perf(frontend): merge_chunk rebuilt the whole backlog on every merge - #2020
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`StreamOutputCollector` folds an arriving chunk into the one still waiting
whenever the consumer lags, and `merge_chunk` rebuilt `token_ids` and
re-concatenated `text` from scratch each time. That is quadratic in the
backlog depth, and it runs on the event loop: the further the frontend falls
behind the engine the deeper the backlog gets, the more every merge costs,
and the further behind it falls. Past the point where the engine outruns
delivery the frontend does not degrade, it collapses.
The first chunk's list is the engine's own `output_tokens` and still must not
be appended to, so the copy happens once, into a list this module owns and
may extend in place from then on. `text` is popped before the concatenation
so the string has a single reference and CPython resizes it in place instead
of allocating a new one per merge.
DSR1 fp4, tp8 with DP attention, c=8192, ISL 1024 / OSL 4096, one wave,
otherwise identical builds:
before after
output throughput 42,970 -> 57,735 tok/s (+34.4%)
total throughput 53,713 -> 72,169 tok/s
mean TPOT 179.9 -> 130.9 ms
median ITL 17,475 -> 912 ms
The engine's last decode step lands at 568.6s in both; the run used to end at
793s and now ends at 581s, so what the quadratic term was buying was 225
seconds of the frontend draining a backlog after the GPUs had gone idle.
Same numbers seen from the other side: reverting the engine-side GC tuning
from ROCm#1980 also gains 10.2% on this config, because a slower engine keeps the
backlog shallow enough that the quadratic term stays cheap. That is worth
noting only as evidence for the mechanism -- the fix belongs here, not there.
gsm8k 3-shot 0.9507 flexible / 0.9492 strict, 1319/1319. Merge output is
byte-identical to the unmerged path at backlog depths from 1 to 500, and to
the previous implementation over 4000 randomized merge sequences, with the
engine's list never mutated.
🏷️ CI GuideRuns automatically on every eligible PR before approval:
Heavy model tests:
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Nice catch on the quadratic term — the mechanism is real and the depth-500 numbers reproduce for me (633 → 217 ns/merge). Four things before this lands. 1. The premise the copy machinery defends doesn't hold. The docstring and commit message say the first chunk's list is the engine's own output_tokens_list = list(new_tokens) if isinstance(new_tokens, tuple) else new_tokens.copy()
2. Copying once in
ns/merge. The shallow-backlog regression matters: depth 1-2 is the common case whenever a merge happens at all, and break-even is around depth 20-30. The variant is already ahead at depth 1 and beats the old code from depth 4 on. Trade-off is honest — it moves the "never touch the producer's list" contract to 3. The docstring now states the opposite of the code. Lines 81-83 still read " The 4. Three behaviours changed that were structurally impossible before. All reachable only via paths that don't exist today, but they were free guarantees: merge_chunk({"text": "a"}, {"token_ids": [], "text": "b"})
# -> {'text': 'ab', 'finished': False} # token_ids key gone; old code left []
eng = [1, 2]
merge_chunk({"token_ids": eng, "text": "a"}, {"text": "b"})
# -> into["token_ids"] is eng == True # empty-ids path no longer de-aliases
merge_chunk({..., "text": "acc"}, {"token_ids": [2], "text": None})
# -> TypeError, into == {'token_ids': [1, 2], 'finished': False} # text key popped, never restoredThe third is worth fixing regardless — Tests. The commit message describes exactly the harness this needs — 4000 randomized merge sequences, depths 1 to 500, engine-list-unmutated assertion — but it didn't land, and the suite is green identically with the PR applied and reverted. The existing One question on attribution, not a blocker. Backing out from your table: 42,970 tok/s / 8192 = 5.25 tok/s/stream → ~43k merges/s total. At depth 100 the old path is 235 ns/merge ≈ 1.0% of one event-loop core; even at depth 2000 it's 4.3%. That doesn't obviously account for +34% throughput and −95% median ITL. Your own #1980 cross-check hints the dominant cost may have been allocator/GC pressure rather than the copy loop — and the 225 s of post-idle drain is hard to square with a backlog of ≤1 chunk per stream, which is all |
…t hold
Review found the premise false. The docstring this change inherited says the
first chunk's list is the engine's own `output_tokens` and must not be appended
to; `scheduler.py` is the only `RequestOutput` site in the repo and already
hands out a per-step copy five lines above it, and the object is unpickled
fresh after crossing ZMQ besides. The ownership tag, the exact-class test and
the conditional copy were all guarding a list nothing else reads.
The copy is kept as defence in depth -- the collector should not extend a list
handed to it across two files and a process boundary on the strength of a
comment -- but it moves to `put_nowait`, which is where the producer's chunk is
first taken and already the only caller. That is one copy per stall rather than
a branch and a type test per merge, and it is faster than the previous
implementation at every backlog depth rather than from depth 20 on.
Three behaviours the old rebuild gave away for free and the tag version had
dropped:
- a delta with no text no longer loses the text already accumulated. The
string is popped to keep its refcount at one, so the write-back is in a
`finally` and a `TypeError` from the concatenation leaves `into` intact.
- `token_ids` is de-aliased from the producer's list on the way in now,
unconditionally, rather than on the first merge that carries ids.
- a chunk that never went through `put_nowait` now raises `KeyError` in
`merge_chunk` instead of silently growing a key. The precondition is
pinned by a test rather than left to be rediscovered.
The docstring said "rebuilt rather than extended" over code that extends; it is
rewritten. The pop carries a comment, since restoring `into["text"] = a + b`
reads as a cleanup and brings the quadratic term back with no test or lint
signal.
Tests: the harness the first commit described now exists. Two of the new cases
fail with the source change reverted, which the suite could not do before.
Agreed. _MergedIds, the exact-class check, the conditional copy and the if ids: early-out are all gone.
Copying once in put_nowait. Adopted verbatim: if waiting is None: merge_chunk is now a single extend, so the depth-1/2 regression in your table is gone.
The text refcount trick carries a comment saying it is not a pointless round-trip, worded after tool_parser/stream.py:53-68.
The third (text=None raising with the key never restored) is fixed with try: / finally:. The other two are now guaranteed by the put_nowait copy — that is the contract migration your point 2 asks for. Stated plainly: calling merge_chunk directly with an into that has no token_ids key now raises KeyError instead of creating it. The unbounded-doubling hazard on aliased or repeated merges is covered by the same copy; into's list is always ours.
Five added; two fail against unpatched source, so "green either way" no longer holds. The 4000-sequence harness did not land and I'd rather not add it: test_merging_is_identical_to_unmerged_delivery already sweeps drain cadences 1/2/3/5/2N over a multibyte payload, the equivalence test is parametrized over depths 1/2/17/500, and all four folded fields (extend, +=, or, last-non-empty-wins) are associative.
Six arms, same commit, two binary factors (merge implementation, ATOM_GC_THRESHOLD). conc 8192, 8192 requests, 1024/4096, default max-num-batched-tokens. py-spy window pinned at bench+260 s for 60 s; instrumentation byte-identical between arms. 
Your arithmetic is right, and so is the objection — but
of the one GIL-bound core, the frontend spends 26.9% folding and 73.1% on real work (unpickle, detokenize, deliver) unpatched, against 4.4% and 95.6% patched. The real work per token is unchanged, so capacity rises by 0.956/0.731 = +30.8%, against +32.2% measured on that pair. The model accounts for 96% of the gain; the remainder is plausibly the drain shortening faster than capacity rises.
Run duration shrinks by 70–80% of the max-lag reduction in both configurations.
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2 participants
Motivation
StreamOutputCollectorfolds an arriving chunk into the one still waiting whenever the consumer lags.merge_chunkrebuilttoken_idsand re-concatenatedtextfrom scratch on every fold, which walks the whole accumulation per merge — and it runs on the event loop, so it feeds back on itself: the further the frontend falls behind the engine, the deeper the accumulation, the more each merge costs.py-spy over 60s of steady decode at c=8192, self time:
merge_chunk27.5% across three lines, thetoken_idsrebuild alone 20.3% and the largest single consumer in the process.Technical Details
Both deltas are extended in place. The producer's list is copied once in
put_nowait, where the chunk is first taken, somerge_chunkis a single unconditionalextendwith no per-merge branch.The copy is defence in depth, not necessity:
scheduler.pyis the onlyRequestOutputsite in the repo and already hands out a per-step copy, and the object is unpickled fresh after crossing ZMQ. The collector should still not extend a list handed to it across two files and a process boundary on the strength of a comment, andtest_put_nowait_gives_merge_chunk_a_list_it_may_extendpins the precondition rather than leaving it to be rediscovered.textis popped before the concatenation so the string has a single reference and CPython grows it in place; the write-back is in afinally, so a non-string delta raises as it always did without losing the text already accumulated.Behaviour differences from the previous code, both only reachable by calling
merge_chunkoutside the collector:token_idsis de-aliased on the way in rather than on the first merge carrying ids, and a chunk that never went throughput_nowaitraisesKeyErrorinstead of silently growing the key.Test Plan
tests/entrypoints/.test_merge_extends_in_place_instead_of_rebuildingandtest_put_nowait_gives_merge_chunk_a_list_it_may_extendboth fail with the source change reverted.test_a_merged_stream_reads_the_same_as_an_unmerged_oneat backlog depths 1, 2, 17 and 500.test_merge_keeps_the_text_it_had_when_the_delta_is_not_a_string.lm_eval.benchmark_serving, DSR1 fp4, tp8 with DP attention, c=8192, ISL 1024 / OSL 4096,--ignore-eos, fresh server per run, otherwise identical builds.lm_evaldrives/v1/completionswithoutstream, so gsm8k does not reach the code this PR touches. The backlog-depth cases were written for that.Test Result
tests/entrypoints/Throughput, measured on the first implementation of this change (the ownership-tag one); being re-measured on the reworked patch and this section will be replaced with those numbers.
One wave, 8192 requests,
--max-num-batched-tokens 4096:Two waves, 16384 requests, default
--max-num-batched-tokens:Five waves, 40960 requests, 48 minutes: 57,514 tok/s, mean TPOT 131.19 ms, p99 TPOT 140.73 ms, 40959/40960 successful, no traceback and no preemption. One, two and five waves land within 0.8% of each other; before the change more waves meant worse, since the wave boundary is where the accumulation is deepest.
The engine's last decode step lands at 568.6s in both one-wave runs, counted from the
decode[bs=...]annotations. The run used to end at 793s and now ends at 581s.Reverting the engine-side GC tuning from #1980, with
merge_chunkuntouched, also gains 10.2% on this config: a slower engine keeps the accumulation shallow enough that the per-merge cost stays cheap. Included as evidence for the mechanism, not as an alternative.Submission Checklist