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Raja Subramanian
345bc5eeb0
* Move ForwardStats aggregation off the packet forwarding path ForwardStats is a process-wide singleton and its Update runs for every forwarded packet. It previously took a shared mutex, updated a windowed aggregate, and observed into a global Prometheus histogram on every call. Update now only buffers the transit sample into a sharded lock-free ring (one atomic add to reserve a slot, one atomic store to publish). A background worker drains the ring every summary interval, observes each sample into the histogram (per-packet fidelity retained) and folds the interval summary into a window ring for the latency/jitter gauges. Under sustained overload the oldest excess samples are dropped and counted, and the count is logged. Ring slots are atomic.Int64 so producer store / consumer load are synchronized (race-clean). Capacity is numShards*shardCap = 131072 samples; at the default 50ms summary interval that sustains ~2.6M samples/s before dropping. Benchmark: BenchmarkForwardStatsUpdate (0 allocs/op both), Update per call: cores before after speedup 1 ~8.6 ns ~1.5 ns ~6x 8 ~175 ns ~22 ns ~8x The before path (mutex + windowed Welford aggregate + per-packet histogram observe) degrades ~20x from 1 to 8 cores under contention; the after path does not block and stays an order of magnitude lower under load. Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com> * Fix ring publish race and report-path double-flush Addresses two review findings on the forward-stats sample buffer. 1. Publish race (reserve-before-store): push reserved a ring slot by advancing writeIdx and stored the value afterwards, so drain could read a slot the producer had reserved but not yet written, getting a stale/zero value; the producer's later store then landed behind the read cursor and was lost. Each slot now carries a publish epoch. push stores the value and then publishes seq = index+1. drain reads a slot only when seq == r+1; a reserved-but-unpublished slot at the cursor stops the drain and is picked up on the next call, so no sample is read stale or lost. A slot overwritten during the read is detected on re-check and counted as dropped. The windowed latency/jitter stats did not permanently drift even before this change (report recomputes from a fixed-size ring that overwrites, not discounts, old buckets, so any error aged out within the report window), but these values feed the capacity manager, so the buffer is made exact. 2. Report-path double-flush: run() flushed on both the summary tick and the report tick, and every flush advances the window ring, so the ring cycled faster than intended and the effective window was shorter than configured (~5% at 50ms/1s/1m). The report tick no longer flushes; the summary ticker keeps the ring current to within one summary interval. Benchmark, Update per call (0 allocs/op), 1 and 8 cores: after fixes: ~5.5 ns / ~29 ns before fixes (this branch): ~1.5 ns / ~22 ns baseline (mutex + per-packet histogram): ~8.6 ns / ~175 ns The publish epoch adds one atomic store to push; the path stays non-blocking, zero-allocation, and well below the baseline under contention. Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com> --------- Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>