perf(sort): give read-ahead depth to the file the merge is actually on - #814
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Included review availability: 0 reviews are currently available. Your included PR review attempts over the past 7 days set your current allowance at 1 review per hour. WalkthroughThe PR adds codec-specific spill-frame sizing and decompression caps. It updates Phase 2 merge consumption, scheduling, source prediction, worker wake selection, progress batching, and diagnostics. ChangesCodec-specific spill sizing
Phase 2 merge pipeline
Merge prediction and reporting
Estimated code review effort: 5 (Critical) | ~90 minutes Merge Risk: 🔵 Low · up to The PR preserves output correctness and passes the supplied checks, but a reused worker pool may retain a stale read-ahead hint, performance diagnostics may misclassify some self-serve events, and one test comment states an incorrect runner-up guarantee. The change is mergeable with explicit owner awareness and follow-up on these bounded risks. Sequence Diagram(s)sequenceDiagram
participant MergeConsumer
participant LoserTree
participant WorkerPool
participant SpillReader
MergeConsumer->>LoserTree: select current winner and runner-up
LoserTree-->>MergeConsumer: return next active source
MergeConsumer->>WorkerPool: publish next-source prediction
WorkerPool->>SpillReader: prioritize awaited or predicted source
SpillReader-->>WorkerPool: queue raw block
MergeConsumer->>WorkerPool: decompress queued block before parking
WorkerPool-->>MergeConsumer: return decoded block
Suggested labels: 🚥 Pre-merge checks | ✅ 3✅ Passed checks (3 passed)
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Stack now has a third: #823 adds the merge floor line (which of three limits binds, and what is recoverable), the clock-overhead correction for the sampled sub-phases, and demotes the breakdown to It also records the arms that failed against that gap, so they are not re-litigated: five that moved decompression off the serial consumer (slope −0.373 s/s, r = −0.975), an adaptive regime gate (+12.5%, and its discriminant moved under its own action), a consumer-side read of the starved block (fired on 0.6% of starved parks), and finer publish granularity on the deep read (0.1%). |
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In `@crates/fgumi-sort/src/external.rs`:
- Around line 1812-1815: Update the Phase 2 merge flow so
serve_self_instead_of_parking does not set parked_yet; set that flag only after
the worker actually parks, preserving stalled_pulls accounting when self-service
occurs before the first park. In merge_chunks_with_index, publish
tree.runner_up() via set_phase2_next_source so indexed coordinate merges receive
predictive read-ahead.
Apply the same fix in `@crates/fgumi-sort/src/external.rs` around lines 5353 -
5366: This is the indexed merge loop that lacks next-source prediction
publication.
In `@crates/fgumi-sort/src/loser_tree.rs`:
- Around line 429-431: Update the documentation for the runner_up test to state
that runner_up identifies the second-smallest active current key and only
predicts the source after the current winner run; do not claim it guarantees the
source that emits the next record, since the winner may remain smaller after
replace_winner.
In `@crates/fgumi-sort/src/worker_pool.rs`:
- Around line 1515-1544: Reset phase2_next_source to NO_AWAITED_SOURCE in
set_phase2_files alongside the other Phase 2 state resets, before activating the
new merge. Preserve the existing prediction behavior after initialization while
preventing a reused pool from carrying the previous merge’s source priority into
the new merge.
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crates/fgumi-sort/src/bgzf_io.rscrates/fgumi-sort/src/external.rscrates/fgumi-sort/src/lib.rscrates/fgumi-sort/src/loser_tree.rscrates/fgumi-sort/src/merge_trace.rscrates/fgumi-sort/src/pooled_chunk_writer.rscrates/fgumi-sort/src/progress_batch.rscrates/fgumi-sort/src/worker_pool.rscrates/fgumi-sort/src/zspill_stream.rs
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Spill frames were flushed at `BGZF_MAX_BLOCK_SIZE`. That is a *BGZF format* requirement -- blocks of at most 64 KiB -- applied to zstd, which has no such limit, for a private temporary format nothing outside this crate reads. The limit was vestigial, and it set the merge's block count: 5,368,249 blocks on an 89-way merge, of which the consumer takes roughly one per round trip. That matters because three scheduling changes have now each moved their target and been absorbed, leaving wall unchanged: merge wall is round-trips x cost-per-round-trip, and `blocks ready at resume` stayed at 0.83-1.81 throughout. Cutting the block count is one of only two levers that touches the round-trip term. `spill_frame_bytes` is keyed on the codec, so BAM output -- which always uses `SpillCodec::Bgzf` -- cannot pick up a spill-only frame size by accident, and BGZF spills stay inside their format. `SPILL_FRAME_BYTES` defaults to `BGZF_MAX_BLOCK_SIZE`, so this changes no behavior; it makes the size settable so the ablation can move it. Both zstd frame caps are now *derived* from it rather than fixed at 256 KiB. This is the part that would have broken silently: - `zstd_decomp_cap` (merge path) sized below the writer's frame fails to decompress *every* frame at that size -- total failure, not slow. - `zspill_stream`'s cap (consolidation path) was held equal to it by a comment reading "kept in sync", which is not enforcement. Consolidation runs only in the spill-heavy configurations, so a mismatch would pass the standard matrix and fail exactly where spill volume is largest. `test_frame_caps_admit_the_largest_frame_the_writer_emits` pins both against the writer, so the two read paths can no longer drift. The slack on the derived cap is **proportional (4x), not a fixed addend**, which the first version of this commit got wrong. A fixed `+ 4 MiB` left the default frame size looking untouched while inflating the per-worker zstd decompress buffer 16x, from 256 KiB to 4.06 MiB. That buffer is scratch touched once per decompressed frame -- 5,368,249 times on the measured cell -- so its size is a cache-locality parameter: the baseline arm measured 249.5s against a 199.2s known baseline (+25%) while peak RSS *fell* slightly, making it invisible to any memory check. It was caught only because the sweep re-measured the default frame size as a control instead of trusting the earlier run. At 4x with a 256 KiB floor the default evaluates to exactly the constant this replaced, so the default path really is unchanged; `test_default_frame_size_preserves_the_original_decompress_cap` pins that. The floor is needed because `BGZF_MAX_BLOCK_SIZE` is 65,280 rather than 65,536, so a bare 4x lands 1 KiB under the tuned value. Verified at a 1 MiB frame size on 721,681 records through 9 consolidation merges -- the path with the previously-stale cap: records in equals records out, and `sort --verify` PASSes with zero order violations. `PooledChunkWriter` pre-flushes the staging buffer when the next record would not fit, so a record never straddles a frame boundary -- which is what lets the merge borrow most records in place instead of reassembling them. That budget was `BGZF_MAX_BLOCK_SIZE` outright, a third copy of the threshold alongside the two zstd decompress caps. So it flushed at 64 KiB whatever the staging buffer was configured for, and the frame size introduced in the previous commit did nothing at all: 5,368,249 blocks measured at both 64 KiB and 256 KiB on an 89-way merge. The arm read as a plausible 4% regression (206.6s against 198.7s) rather than as a no-op, so nothing in the wall time revealed that the knob was inert -- only the `Spill compress [N blocks]` count did. Resolve the budget once per writer from the codec and store it, so the pre-flush and the staging flush cannot disagree, and pin it with `test_spill_writer_pre_flushes_at_the_staging_frame_size`. Verified: on 721,681 records the block count now scales as it should -- 5,700 blocks at 64 KiB against 360 at 1 MiB, a 15.8x cut -- with `sort --verify` PASSing at zero order violations and the record stream byte-identical to the 64 KiB output. Three duplicated copies of this threshold have now been found, each by a different failure: two decompress caps held together by a comment reading "kept in sync", and this one. All three now derive from `spill_frame_bytes` and each is pinned by a test.
Measured with paired controls in one session: -2.8% at t8 (199.1s mean of five against 193.5 / 193.8s) and -5.2% at t16 (178.9 -> 169.6s). Output compares IDENTICAL. The earlier sweep that set 2 MiB called 4 MiB a 3s gain and left the knee at 2 MiB; re-run against the same cell it is worth more than that, and the t16 half was never measured at all. Bounded from above at the same time, which is the more useful half. At 32 MiB the merge is *slower* than at 4 MiB (199.3s) and peak RSS goes 4793 -> 7562 MB, because raw-lock contention climbs with batch depth -- 29% -> 52% -> 38% of awaited-file skips as more workers collide on one file's raw_blocks mutex. Wall clock alone would have called that arm harmless, so the constant now carries the memory column beside the timing. Why it helps t16 more, which is not a scheduling improvement and should not be read as one: critical-path worker discovery lag halves (98.5 -> 59.8s) and finds arriving after 320us drop from 23% to 16%. Deeper read-ahead does not make recruiting a worker faster, it needs fewer recruitments -- so it pays most in the regime with the most idle capacity, where each recruitment is most expensive. That underlying cost is untouched and is measured separately. Two constants carry this target, not one: `PHASE2_STARVING_READ_TARGET_BYTES` and its `_USIZE` twin, added when #806 gained a byte bound on the raw FIFO. Both move together here. `PHASE2_STARVING_FIFO_BYTE_BUDGET` is derived from the target times `PHASE2_STARVING_CAP_MULTIPLE`, so the secondary guard scales with this change rather than silently clamping it -- 32 MiB becomes 64 MiB, and it still only bites when the frames actually queued run larger than the measured mean.
…ing for one
When the block the merge is waiting for has already been read and nobody has
claimed it, the consumer now decompresses it on its own thread rather than going
to sleep. That is 12% of parks by the park-supply census, and it trades ~53us of
work for a ~190us wait on a worker that has to be woken first.
The reason to remove the handoff rather than schedule around it is the cap sweep,
run at t16 with the park census:
cap merge parks park time decomp-capped
8 167.0s 978,325 89.6s 61%
32 170.9s 633,514 92.4s 34%
128 172.7s 400,487 89.7s 10%
Parks fell 2.4x and decomp-capped collapsed 61% -> 10%, so the cap really is the
prefetch limit -- and total park time did not move at all, while wall clock got
monotonically worse. A cost that survives a 16x change in buffer depth is not a
buffering problem, and eight scheduling interventions establish it is not a
signalling one either: awaited-source scan hint (0%), wake targeting (268,205
misdirected wakes removed, -0.9%), grab-N (+36%), priority reorder (0.06%), cap 32
(+2.3%), cap 128 (+3.4%). Divided out, the consumer's exposed latency is ~17us per
block and invariant, which is the shape of a producer-to-consumer handoff. The only
way to remove a handoff is for one thread to do both halves.
It loops while it keeps succeeding. The same argument applies to the blocks after
the one immediately needed, and serving them here is depth on precisely the file
being drained -- where >=94% of consumed blocks come from, a bound from the
measured run lengths (n=167,624 runs over 5,306,698 blocks, median 1 but mean 31.7
and p99 512). Uniform depth is what the cap sweep already showed does not work.
Only on the way into a park, never in place of merging: this thread is the serial
bottleneck at 76s of a 166s merge, so taking work it could have delegated while it
still has records to emit would be strictly worse.
Structurally it goes through the same try_pop_raw_for_decompress and
decompress_and_publish a worker uses, so the in-flight counter and the
reorder-buffer protocol cannot drift between the two paths, and the block is
published rather than consumed directly so serial ordering keeps exactly one
implementation. Sharing that path required lifting the three decompressor fields
off SortWorkerState into a DecompressorSet the consumer can own too; the zstd
scratch is still pre-allocated only for zstd spills, so a BGZF-only sort pays
nothing. The consumer holds no locks while parking, so it cannot join a lock cycle
with a worker.
consumer_self_served is the validity gate: near zero means the consumer never found
an unclaimed block and this is inert whatever the clock says.
`merge_progress.log_if_needed(1)` runs once per record, and ProgressTracker::log_if_needed does `count.fetch_add(additional, Relaxed)`. On aarch64 Rust defaults to outline-atomics, so that is not an inline LDADD but a call into __aarch64_ldadd8_relax, which runtime-checks for LSE and can fall back to an ldxr/stxr loop. A per-record atomic through a function call is an anti-pattern regardless of what it costs. So the counting is batched locally and forwarded every 4096 records. log_if_needed already takes an `additional` count, so the tracker's totals and milestones are unchanged; against a 1,000,000-record interval a milestone is noticed at most 0.4% late, and 99.98% of the atomic traffic disappears. All three per-record tick sites get it -- the pooled Phase 2 consumer, its non-pooled variant, and `fgumi merge`'s own loop -- since it is one anti-pattern in three places. flush() before log_final() is the one way this can corrupt a total, so it is called at all three sites and tested directly, including that it is idempotent. Measured on 1kg-wgs-HG00096, coordinate -> template-coordinate, --max-memory 512M --max-temp-files 1024, 779,820,469 records. At t8, where the merge consumer is the serial bottleneck at 99% busy, merge wall 190.8 -> 187.9s (-1.5%), control and arm in one boot. At t16, where the consumer still parks, 162.6 -> 162.9s -- a null, inside run-to-run spread. The win is real only where serial consumer CPU is what binds, which is the shipping configuration. A caution about the profile that motivated this, because it is the more useful result. perf put __aarch64_ldadd8_relax at 28.25% of consumer-thread self-time, ahead of LoserTree::replay at 11.44% and the record memcpy at 8.26%, which reads as ~40ns of a 145 ns/record budget over 776M records -- tens of seconds. Removing it recovered 1.2s at t16. `cycles:u` is not a precise event on aarch64, and samples skid onto whatever small, frequently-called function sits downstream of the real stall; a tiny leaf routine called once per record is the ideal skid target. Do not size a change on this profile's flat self-time without changing the code and re-measuring. Two further notes on reading it. The 7.57% against SortPhaseTimer::time_merge is NOT timing overhead: it wraps the entire merge in one call, and with -C strip=debuginfo perf attributes the inlined loop body to the enclosing symbol. And the ~29% across the ZSTD and HUF symbols is real, expected work -- the consumer decompresses blocks itself now -- which confirms that accounting rather than questioning it.
Every read-ahead signal the pool has is reactive. phase2_awaited_source is set only once the consumer has already parked, and the frontier is a proxy that is correct only while sources drain in index order. Neither can start a read early. The measurement says early is the only thing left. Of the residual 49.8s of park time at t16, 99% is `starved` -- nothing buffered, nothing in flight, nothing read at all -- while the disk runs at 323 of 1000 MB/s provisioned and workers sit 45% idle. And it is rare and expensive rather than steady: only 2,475 of 167,624 source switches starve, each costing ~20ms against an 8.4ms read latency. That is 2.4 read times, which is not a buffer that was too shallow; it is a read that had not been started when the consumer arrived. What moves this merge is which file gets depth, not how much depth exists. Raising the caps uniformly has been measured four times and never paid: PHASE2_RAW_CAP 16 and 32 moved merge wall 162.9 -> 162.6 -> 161.9s and left the starved share of park time at 99%, 99%, 99%; PHASE2_DECOMP_CAP 32 and 128 cost 2.3% and 3.4%; filling the cap on every claim cost 36%. Every gain in this stack instead targeted the one file the consumer was actually blocked on. A uniform floor cannot help here because `starved` means the FIFO is EMPTY -- the work had not been created yet -- and it cannot know which of K files to create it for. So the loser tree now names the source it will consume next, and the pool gives that file the same deep read allowance the drained file gets. Publication happens on source change rather than per record: 25,003,410 times over 779,820,469 records, one per ~31 records. At the 11.63 ns/call measured by the loser-tree benchmark that is ~0.29s of serial consumer time on a 156s merge -- 0.19%, on the thread that binds. LoserTree::runner_up walks the losers along the winner's root-to-leaf path, O(log k) over existing state, ~6 comparisons on a 44-way merge, no new bookkeeping. `losers[1]` alone is NOT the answer, which is the tempting O(1) shortcut: after a replay that node holds whoever lost the final comparison of that replay, not the global second-smallest. With keys [50, 100, 30, 20] the winner is source 3 and losers[1] is source 0 (key 50) while the true runner-up is source 2 (key 30). A test covers exactly that case and fails against the shortcut. Exhausted sources are never predicted -- a read for a drained file is pure waste -- and a wrong prediction costs one deep read on a file the merge reaches eventually. There is no correctness component: this only changes which file a worker reads ahead on. Measured by isolation rather than by comparing builds: the arm and its control are the same binary with `runner_up()` patched to return `None`, one boot, one host. 1kg-wgs-HG00096, coordinate -> template-coordinate, t16, --max-memory 512M --max-temp-files 1024, 44 spilled runs, 779,820,469 records. arm merge wall consumer park peak RSS prediction off 163.5s 50.9s 9.594 GB prediction on 156.0s 43.3s 11.038 GB -7.5s, **-4.6%**, and the entire gain is park: -7.6s of park for -7.5s of wall, which is the mechanism this targets and not something else. `compare bams --command sort` reports IDENTICAL on the full 779,820,469 records. **It costs 1.44 GB of peak RSS, +15%, and that is the whole of this branch's memory growth at t16** -- the prediction-off arm sits at 9.594 GB against an uninstrumented baseline's 9.572 GB. Phase 2 read-ahead is not charged against `--max-memory`, so this is real additional footprint: a third file now gets the deep allowance alongside the frontier and the awaited source. On a host sized for `--max-memory` alone that matters, and it scales with the number of spill files rather than with the memory budget. `phase2_predictions` is the validity gate, and it exists because this change has a silent failure mode: `runner_up()` returns None whenever fewer than two sources are active, so a version that returned None always would produce a plausible null wall-clock result with nothing in the log to say the mechanism never ran. The counter is printed beside the self-serve gate. A test covers that a published source is counted and that clearing the prediction is not. Also reconciles `record_source_run`'s documentation, which argued from the median run length of 1 block that "any fix framed as prefetch the file the consumer needs next is answering a question the workload does not pose". That reading is count-weighted. Weighted by blocks, at least 94% of blocks come from the top decile of runs, which is what predicted this result -- the same count-versus-time trap that sent an earlier change at get_sort_priorities for 0.06%. Both readings are now recorded there, with the bound that separates them.
Three scheduling changes, kept together because they share a result: each fixes something real, each is defensible on its own terms, and none of them moved wall clock. They are here for the code, not for the clock, and grouping them keeps that honest rather than letting three near-zero commits imply three wins. **Start a Phase 2 scan at the awaited source, not just the frontier.** The scan walked from a rotating cursor, so the file the consumer was actually blocked on was reached only after passing over every other one. Measured at 0%: the walk was never the cost, because the block was not read yet whatever order the scan visited files in. **Wake a parked worker, not the next one in rotation.** `wake_one_worker` targeted a rotating index without checking whether that worker was awake, so a wake aimed at a running worker did nothing and the consumer then waited for some other worker's backoff to expire -- bounded by `MAX_BACKOFF_US`, not by unpark cost. This took misdirected wakes from 268,205 to 30, and merge wall by 0.9%. The falsifier was run before believing it: at 8 threads the same counters look *worse* (100% of wakes hit a running worker, 0% recoverable) while the consumer waits 10us against 72us, because at 91% utilization there is nobody parked to hit. "Wakes land on busy workers" is what a healthy pool looks like, not a defect. Fallback to the plain rotation is preserved, and tested, so a saturated pool behaves exactly as before and the spread an interleaved merge needs is not lost. **Rank the parked consumer's file ahead of draining compression.** `get_sort_priorities` puts `Compress` ahead of `Phase2FileWork` whenever the compress queue is non-empty, and output compression is 68% of merge worker busy, so the one block the consumer is blocked on could wait behind output work. The inversion is real and the park census measured it at 13% of park time. Reordering it measured 0.06%. `consumer_parked` is a separate flag rather than a reuse of `phase2_awaited_source`, which is never cleared between parks and would therefore rank Phase 2 work first permanently, undoing the compress-first policy entirely. Why keep changes that measured nothing: each removes a wrong behaviour that would otherwise have to be re-derived by the next person reading these paths, and the counters that prove they are inert are themselves the evidence for where the cost actually is. The park census reading that motivated the third one is also the clearest instance of the count-versus-time trap recorded in this branch.
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Stacked on the instrumentation PR. The changes that moved wall clock, plus four pieces of hygiene found along the way — and an honest account of how much is left, because it turns out to be the more useful result.
Every number below is a paired control and arm in one boot on one host.
c7g.4xlarge,1kg-wgs-HG00096, coordinate → template-coordinate,--max-memory 512M --max-temp-files 1024 --temp-compression 1 --compression-level 1, 779,820,469 records.--max-memoryis per thread, so t8 spills 89 runs and t16 spills 44 — different merges, whose absolute walls are not comparable to each other.Result
Control is
main, which now contains #806.mainmainfgumi compare bams --command sortreports IDENTICAL on the full 779,820,469 records — which is what licenses reading these gaps as overhead removed rather than work skipped.These are marginal figures for this PR alone. An earlier draft of this work quoted −42.5% at t8; that was cumulative from before #806, which did the heavy lifting (326.5s → ~200s). This PR is the next 4.3% at t8 and 10.8% at t16.
What it costs
Peak RSS grows, and at t8 it nearly doubles: 4.813 → 9.344 GB (+94%). At t16 it is +10% (9.599 → 10.568 GB). Phase 2 read-ahead is not charged against
--max-memory, so this is real footprint on top of the budget, and it scales with the number of spill files rather than with the budget — t8 has 89 files against t16's 44, which is why the same change costs so much more there.At t16 the growth is attributable: disabling only the predictive read-ahead returns RSS to baseline (9.594 GB against an uninstrumented 9.572 GB, measured on #806's pre-merge head). So top-k is essentially all of the t16 growth — for −4.6% of wall. The t8 split is not isolated; if a reviewer wants it attributed before merge, say so and I will run it.
Anyone sizing a host on
--max-memoryalone should know this. It is the hazard the sort sweep's own documentation warns about: a tighter budget spills more runs, so the widest merges land on the hosts least able to absorb them.The one finding worth carrying forward
Targeted depth wins; uniform depth fails. Fifteen interventions were measured. Three paid, and all three gave depth to the single file the consumer was blocked on or about to reach:
Everything that spread depth uniformly failed: decompress cap 32/128 (+2.3%/+3.4%), raw cap 16/32 (~0%/−0.6%), filling the cap on every claim (+36%). Everything that tried to signal workers better also failed: wake targeting (−0.9%), priority reorder (0.06%), scan-start redirect (0%).
Both failure modes share one cause. 98-99% of residual park time is
starved— nothing buffered, nothing in flight, nothing read at all — while the device runs at 323 of 1000 MB/s provisioned. Uniform depth offers somewhere to put work that does not exist yet; better signalling wakes a worker to do work that does not exist yet. Only naming which file to start reading changes anything.Headroom left, stated plainly
This is the part I would want to read first as a reviewer, and it is not flattering to the PR.
At t8 the merge is now at its floor — but the floor moved. Merge 189.8s = consumer serial CPU 187.8s (98.9%) + park 1.4s, with a worker floor of 1410.5s ÷ 8 = 176.3s. Recoverable without doing less work: 2.0s, 1.1%.
That is worth reading carefully, because it is not the same as "we captured the available headroom". On
mainat t8 the merge was worker-bound: loop 197.2s against a 175.4s worker floor, 21.8s recoverable. This PR lowered the loop by 7.4s and simultaneously raised the consumer floor from 75.4s to 187.8s, because self-decompression moves work onto the serial thread. The merge is now consumer-bound where it was worker-bound. ~13.5s (7%) sits between the current 189.8s and the 176.3s worker floor, and it is reachable only by giving that decompression back to the workers without reintroducing the park. That is the clearest next target this work produces.At t16, 44.7s (28%) is recoverable, all of it consumer park, 99%
starved, with workers 43% idle and a worker floor of only 89.8s. That is the largest identified prize. But a control says most of it is a property of the input shape: the same records sorted queryname → coordinate merge in 126.7s with the consumer 97% busy and park ~3s. Sorting an already-SO:coordinatefile into template-coordinate is what produces the starving run structure — and that is also the production path for dedup, so it is worth attacking; it is just not general engine headroom.And the number that actually binds has never been decomposed. Consumer serial CPU is the floor in all three configurations — 186.5s at t8 (239 ns/record), 112.1s at t16 (144 ns/record), 123.2s on the queryname→coordinate control (158 ns/record). Same records, same block count, near-identical worker-side decompress, and a 66% spread in per-record cost that is unexplained. A loser-tree microbenchmark accounts for ~5% of it. The other ~95% is unattributed, and it is larger than everything this PR delivers.
This PR also demonstrates why: at t8 it removed 119.4s of park and gained 8.2s of wall, because park converted almost 1:1 into consumer serial CPU (74.6 → 186.5s). The work was not removed, it was relocated onto the critical path. That is a real win where the consumer had idle capacity and nearly a no-op where it did not.
Reading order
The three wins:
3d08ea8d(read-ahead 4 MiB),8b66d5d1(consumer self-decompress),5f52205c(top-k prediction).dca835cfbatches a per-record atomic off the consumer path (−1.5% at t8, null at t16).Hygiene, near-zero on wall, kept for correctness or clarity:
ff7824admakes the zstd spill frame size a knob and fixes a latent bug it exposed — the spill writer pre-flushed at a hardcoded 64 KiB rather than the configured frame size, so the knob was inert and a frame-size sweep measured a no-op as a 4% regression.bc0ab690squashes three scheduling tidy-ups that measured 0%, −0.9% and 0.06%, kept because each removes a wrong behaviour the next reader would otherwise re-derive.Notes for review
losers[1]is not the runner-up, which is the tempting O(1) shortcut in the top-k commit. After a replay that node holds whoever lost that replay's final comparison, not the global second-smallest.runner_up()walks the losers along the winner's root-to-leaf path instead, O(log k). There is a test on exactly the case the shortcut gets wrong, and it fails against the shortcut.runner_up()is called 25,003,410 times, once per ~31 records, not once per run as an earlier draft claimed. At 11.63 ns/call that is ~0.29s of serial consumer time — 0.19% of merge wall, so the design holds, but with less margin than "once per run" implied.Consumer served itselfandNext-source predictions publishedeach guard a silent failure mode: the mechanism can fail to run at all and still report a plausible wall time. During this work the gate function was briefly defined but never called, so both were dead-code eliminated — and neitherclippy -D warningsnor rustc'sdead_codereported it. Grepping the built binary is what caught it.Verification
cargo ci-fmt,ci-lintclean;cargo ci-test7559 passed / 30 skipped; every commit builds independently. Correctness gated oncompare bams --command sortthroughout — IDENTICAL every time, 0 sort-order violations, 0 run-multiset mismatches.Risk: output changes: none; identical sorted output and sort-order checks pin behavior.
unsafechanges: none; CLAUDE.md allowlist update: none. Memory and scheduling policy changes: yes; targeted read-ahead increases peak RSS outside--max-memory.Fix: steer read-ahead to blocked and predicted merge sources, and let the consumer decompress queued blocks before parking.
Merge wall time decreases by 9.7% at t16 and 3.8% at t8. Peak RSS increases by 10% at t16 and 94% at t8. Correctness, formatting, lint, and CI checks pass.