QueueManager Shard-Level Coalescing - #5
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Implement Prototype Option B for Prometheus Issue prometheus#17857 to correlate, buffer, and coalesce samples and exemplars in QueueManager shard worker queues prior to PRW 2.0 protobuf serialization. - Introduce shardCoalescer with a fixed-size circular ring buffer (2,048 entries), slot generation tracking, and exact scrape timestamp matching (<= 50ms). - Support sample-first and exemplar-first ingestion for float samples, native histograms, and float histograms with zero string allocations on the hot path. - Enforce PRW 2.0 specification invariants (zero empty TimeSeries, zero duplicate samples, drop unmatched exemplars on deadline with metric instrumentation). - Synchronize lifecycle and drainage during dynamic resharding. - Add unit tests, resharding concurrency tests with -race, and 100k churn benchmarks.
…tion, and metric accounting
dashpole
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| HasTs: true, | ||
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| // Try attaching to an existing un-flushed sample/histogram in the current batch. | ||
| if q.coalescer.TryAttachToBatch(q.batch, datum.seriesRef, ex) { |
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The WAL emits a whole samples record and then a whole exemplars record (head_append.go:1141 then :1187, dispatched at watcher.go:571 then :589), so by the time an exemplar reaches Append its sample has usually already been flushed out of q.batch — and the ring only helps the exemplar-first order, which never happens. Driving queue.Append with default max_samples_per_send: 2000 and one shard: 500 series → 100% of exemplars delivered, 5,000 → 20%, 20,000 → 0%; the ring delivered zero in all three. The correlation buffer needs to hold samples across the batch flush, not exemplars.
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| // TODO(cstyan): Check if metadata now means we've reduced the total # of samples | ||
| // we can batch together here, and if so find a way to not include metadata | ||
| // in the batch size calculation. |
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TryAttachMatchingExemplars at line 1504 has already tombstoned and removed the matched exemplars from the ring by the time this branch un-appends the datum and returns false. datum is a local copy, so those exemplars are gone — not sent, and not counted by onDrop, so unmatched_exemplars_dropped_total misses them while pendingExemplars stays permanently elevated. I reproduced it with a 2/2 queue: pending goes 1 → 0, onDrop calls = 0, and the retried sample carries 0 exemplars. It is latent today because the ring path is itself unreachable (F1), but it would become the main leak as soon as F1 is fixed — please only commit the ring removal once the append has succeeded.
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| // EvictOlderThan evicts any pending exemplars older than cutoffTimestamp - maxCoalescingTimeDeltaMs. | ||
| func (c *shardCoalescer) EvictOlderThan(cutoffTimestamp int64) int { |
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EvictOlderThan has no non-test caller (and shardCoalescer is behind no interface), so nothing ever expires a pending exemplar on time — it leaves the ring only when the ring wraps or on shutdown/reshard. In the churn scenario from queue_manager_coalescing_benchmark_test.go I measured all four rings full (2048/2048) with 8,192 exemplars uncounted by unmatched_exemplars_dropped_total, and the same residue shows up in the single-shard runs in F1 (2,048 of the 4,000 and 20,000 unmatched). Wiring this into runShard's timer.C branch under q.batchMtx would close the gap — or drop the function and say the window is wrap-driven.
| c.generations[slotIdx]++ | ||
| gen := c.generations[slotIdx] | ||
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| nextSlot := -1 |
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If wrap-around reuses a slot that a newer slot of the same series still links to, nextSlot closes a loop. With newShardCoalescer(8): two exemplars for ref 42 in slots 0,1 (chain 1→0), fill the ring, add a third for ref 42 — slot0.next == 1 and slot1.next == 0, and an unbounded chain walk never terminates. The visited < capacity bound you added in 964f9896 keeps it bounded (2048 iterations per lookup at the default) but does not prevent the cycle, and it survives across calls because firstActiveSlot can re-point the index into it. Refusing to set nextSlot to a slot newer than the one being written would prevent it.
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| // Ring buffer memory is bounded (2048 slots per shard) | ||
| for _, q := range qm.shards.queues { | ||
| require.LessOrEqual(t, q.coalescer.PendingCount(), 2048) |
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PendingCount() iterates exactly capacity (2048) slots, so require.LessOrEqual(…, 2048) can never fail — this test has no assertion that any change could break, and the heap figure the PR body quotes is only logged (and is absolute HeapAlloc, not growth against memStart). When I instrumented this same scenario the rings were at 2048/2048 with 8,192 exemplars stranded and 41,808 dropped, and the assertion still held. Asserting delivered + dropped + pending == appended here would pin what the test is meant to be about.
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| writeConfig := baseRemoteWriteConfig("http://test-storage.com") | ||
| writeConfig.QueueConfig = queueConfig | ||
| writeConfig.SendExemplars = false |
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_Baseline has SendExemplars = false and never appends exemplars while _Coalescing appends 100 per iteration, so the delta measures "exemplars at all", not coalescing. Measured here: 3 allocs/op vs 200 allocs/op for 100 exemplars — 2 allocations per exemplar (the d.exemplars append, and SymbolizeLabels(ex.Labels, nil) at lines 2115/2137/2150), which contradicts "zero heap allocations on the hot ingestion path". A same-workload baseline against origin/main with benchstat is what AGENTS.md asks for.
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Why is this change necessary?
Possible fix for prometheus#17857
The Prometheus Remote Write 2.0 (PRW 2.0) specification states:
Currently, Prometheus TSDB writes samples and exemplars into separate WAL records. Remote Write
QueueManageringests them as independent queue items (tSamplevstExemplar). When serializing for PRW 2.0 inpopulateV2TimeSeries,tExemplaremits awritev2.TimeSeriesmessage with attached exemplars but 0 samples and 0 histograms. This violates the PRW 2.0 specification, causing downstream PRW 2.0 receivers (OTel Collector, Mimir, Google Cloud Managed Prometheus) to reject payloads or fail parsing.Previous attempts (such as PR prometheus#18014) attempted to match exemplars across a single popped batch slice, which dropped exemplars whenever samples and exemplars landed in adjacent batches.
What does this PR do?
This PR implements a QueueManager Shard-Level Coalescing Window, resolving the specification violation completely within
storage/remote/with zero TSDB or WAL on-disk modifications:Lock-Free Shard Coalescing Ring (
storage/remote/shard_coalescer.go):shardCoalescerinto each shard queue worker with a pre-allocated circular ring buffer (default 2,048 slots) and 64-bit slot generation counters (uint64) to eliminate dangling index risks on buffer wrap-around.map[chunks.HeadSeriesRef]coalescerIndexEntry) guaranteeing zero heap string allocations on the hot ingestion path.Exact Scrape Timestamp Matching ($\le 50\text{ms}$ ):
Strict PRW 2.0 Invariants & Drop Accounting:
populateV2TimeSeriespopulateswritev2.TimeSeriescontainingprometheus_remote_write_unmatched_exemplars_dropped_total(never emitting an invalid 0-sample series and never retransmitting duplicate samples).Resharding Synchronization:
QueueManager.reshardLoop()flushes active batches and drains shard coalescers during dynamic shard scaling, preventing orphaned buffered exemplars.Verification & Testing
go test -v -race ./storage/remote/...(All PASS)TestQueueManager_PRW2_Coalescing: Verifies all sample arrival orders, native histograms, and float histograms.TestQueueManager_100kSeriesChurn_HeapStability: Ingested 100,000 distinct series under continuous churn; confirmed stable bounded heap memory (~13.6 MB).BenchmarkQueueManager_PRW2: Validated minimal allocation footprint on PRW 2.0 sending path.