to 0 in the environment, this head gives 7922.0 / 2,006,732, the same as main.
A finite bound does not separate this workload from the GB200 case. Instrumented runs of the bug's case show why small bounds behave exactly like main. All 514 of main's teardowns there come from a request that is itself suspended, and not one of them lets that request allocate, because its victims are newer requests that are suspended too. Under TP2 such a request is retried for up to 11 consecutive iterations, and under pp:2 a bound of 32 still lets most teardowns through. A shorter bound tears down the same 514 victims, only later: with k=4 the run matches main in all 7,942 iterations.
On the GB200 case above, a bound of 2 or 4 gives main's behavior. Bounded runs prefill 12.6-12.9M context tokens and schedule 202-208 requests per iteration, with no collapsed decode batch; main gives 12.5-12.9M and 207. This PR's default (the previous head's runs above) gives 4.55M and 187, with 13.5% of the busy iterations collapsed (fewer than a quarter of the active requests scheduled). Every run completed 10,240/10,240 requests with no stall. Same-node output throughput, in run order: k=4 20,328.3 then main 20,721.0, main 20,357.4 then k=4 23,635.5, and main 23,816.3 then k=2 22,654.6 tok/s. Within a node, the second run differs from the first by -5% to +16% whichever arm runs second, so these pairs do not resolve a throughput difference; the scheduling statistics match main's. For that regime, setting TLLM_V2_RECOMPUTE_PAUSE_MAX_WAIT_ITERS=0 restores main's scheduling, and so does a bound of 2 or 4.
new test_recompute_pause_waits_while_generation_progresses and its GPU-only variant ..._gpu_only (both fail on main), test_first_generation_request_recompute_pauses_gpu_only (the first-request exception in a pool where suspension frees nothing), and test_requests_in_flight_do_not_hold_back_recompute_pause (fails if requests in flight withhold the teardown).
test_secondary_tier_recompute_pause_retries_immediately and test_recompute_frontier_survives_ordinary_frontier_shrink expected a recompute pause after another generation request had been scheduled in the same pass, which this PR removes. They now start the pass with the blocked request and keep their checks: the immediate retry after teardown, and the recompute frontier reaching a suspended tail behind an ordinary eviction.
TLLM_V2_RECOMPUTE_PAUSE_MAX_WAIT_ITERS: test_recompute_pause_after_bounded_wait (waits k iterations, then tears down; host-tier and GPU-only pools), test_zero_wait_recompute_pauses_right_away (0 behaves as main), test_negative_wait_lasts_while_generation_progresses, test_wait_restarts_after_the_request_runs (only iterations waited since the request last ran count), and test_wait_restarts_after_the_request_is_torn_down (a request torn down while it waits starts a fresh wait). All but test_negative_wait_lasts_while_generation_progresses fail against the previous head, and every test in which a request waits fails against main. With the reset on scheduling removed, only test_wait_restarts_after_the_request_runs fails; with the reset on teardown removed, only test_wait_restarts_after_the_request_is_torn_down fails.
558 passed, 0 failed across test_kv_cache_v2_scheduler.py, test_py_scheduler.py, test_recompute_pause.py, test_scheduler_serializable_output.py, test_iter_stats_populate.py and test_kv_cache_manager_v2_helix_superblock.py, run on H100 with this PR's own pre-merge wheel (L0_MergeRequest_PR #67892).
PR Checklist
Please check this after reviewing the above items as appropriate for this PR.
The scheduler now disables recompute-pause for a blocked generation request after another generation request has been scheduled in the same pass. The blocked request can wait or use the existing self-suspend/stop handling. Earlier in-flight work does not disable recompute-pause. The change preserves the existing recompute-pause setting and does not change public APIs.
No current review findings or severity counts were supplied. Test execution results are not verified here.
QA Engineer Review
The modified scheduler test file adds coverage for both sides of the scheduling gate: defer teardown when generation work progresses, and allow recompute-pause when the blocked request is the first generation request to need pages. Tests also cover in-flight work, disabled recompute-pause, victim exclusions, allocation retry, and recompute-frontier behavior.
The changed tests are not listed by their new test IDs in the inspected test-db entries. That CI list contains other tests from the same test file. No manual-QA list entry was found. Coverage verdict: sufficient for the gate behavior, with test-list coverage for the new unit tests not confirmed. The PR objectives report 550 passing tests across six files; that result was not independently verified.
Per-File QA Perspective
tensorrt_llm/_torch/pyexecutor/scheduler/scheduler_v2.py — Verify that a scheduled generation request prevents destructive recompute-pause for later requests in the same pass, while earlier in-flight work does not. Confirm the blocked request follows the intended wait or self-suspend path.
tests/unittest/_torch/pyexecutor/kv_cache/test_kv_cache_v2_scheduler.py — The tests cover recompute-pause gating, victim eligibility, retries, and frontier behavior. The inspected test-db list contains other tests from this file, but not the new test IDs; no manual-QA entry was found.
…when no other generation work progresses
Since NVIDIA#15252, when suspending other requests does not free enough KV pages
for a generation request, KVCacheV2Scheduler frees a started request's KV
cache and re-prefills it later. It did so even when other generation
requests were already scheduled in the same pass, which release pages as
they finish. Under sustained KV pressure that repeatedly discards and
recomputes work.
Measured on qwen3.5_27b bf16, maxbs 512, maxnt 2048, ISL/OSL 1000/2000,
2x H100 (test_perf, trtllm-bench throughput), 3 interleaved reps each on one
node:
- parent of NVIDIA#15252: 9331.4 tok/s; NVIDIA#15252: 7984.7 tok/s (-14.4%)
- main 35cfb8c: 7913.9 tok/s; with this change: 9317.9 tok/s (+17.7%)
- prefill tokens processed: 516,000 before NVIDIA#15252 and with this change;
995,002 at NVIDIA#15252 and 1,007,366 on main without this change.
Allow the destructive recompute pause only when no generation request has
been scheduled earlier in the pass and none is in flight. Otherwise the
request self-suspends and waits, as before NVIDIA#15252. A pass in which nothing
else can run still recompute-pauses, so the stall NVIDIA#15252 resolves stays
resolved.
Tests: two scheduler tests that expected a recompute pause after another
generation request was scheduled now start the pass with the blocked
request, keeping what they check (immediate retry, recompute frontier
behind an ordinary eviction). Two new tests check that no recompute pause
happens while generation progresses or a request is in flight.
Signed-off-by: trtllm-agent <296075020+trtllm-agent@users.noreply.github.com>
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Generation scheduling can wait for pages before recompute-pausing a blocked request. The wait limit is configurable, and the scheduler clears the wait state when that request is scheduled or preempted. Tests cover wait limits, allocation conditions, and recompute frontier behavior.
Changes
Generation scheduling
Layer / File(s)
Summary
Configure and apply page-wait policy tensorrt_llm/_torch/pyexecutor/scheduler/scheduler_v2.py
The scheduler reads TLLM_V2_RECOMPUTE_PAUSE_MAX_WAIT_ITERS, which defaults to -1. When another generation request has been scheduled in the current pass, a blocked request waits for pages until the configured limit is reached. The scheduler clears the wait state when that request is scheduled or preempted.
Verify waits and recompute fallback tests/unittest/_torch/pyexecutor/kv_cache/test_kv_cache_v2_scheduler.py
Tests cover bounded, zero, and unlimited waits; wait reset after scheduling; GPU-only pools; prior-pass in-flight work; allocation retries; and recompute frontier behavior.
Under pipeline parallelism and KV pressure, a request can reach a worker without its required KV pages. Resolve this before merging.
Pre-merge checks: 4 · 1
Failed checks: 1 warning
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Passed checks: 4 passed
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The title names the main change: recompute-pausing a started request is restricted to when no other generation work progresses. It follows the required [NVBug ID][type] Summary format. It matches the …
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Actionable comments posted: 1
🪄 Fix comments: Fix CodeRabbit comments on this PR
Prompt to fix review comments
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only still-valid issues, skip the rest with a brief reason, keep changes
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Inline comments:
Review comments at @tensorrt_llm/_torch/pyexecutor/scheduler/scheduler_v2.py:
- Around line 575-580: Make the `allow_recompute_pause` decision consistent
across pipeline-parallel ranks before the nonzero-rank scheduler rerun, using a
synchronized in-flight request snapshot or shared decision. Reconcile any KV
allocation and victim-state mutations from a rerun whose result is discarded
with the propagated batch that will execute.
After applying the fix, consider running `coderabbit review --agent` for local
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…cro-batches are in flight
Requests in flight from an earlier pass no longer withhold the
recompute-pause fallback. Under pipeline parallelism every scheduled
micro-batch stays in inflight_request_ids until its step returns, so the
set is rarely empty there and the pause would almost never fire. Those
requests keep their pages; without the teardown this micro-batch would
run empty. The pause is still withheld once a generation request has
been scheduled earlier in the same pass.
test_recompute_pause_waits_while_requests_in_flight is replaced by
test_requests_in_flight_do_not_hold_back_recompute_pause.
Signed-off-by: chenfeiz0326 <203214996+chenfeiz0326@users.noreply.github.com>
Updated per BowenFu's review: 7c26567 scopes the recompute-pause gate so that requests in flight from another pipeline-parallel micro-batch no longer withhold it, and ad8cc8c merges main. H100 results for the bug's case and for the same workload under pp:2 are in the review thread. The GB200 attention-DP case of NVBug 6602927 is running and will be posted there.
@yizhang-nv@eopXD (owners of tests/unittest/_torch/executor/kv_cache) and @cascade812@lori-ren (owners of tensorrt_llm/_torch): could one of each team review?
…only KV pool
Add GPU-only variants (no cache tier below GPU, so suspension frees
nothing) of the gate test and of the first-request exception. Once a
generation request is scheduled in the pass, a blocked request does not
tear down a request suspended in an earlier pass; with nothing scheduled
yet, it does.
Signed-off-by: chenfeiz0326 <203214996+chenfeiz0326@users.noreply.github.com>
CI passes on 2b76364, which merges main d870ded. That commit moved the test file to tests/unittest/_torch/pyexecutor/kv_cache/; the PR's own changes are unchanged. lori-ren has approved for tensorrt_llm/_torch.
@yizhang-nv@eopXD (owners of tests/unittest/_torch/pyexecutor/kv_cache): could one of you review? The open question is in the review thread above. On the GB200 attention-DP case of NVBug 6602927, this PR is 9.7% slower than main across four same-node pairs. It is 17.4% faster on the bug's case and 10.8% faster under pp:2. Should it land as is, or should the gate keep main's behavior in that case?
@NVIDIA/trt-llm-kv-cache-manager-devs, a reminder: CI passed on 49a5558, and the change your team asked for (TLLM_V2_RECOMPUTE_PAUSE_MAX_WAIT_ITERS) is in. The PR still needs a code-owner review for tests/unittest/_torch/pyexecutor/kv_cache. Could one of you take a look?
@BowenFu, if the reply in your review thread covers your concern, could you resolve it?
@NVIDIA/trt-llm-kv-cache-manager-devs, a reminder: this PR has two approvals and includes the change your team asked for (TLLM_V2_RECOMPUTE_PAUSE_MAX_WAIT_ITERS). It still needs a code-owner review for tests/unittest/_torch/pyexecutor/kv_cache. Could one of you take a look?
CI is re-running on 3788f7f, which merges main. The previous run failed only on two B300 tests that also failed on main, and main now waives both.
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Review summary: 1 blocker, 3 possibly blocking, 6 required, 7 nice to have. The fix is well motivated, and the host-tier results are convincing. The main asks are:
the stale wait count after a deferred preemption (blocker);
a measurement, or a different default, before the wait ships for GPU-only pools;
the knob's semantics, documentation and validation;
the PR description.
Items that have no anchor in the diff:
[Possibly blocking] +1 to BowenFu's thread on scheduler_v2.py (the in-flight gate thread). With the unbounded default, the author's own same-node pairs on GB200 DeepSeek-R1 FP4 DEP4/MTP1 1k/8k still lose about 9.7% output throughput: 4 of 4 pairs are slower, and TPOT goes from 75.0 to 92.9 ms in pair 1. That case is in no CI or QA list, and the question to @yizhang-nv about the default is still open. Please get the KV-cache-manager owners' sign-off on the unbounded default, or default to a small bound for attention-DP/MTP. Also add the case to a QA perf list so the trade-off stays tracked. To decide: run that perf-sanity case on 4x GB200 for main, this default and =0, in at least 4 same-node pairs with alternating order. If the default is still 5% or more below main, this stays blocking.
[Required] PR description:
Merging main (e1f2b75, which brought in #21122) made this gate also control aggregated helix's only page-reclaim path. Helix skips ordinary eviction and goes straight into the gate (scheduler_v2.py:2037-2063). The description neither mentions nor measures this. Please describe it and get an ack from the author of #21122/#21455.
The "558 passed ... (L0_MergeRequest_PR #67892)" claim predates that merge; please cite the run on the current head (#68360).
The CodeRabbit block is stale: it says 550 tests and "test-list coverage not confirmed", and does not mention the env var. The template asks that a CodeRabbit summary make sense; please delete or regenerate it.
#21455 on current main rewrites the same helix branch, so please re-run the helix tests after rebasing.
[Required] Please coordinate with the open PR #21827 (same class, same test file). Its _admits_within_replay_reservation admits contexts in GPU-only pools without chunked prefill on the premise that "a growing request can always get pages by preempting the preemptible ones". With this PR's default, only the first blocked generation request of a pass preempts. The two PRs merge cleanly as text, so nothing will flag this. Please link the PRs, agree on the merge order, and restate or adjust that guarantee for the gated case.
[Required] release/1.3 contains #15252 (6afe08d), with the same ungated _try_recompute_pause_for_gen call (scheduler_v2.py:1306-1307 on that branch). Please say whether NVBug 6701568 also applies to release/1.3, and if it does, plan the cherry-pick together with the env-var escape hatch.
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[Blocker] This reset is skipped when the preemption is deferred. _preempt_or_defer returns early at L2473-2478 when KVCacheManagerV2.preempt_request() returns False because a KV connector save is still reading the pages (kv_cache_manager_v2.py:5662-5664; with a saving connector such as mooncake_store that is the normal path). _recompute_pause_waiter / _recompute_pause_wait_iters then keep the torn-down request's count. The release is finished later outside the scheduler (try_complete_preemption, PyExecutor._resume_preempted_request), and the parked request never goes through _preempt_or_defer again.
When that request has re-prefilled and is blocked on its first generation step behind a scheduled request, with no other request waiting in between, _waits_for_pages reads the stale count. With TLLM_V2_RECOMPUTE_PAUSE_MAX_WAIT_ITERS=k > 0 it then tears down another started request up to k passes earlier than configured. That contradicts the comment at L226-228 ("ends when ... its KV cache is given up").
Reach: a GPU-only pool with a saving KV connector and a finite k. The default (-1) and k=0 are not affected.
test_wait_restarts_after_the_request_is_torn_down only covers the immediate-release path.
Fix: clear the waiter before calling preempt_request, since the request gives up its KV cache either way:
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[Possibly blocking] In a GPU-only KV pool this wait can collapse the decode batch. GPU-only is a supported setup in three cases:
aggregated helix (kv_cache_manager_v2.py:2348-2356: "relieves KV pressure by recompute pause");
a KV connector that registers page addresses (L2357-2369: "pages are reclaimed by preemption instead");
host_cache_size=0.
Here is what happens in such a pool:
_try_evict_for_gen suspends every active request behind the blocked one and frees nothing. Suspended pages stay HELD at the last level (storageManager.cpp:850), so they still count toward utilization.
The blocked request waits and self-suspends.
None of these requests can come back. KvCache::resume refuses while hot-level utilization is above max_util_for_resume (kvCache.cpp:553, default 0.95), and V2 does not free paused_requests (py_executor.py:6176-6181).
Each request ahead that later needs a page, and is not first in its pass, does the same, so the batch shrinks toward the head.
Only the first generation request of a pass still tears down: one victim per page it needs. That never brings utilization back under 0.95.
On main, every blocked request tears down until it can resume, so the batch recovers, at a re-prefill cost. A quick simulation of this ladder (GPU-only, 200 requests, 1k/2k) gave about 2x the decode iterations of main, with long stretches at batch size 4 or less; k=2 and k=4 matched main. Every measurement in the description uses a host tier.
Please do one of these:
Keep the wait only where suspension moves pages off the GPU. When the env var is unset, use -1 if kv_cache_manager.has_cache_tier_below_gpu else 0, and update the two _gpu_only tests.
Or measure it. Run the bug's workload (qwen3.5_27b, 2x H100, 1k/2k) with host_cache_size=0, and once with aggregated helix or a registering connector. Compare main, this default and =0 on throughput and scheduled generation requests per iteration. If the default is more than 5% slower than main, or shows collapsed batches that main does not, it should not be the GPU-only default.
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[Possibly blocking] With a finite TLLM_V2_RECOMPUTE_PAUSE_MAX_WAIT_ITERS, the bound applies to the single waiter slot, not to each request. The description promises that "a blocked request waits at most k consecutive scheduling iterations". But there is only one _recompute_pause_waiter, and any other request that starts waiting resets the count (L226-228 says so).
Example with k=1 and requests [0, C, A, V]:
Pass 1: A waits (count 1).
Pass 2: C is blocked ahead of A and takes the slot. A's count is lost.
Pass 3: C hits the bound, tears down and runs. A starts again at 0.
Pass 4: A tears down.
A sat out 3 passes with k=1. Users who set k=2 or k=4 for the GB200 regime, as the description suggests, can see some requests held longer than configured.
Please do one of these, and add a test for the takeover case:
Keep a count per request: a dict[int, int] cleared on schedule and on teardown, pruned to the requests of the pass.
Or document the real rule ("k consecutive passes as the first blocked generation request; the count restarts when another request starts waiting") in the description, the docs and the __init__ comment.
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[Required] Please document this knob, and the default change, in the user docs.
Every aggregated KVCacheManagerV2 deployment now waits instead of recompute-pausing once a generation request is scheduled earlier in the pass; V2 is the default manager. The only way back is TLLM_V2_RECOMPUTE_PAUSE_MAX_WAIT_ITERS, and the description recommends it for the GB200 attention-DP/MTP case. It appears nowhere under docs/source. The PR template asks for "Documentation updated as needed", and env vars with their defaults belong in the user docs.
docs/source/features/kvcache.md already lists the KVCacheManagerV2 env vars. A short subsection there, or in docs/source/torch/scheduler.md, should cover:
what happens when a generation request cannot get pages: suspend, then wait if another generation request is scheduled earlier in the pass, otherwise recompute pause;
the default: unset or negative means wait without limit;
that 0 restores the previous behavior;
the exact counting rule for k > 0;
that only generation requests are affected (context-path preemption in GPU-only pools is unchanged);
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[Required] The value is not validated:
int("") or int("4.0") raises a bare ValueError: invalid literal for int() that does not name the variable. It comes from KVCacheV2Scheduler.__init__, on every rank, after the model has loaded.
Any negative value silently means "unbounded".
0 here means "tear down right away", while the sibling TLLM_V2_CTX_DEMOTE_MAX_ITERS (L318-322) uses 0 for "no bound". A user who follows that convention silently gets main's behavior back.
The startup log prints -1 instead of unbounded.
Please parse the value once with an error that names the variable, and store None for unbounded (bound is not None and waited >= bound). Log unbounded, and spell out what 0 means in the comment. Alternatively, make it a validated SchedulerConfig prototype field (Optional[NonNegativeInt]) next to context_starvation_iterations, which already reaches this constructor.
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[Nice to have] The gate also applies to aggregated helix: helix does no ordinary eviction and goes straight into the gate at scheduler_v2.py:2063. Nothing tests the helix wait branch. The helix superblock tests call _try_schedule_generation without generation_scheduled, and the test_helix_agg_preemption.py scenarios behave the same with or without the wait.
Consider a helix variant of these tests: mgr._has_cp_helix = True with requests [0 fits, 1 blocked, 99 started], asserting gen == [0], no recompute pause, no suspend and py_batch_idx is None, plus a =0 variant. Also update the docstring in test_kv_cache_manager_v2_helix_superblock.py that says an allocation failure "goes straight to recompute-pause".
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[Nice to have] Nothing is logged when a request waits instead of recompute-pausing. The evict, recompute-pause and self-evict steps of the same ladder all log. A debug line before return True would make the knob tunable from logs; it fires at most once per pass:
logger.debug(f"[V2Scheduler] Request {req.py_request_id} waits for pages "f"({waited+1}/{boundor'unbounded'}) instead of recompute-pausing a started request")
The helix message at L2088-2090 ("found no pages or victim; waiting") is also wrong on this path, because no victim search ran.
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[Nice to have]_waits_for_pages reads like a predicate (the _is_* / _has_* methods in this file are pure), but it advances the wait state, and it is called inside not (generation_scheduled and ...), where operand order matters. Suggestions:
Rename it _try_wait_for_pages and evaluate it as its own statement: waiting = not success and generation_scheduled and self.enable_recompute_pause and self._try_wait_for_pages(req).
Rename _recompute_pause_waiter to _recompute_pause_waiter_id.
Use one helper for the two resets (L2076, L2483).
Make generation_scheduled keyword-only without a default; the default only serves the helix test stand-in.
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[Nice to have] This test and its _gpu_only copy rebuild what _bounded_wait_case(-1, gpu_only=...) builds, and test_negative_wait_lasts_while_generation_progresses is the same case in a loop. Consider:
one test parametrized over gpu_only and the env value (unset, -1);
a TestRecomputePauseWait class for the knob tests, like TestLongContextDemotion;
an autouse monkeypatch.delenv("TLLM_V2_RECOMPUTE_PAUSE_MAX_WAIT_ITERS", raising=False), so an exported value does not fail the default-path tests;
dropping the attempts dict in test_recompute_frontier_survives_ordinary_frontier_shrink, which is now written but never read.
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[Nice to have] "without a teardown this pass would run empty" is not quite right: phase 2 still runs and can schedule context or encoder work. Suggest "would schedule no generation request".
In the _waits_for_pages docstring, the two "that request"s refer to different requests, and "it recompute-pauses" reads as if req itself is paused. Suggested wording: "Scheduled requests free their pages when they finish, so req waits rather than tearing down a victim, which would cost the victim a re-prefill ... it then tears down a victim as it would with nothing else scheduled."
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Description
Fixes https://nvbugs/6701568: qwen3.5_27b bf16 (2x H100, maxbs 512, maxnt 2048, ISL/OSL 1000/2000) Total Token Throughput dropped from 9333.1 to 7987.3 tokens/sec (-14.4%) between the bug's good commit 22d1def and bad commit b024d1d (3 reps each, measured alongside the first table below).
Bisecting the first-parent range lands on #15252 ("Add KV cache V2 recompute pause path"). When suspending other requests does not free enough KV pages for a generation request,
KVCacheV2Schedulernow frees a started request's KV cache and re-prefills it later. It does this even when other generation requests were already scheduled in the same pass, which release pages as they finish. In this benchmark the KV pool fills up (kv_cache_util reaches 0.999) and the run prefills 995,002 context tokens instead of 516,000, while generation-only steps cost the same (26.0-26.8 ms mean host step time in every build). The extra prefill matches started requests being discarded and recomputed.This change withholds the destructive recompute pause once a generation request has been scheduled earlier in the same pass; the blocked request self-suspends and waits instead, as it did before #15252. A generation request that suspension cannot make room for still recompute-pauses when no generation request has been scheduled earlier in the pass, so a pass in which nothing else runs still resolves the stall #15252 addresses. Requests in flight from an earlier pass (another pipeline-parallel micro-batch) do not withhold it: they keep their pages, and without the teardown that micro-batch would schedule no generation request.
The wait is configurable with
TLLM_V2_RECOMPUTE_PAUSE_MAX_WAIT_ITERS. Withk> 0, a blocked request waits at mostkconsecutive scheduling iterations and then recompute-pauses as on main.0recompute-pauses right away, as on main. The default (unset or negative) waits for as long as other generation requests are scheduled. The default stays unbounded because no finite bound kept this fix on the bug's workload (see "Bounding the wait" below).Measurements
Root cause,
perf/test_perf.py::test_perf[qwen3.5_27b-bench-pytorch-bfloat16-maxbs:512-maxnt:2048-input_output_len:1000,2000-gpus:2], 2x H100 80GB HBM3, all arms on one node, 3 interleaved rounds, wheels built from each commit. Total Token Throughput (tokens/sec):On current main, with the CI wheel of L0_PostMerge #3042 (main 49b3197) and only
scheduler_v2.pyswapped per arm, all arms interleaved on one node (this harness version counts 1,024,000 prefill tokens when nothing is re-prefilled):pp:2, 2x H100Both cases fill the KV pool. Every rep completed 512/512 requests with no deadlock warning.
The GB200 attention-DP case whose deadlock #15252 fixed (NVBug 6602927),
perf/test_perf_sanity.py::test_e2e[aggr-deepseek_r1_fp4_v2_grace_blackwell-r1_fp4_v2_dep4_mtp1_1k8k](DEP4, MTP1, con2048 x 5, 1k/8k; in no CI list since #20677), same wheel and swap, four same-node pairs in alternating order: every run completed 10,240/10,240 requests with no stall or deadlock. Output throughput is lower with this PR in all four pairs: main 21,199.7 / 24,115.5 / 19,655.5 / 24,906.6, this PR 19,798.7 / 21,178.6 / 19,012.7 / 21,192.9 tok/s (means 22,469 vs 20,296, -9.7%). There main's teardown re-prefills about three times the prompt tokens (12.0-12.9M vs about 3.1M per rank) but keeps a larger decode batch (207-213 vs 187 scheduled requests per iteration), which pays off in that case.Bounding the wait
Same harness, one rep per arm on a shared H100 node. Scheduling in these cases is deterministic run to run, so the prefill counts are exact, and main and the unbounded default reproduce the numbers above. Total Token Throughput (tok/s) / prefill tokens:
TLLM_V2_RECOMPUTE_PAUSE_MAX_WAIT_ITERSpp:2With the variable set to 0 in the environment, this head gives 7922.0 / 2,006,732, the same as main.
A finite bound does not separate this workload from the GB200 case. Instrumented runs of the bug's case show why small bounds behave exactly like main. All 514 of main's teardowns there come from a request that is itself suspended, and not one of them lets that request allocate, because its victims are newer requests that are suspended too. Under TP2 such a request is retried for up to 11 consecutive iterations, and under
pp:2a bound of 32 still lets most teardowns through. A shorter bound tears down the same 514 victims, only later: with k=4 the run matches main in all 7,942 iterations.On the GB200 case above, a bound of 2 or 4 gives main's behavior. Bounded runs prefill 12.6-12.9M context tokens and schedule 202-208 requests per iteration, with no collapsed decode batch; main gives 12.5-12.9M and 207. This PR's default (the previous head's runs above) gives 4.55M and 187, with 13.5% of the busy iterations collapsed (fewer than a quarter of the active requests scheduled). Every run completed 10,240/10,240 requests with no stall. Same-node output throughput, in run order: k=4 20,328.3 then main 20,721.0, main 20,357.4 then k=4 23,635.5, and main 23,816.3 then k=2 22,654.6 tok/s. Within a node, the second run differs from the first by -5% to +16% whichever arm runs second, so these pairs do not resolve a throughput difference; the scheduling statistics match main's. For that regime, setting
TLLM_V2_RECOMPUTE_PAUSE_MAX_WAIT_ITERS=0restores main's scheduling, and so does a bound of 2 or 4.Test Coverage
tests/unittest/_torch/pyexecutor/kv_cache/test_kv_cache_v2_scheduler.py:test_recompute_pause_waits_while_generation_progressesand its GPU-only variant..._gpu_only(both fail on main),test_first_generation_request_recompute_pauses_gpu_only(the first-request exception in a pool where suspension frees nothing), andtest_requests_in_flight_do_not_hold_back_recompute_pause(fails if requests in flight withhold the teardown).test_secondary_tier_recompute_pause_retries_immediatelyandtest_recompute_frontier_survives_ordinary_frontier_shrinkexpected a recompute pause after another generation request had been scheduled in the same pass, which this PR removes. They now start the pass with the blocked request and keep their checks: the immediate retry after teardown, and the recompute frontier reaching a suspended tail behind an ordinary eviction.TLLM_V2_RECOMPUTE_PAUSE_MAX_WAIT_ITERS:test_recompute_pause_after_bounded_wait(waitskiterations, then tears down; host-tier and GPU-only pools),test_zero_wait_recompute_pauses_right_away(0behaves as main),test_negative_wait_lasts_while_generation_progresses,test_wait_restarts_after_the_request_runs(only iterations waited since the request last ran count), andtest_wait_restarts_after_the_request_is_torn_down(a request torn down while it waits starts a fresh wait). All buttest_negative_wait_lasts_while_generation_progressesfail against the previous head, and every test in which a request waits fails against main. With the reset on scheduling removed, onlytest_wait_restarts_after_the_request_runsfails; with the reset on teardown removed, onlytest_wait_restarts_after_the_request_is_torn_downfails.test_kv_cache_v2_scheduler.py,test_py_scheduler.py,test_recompute_pause.py,test_scheduler_serializable_output.py,test_iter_stats_populate.pyandtest_kv_cache_manager_v2_helix_superblock.py, run on H100 with this PR's own pre-merge wheel (L0_MergeRequest_PR #67892).PR Checklist
🤖 Generated with Claude Code
Dev Engineer Review
The scheduler now disables recompute-pause for a blocked generation request after another generation request has been scheduled in the same pass. The blocked request can wait or use the existing self-suspend/stop handling. Earlier in-flight work does not disable recompute-pause. The change preserves the existing recompute-pause setting and does not change public APIs.
No current review findings or severity counts were supplied. Test execution results are not verified here.
QA Engineer Review
The modified scheduler test file adds coverage for both sides of the scheduling gate: defer teardown when generation work progresses, and allow recompute-pause when the blocked request is the first generation request to need pages. Tests also cover in-flight work, disabled recompute-pause, victim exclusions, allocation retry, and recompute-frontier behavior.
The changed tests are not listed by their new test IDs in the inspected
test-dbentries. That CI list contains other tests from the same test file. No manual-QA list entry was found. Coverage verdict: sufficient for the gate behavior, with test-list coverage for the new unit tests not confirmed. The PR objectives report 550 passing tests across six files; that result was not independently verified.Per-File QA Perspective
tensorrt_llm/_torch/pyexecutor/scheduler/scheduler_v2.py— Verify that a scheduled generation request prevents destructive recompute-pause for later requests in the same pass, while earlier in-flight work does not. Confirm the blocked request follows the intended wait or self-suspend path.tests/unittest/_torch/pyexecutor/kv_cache/test_kv_cache_v2_scheduler.py— The tests cover recompute-pause gating, victim eligibility, retries, and frontier behavior. The inspectedtest-dblist contains other tests from this file, but not the new test IDs; no manual-QA entry was found.