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@LucasWilkinson @WoosukKwon this does the close-upper-bound idea from #29134 for FlashInfer on Model Runner V2: plan from the CPU upper bound, then hand fa2 the exact lengths on the GPU. Could one of you please take a look and add @yavarb your sm120 MTP setup from #29134 takes exactly this path. If you have time, a run on this PR would be very welcome. |
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This pull request has merge conflicts that must be resolved before it can be |
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The FlashInfer builder copied seq_lens from the GPU on every build unless TRTLLM served all rows. The copy waits for all queued GPU work. Plan from the CPU upper bound instead. The runner adds a lower bound: the upper bound minus the drafts that the step in flight may reject. fa2 wrappers get the exact lengths on the GPU after plan(); other kernels skip the copy only when both bounds are equal. The check reads the wrapper about to be planned, so a wrapper that has not picked its kernels yet keeps the copy. Each wrapper stages plan() through a ring of pinned buffers, so a buffer with a pending copy is never reused; a builder adds at most 8 buffers. Co-authored-by: Claude Opus 5 <noreply@anthropic.com> Signed-off-by: gf239 <gf239@users.noreply.github.com>
The page-indices kernel wrote paged_kv_last_page_len_exact only when the build planned from the upper bound, through a constexpr variant. The exact seq_lens are on the device on every build, so the kernel now writes them always and the variant goes away. The buffer is read as before, only after a plan from the upper bound. Warm-up now compiles the only variant of the page-indices kernel; before, the variant used after a plan from the bounds compiled on the first live step. _plan() records the ring event in a finally block, so a plan() that raises does not leave its pinned buffer marked free. Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com> Signed-off-by: gf239 <gf239@users.noreply.github.com>
…ounds on the device Planning from the CPU bounds relies on lower <= seq_lens <= upper. With VLLM_DEBUG_SEQ_LENS_BOUNDS=1 the builder asserts this on the device where it consumes the bounds, through torch._assert_async. It does not synchronize; a violation surfaces as a device-side assertion. Off by default, at no cost. The helper lives in attention/backends/utils.py so tests import it without flashinfer. A GPU test builds with the check on under sync debug mode "error". Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com> Signed-off-by: gf239 <gf239@users.noreply.github.com>
…U tests The runner and the speculator computed their lower bounds inline. compute_seq_lens_cpu_lower_bound (numpy) and compute_draft_seq_lens_cpu_lower_bound (torch) do the same arithmetic in the same modules, called from the same sites. tests/v1/worker/test_seq_len_bounds.py checks both against known values, so a +-1 change of either formula or clamp fails, and runs check_seq_lens_bounds on CPU tensors: in bounds passes, out of bounds raises. No GPU needed. Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com> Signed-off-by: gf239 <gf239@users.noreply.github.com>
…what it does fixed_split_size counts pages, not tokens: fixed_split_size=block_size splits KV into chunks of block_size pages, not into one-page chunks. Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com> Signed-off-by: gf239 <gf239@users.noreply.github.com>
…d prompt tail test Since vllm-project#57214 the FlashInfer builder takes the upper bound as exact when there is no speculative decoding. The bounds in the builder tests count drafts, so those builders now report NUM_SPEC speculative tokens. test_padded_prompt_tail_builds_as_spec_decode builds its input batch by hand; give it the lower bound field the attention metadata now reads. Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com> Signed-off-by: gf239 <gf239@users.noreply.github.com>
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@vadiklyutiy @mgoin thanks for extending #57214 to all models without speculative decoding. |
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Heads-up from #40756: the I measured it with this PR merged at 5cf5be4 (Qwen3.5-0.8B, MTP, FlashInfer, fp8 KV, RTX 4080): 40-62% of plans were issued while the wrapper's previous copy was still queued, against 0 on main. #59493 keeps that staging buffer pageable, which removes the race without a sync. With both applied, outputs and acceptance matched this PR alone in every run, with throughput within run-to-run noise, so the two should compose. |
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@Sahil170595 thanks for checking this.
So a queued copy never reads a buffer that is being rewritten. #59493 also covers DCP prefill and cascade, which call |
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Purpose
On Ampere and Ada, with speculative decoding, the FlashInfer builder copies
seq_lensfrom the GPU on every build.The copy waits for all queued GPU work.
Meanwhile the CPU cannot prepare the next step.
#57214 removed this copy without speculative decoding.
There the CPU upper bound on
seq_lensis exact.With drafts it is not: it also counts drafts that may be rejected.
This PR covers speculative decoding.
The runner adds a CPU lower bound.
fa2 plans from the upper bound and reads the exact lengths from the GPU.
Without speculative decoding, only the pinned-buffer handling changes (see How it works).
Output tokens/s, this PR vs sham (main with comment-only edits in the same files):
n.s. = not significant. Median ITL is lower in every cell.
Not run on Hopper or Blackwell.
@WoosukKwon suggested planning from a close upper bound in #29134.
AI assistance (Claude Code) was used.
How it works
plan()uploads from a pinned buffer. While an upload is pending, the wrapper takes a fresh buffer. A builder adds at most 8 buffers (64 MB); thenplan()waits. This also covers the path from [Perf][Pooling] Avoid blocking seq_lens GPU-to-CPU copy for pooling in FlashInfer metadata builder #57214.backend="auto"picks its kernels on its firstplan(). That first build keeps the copy.plan()stages through_pin_memory_int_workspace_buffer, the kernel reads_paged_kv_last_page_len_bufon the device, and a non-positive last-page length is an empty page. Without these fields the copy stays.test_fa2_plan_from_upper_bound_matches_exact_planfails if FlashInfer changes them.VLLM_DEBUG_SEQ_LENS_BOUNDS=1assertslower <= seq_lens <= upperon the device in every build that uses the bounds. Off by default.Answers to #29134
@yavarb reported three problems with planning drafts from the upper bound:
plan()overwrote its pinned buffer during an upload. Here the wrapper takes a fresh buffer instead.Test Plan
tests/v1/attention, the FlashInfer kernel tests, four runner test files andtests/v1/spec_decode. Every failure re-run on main.vllm bench serve: main, sham and this PR, 6 rounds.Commands
gsm8k uses the prompts and scoring of
tests/evals/gsm8k/gsm8k_eval.py: 1319 questions, 5-shot, greedy, 256 tokens.Test Result
VLLM_DEBUG_SEQ_LENS_BOUNDS=1under load: no assertion.Tests
RTX 4090, torch 2.13.0+cu130, flashinfer 0.7.0, main af5b485.
This PR: 2103 passed, 302 failed, 2709 skipped.
The same 302 fail on main: 260 Triton kernels over this GPU's shared memory, 42 need checkpoints that are gated or not available offline here.
The tests from #57214 and #57075 pass.
Accuracy
gsm8k at c=32, RTX 3080. "Same answer" counts questions whose extracted answer matches the first main run.
Offline gsm8k, one
LLM.generateover the 1319 prompts, RTX 3080.For scale, on main:
Planning from the upper bound can give a row one more page.
fa2 splits KV by the planned lengths, so the same terms are summed in another order.
Speed details
vllm bench serve, random dataset, 1024 input and 256 output tokens,--max-num-seqs 32,--max-model-len 4096, fp8 KV cache.6 rounds; each runs main, sham and this PR with one seed, in rotating order.
Paired differences over rounds, 95% intervals. Sham vs main: every tokens/s interval covers zero.
Measured on main 386ac25. After the rebase on af5b485, 3 rounds of the first row give +15.0% / +17.7% / +15.7%.
Where tokens/s is n.s., its interval is wide; median ITL is still lower.
The RTX 4050 Laptop ran c=32 at
--gpu-memory-utilization 0.70; at 0.85 all three arms ran out of memory.A larger model, same protocol:
Qwen3.8-27B loads on main only with two local loader fixes, applied to all three arms:
quant_configfor its quantizedembed_tokens(cf. #54304), and skipping an extra draft head the checkpoint ships.Its KV cache holds about 19 of these requests, so c=32 runs about 19 at a time.
Essential Elements of an Effective PR Description Checklist
supported_models.mdandexamplesfor a new model.Generated with Claude Code and Human In The Loop 🙈