feat(cache): make contiguous evictor unlink concurrency and sweep bound configurable - #909
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…nd configurable The chunk evictor already derives its unlink fan-out from the thread pool (CHUNK_DATA_CACHE_INDEX_UNLINK_CONCURRENCY, max(1, UV_THREADPOOL_SIZE/8)), with a comment explaining that a hard-coded 50 "takes the entire pool on a stock node and queues every chunk read behind it -- on a device that is, by definition, already saturated when the evictor is running." The contiguous evictor still hard-codes both UNLINK_CONCURRENCY = 50 and MAX_BATCHES_PER_SWEEP = 50, so with the default batch size of 1000 a single 60s sweep can issue up to 50,000 unlinks at 50-way concurrency, and an operator has no way to pace it. Measured on a production gateway (20 TB btrfs cache on HDD, 88% full, UV_THREADPOOL_SIZE=64): during eviction bursts of 6,000-39,000 deletions per 10 minutes the disk sits at 100% utilisation and in-flight libuv requests reach a median of 3,376 (peak 24,483) against the 64-thread pool, versus 43 outside those windows. Everything file-backed queues behind it, including CDB64 root-tx index lookups on a separate NVMe device, which then hit their 60s circuit-breaker timeout. All 14 of the highest in-flight slots over 48h coincided with eviction bursts; public request volume, chunk ingest and the filesystem-walk cleanup worker showed no correlation. Adds, mirroring the chunk evictor: - CONTIGUOUS_DATA_CACHE_INDEX_UNLINK_CONCURRENCY, default max(1, UV_THREADPOOL_SIZE/8) - CONTIGUOUS_DATA_CACHE_INDEX_MAX_BATCHES_PER_SWEEP, default 50 (unchanged) The concurrency default drops from 50 to 8 on a 64-thread pool (1 on a stock 4-thread node), which is the behaviour change here: the same work is done per sweep, just without occupying the whole pool at once. Tests: unlink fan-out never exceeds the configured limit; a sweep stops at batchSize * maxBatchesPerSweep instead of draining the index. Co-Authored-By: Claude Opus 5 <noreply@anthropic.com> Claude-Session: https://claude.ai/code/session_012NWDKc9pST69qTEha4AGaB
📝 WalkthroughWalkthroughThe contiguous data cache evictor now supports configurable unlink concurrency and per-sweep batch limits. New environment variables provide defaults. Tests verify both limits, and the environment documentation describes the controls. ChangesCache eviction limits
Priority: ⬇️ Low Estimated code review effort: 2 (Simple) | ~10 minutes Change: Feature Merge Risk: 🟡 Moderate · up to Invalid explicit eviction limits can leave cache blobs unlinked or remove intended work bounds, so validation should be added before merge. The environment documentation should also state the integer rounding used by the default. 🚥 Pre-merge checks | ✅ 5✅ Passed checks (5 passed)
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Inline comments:
In `@docs/envs.md`:
- Line 240: Update the default description for
CONTIGUOUS_DATA_CACHE_INDEX_UNLINK_CONCURRENCY to explicitly state that
UV_THREADPOOL_SIZE/8 is floored before applying the minimum of 1, preserving the
documented positive-integer behavior.
In `@src/workers/contiguous-data-cache-evictor.ts`:
- Around line 78-79: Validate unlinkConcurrency and maxBatchesPerSweep in the
constructor before assignment, rejecting non-integer or non-positive values with
clear errors; assign valid values unchanged instead of clamping them via
Math.max.
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Addresses CodeRabbit's review of the constructor options. Math.max(1, x) does not guard the values that matter. NaN clamps to NaN, so `batch < this.maxBatchesPerSweep` is false and a sweep evicts nothing while the cache keeps filling. Infinity removes the sweep bound entirely. A fractional unlinkConcurrency is rejected by p-limit mid-sweep -- after the index rows are deleted but before the blobs are unlinked, leaving orphans for the reconciler to find. All three fail silently or half-way through, which is the worst place for a configuration error to surface. Explicit limits are now validated as positive integers and throw at construction. Env-supplied values already go through positiveIntOrDefault, so this only affects direct callers passing bad values. Also documents that the derived default floors the division, per the same review: UV_THREADPOOL_SIZE=15 yields 1, not 1.875. Tests: NaN, Infinity, 0, -1 and 2.5 are each rejected for both options. Co-Authored-By: Claude Opus 5 <noreply@anthropic.com> Claude-Session: https://claude.ai/code/session_012NWDKc9pST69qTEha4AGaB
First production data pointThe gateway running the stopgap (
Disk drained 88.0% → 86.98% during the sweep, so the smaller batch still keeps up with pressure — that was my main worry about lowering the fan-out, and at least on this workload it is not a problem. The sibling gateway, left on the defaults, evicted nothing in the same window (it is at 84.6%, below its watermark), so it is not a controlled comparison yet. What this is not: proof. It is a single sweep, and the 72h history shows sweep size alone does not determine whether the pool floods — there are sweeps of 75,000 evictions with 52 in-flight requests and no timeouts, presumably because little else was reading at the time. I will report again when both nodes have swept under comparable load, and will post it here even if it shows no difference. Worth noting this PR's default ( |
Correction to my evidence above — the causal claim does not hold upTwo errors of mine, both against this PR's own argument. 1. The disk figure in my previous comment was the wrong volume. I quoted "disk drained 88.0% → 86.98% during the sweep" from 2. More importantly: eviction bursts are not sufficient to flood the pool. Both nodes have now swept under normal load, and neither showed any of the behaviour I attributed to eviction:
A 56k-eviction sweep on the unmodified node did nothing at all. That is the configuration this PR changes, behaving perfectly. So the correlation in the PR description — every one of the 14 highest in-flight windows coincided with an eviction burst — was real, but I over-read it as causation. The 72h history contains the counter-examples too (75,000 evictions with 52 in-flight); I noted them and still drew the strong conclusion. Eviction is at most a contributing factor to the stalls we see, and what actually distinguishes a harmful window from a harmless one is still unexplained on our side. What I think still stands, on design grounds rather than my measurements:
I have edited the PR description to match this. If you would rather see this land as configurability only, leaving the default at 50, I am happy to make that change — the default is the part my evidence no longer supports. |
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Why
The chunk evictor already derives its unlink fan-out from the thread pool, and the comment on it says why:
The contiguous evictor never got the same treatment. It still hard-codes
UNLINK_CONCURRENCY = 50andMAX_BATCHES_PER_SWEEP = 50, so with the defaultCONTIGUOUS_DATA_CACHE_INDEX_EVICTION_BATCH_SIZE=1000a single 60 s sweep can issue up to 50,000 unlinks at 50-way concurrency, and an operator has no way to pace it.What we measured, and what it turned out not to show
Production gateway, 20 TB btrfs contiguous cache on a spinning disk at 88% full,
UV_THREADPOOL_SIZE=64:All 14 of the highest in-flight slots over 48h coincided with eviction bursts of 6,000–39,000
deletions, with the disk at 100% utilisation. Public request volume (r = −0.00), chunk ingest
(−0.03) and the filesystem-walk cleanup worker (+0.04) showed no correlation. But the same window
also contains sweeps of 75,000 evictions with 52 in-flight requests and no timeouts, so eviction
clearly is not the whole story — what separates a harmful sweep from a harmless one is still
unexplained.
Raising
UV_THREADPOOL_SIZEis still not the answer: libuv's pool is a single FIFO queue, so workqueued behind a backlog waits regardless of thread count.
What this changes
Two new settings, mirroring the chunk evictor exactly:
CONTIGUOUS_DATA_CACHE_INDEX_UNLINK_CONCURRENCYmax(1, UV_THREADPOOL_SIZE / 8)CONTIGUOUS_DATA_CACHE_INDEX_MAX_BATCHES_PER_SWEEPThe behaviour change is the concurrency default: 50 → 8 on a 64-thread pool, or 1 on a stock 4-thread node. The same work happens per sweep; it just no longer occupies the whole pool at once. Both are constructor options too, so the evictor stays testable without env.
Documented in
docs/envs.md.Testing
yarn test:file src/workers/contiguous-data-cache-evictor.test.ts— 6 pass, 2 new:delete, asserts peak concurrency)batchSize * maxBatchesPerSweeprather than draining the indexyarn test:file src/workers/chunk-data-cache-evictor.test.ts— 15 pass (sibling untouched).yarn lint:checkclean;tsc --noEmitdiffed against an unmodified tree, no new errors.Operationally:
CONTIGUOUS_DATA_CACHE_INDEX_EVICTION_BATCH_SIZE=200is live on one of our two gateways, with the other unchanged as a control. Both have now swept without incident, which is what prompted the correction above.So this PR should be judged on design grounds, not on my incident. The case for it is the one already made in the codebase for the sibling chunk evictor: a hard-coded fan-out of 50 unlinks takes the entire thread pool on a stock node, and an operator currently has no way to pace the contiguous evictor at all. If you would prefer configurability without the default change, say so and I will adjust.
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