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feat(bench): record relay worker lock waits with perf - #5277
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Co-Authored-By: GPT-6 <noreply@openai.com>
Co-Authored-By: GPT-6 <noreply@openai.com>
Co-Authored-By: GPT-6 Astra <noreply@openai.com>
Co-Authored-By: GPT-6 Astra <noreply@openai.com>
# Conflicts: # .github/justfile # bench/README.md
Replace the bpftrace/perf capability probe with a capture path that works on hosts with perf_event_paranoid=4 and unprivileged BPF disabled, and in CI: the relay starts with PR_SET_PTRACER naming the harness, eu-stack samples worker stacks at 20 Hz, and addr2line symbolizes once with inlined frames. `just bench-lock` pairs each profiled run with an unprofiled control and nightly smoke-tests it. Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
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| if not any(re.search(r'\bmoq_|\bkio::', frame) for sample in samples for tid in names for frame in sample.get(tid, [])): | ||
| raise ValueError('worker stacks contain no resolved MoQ frames; build with the profiling profile') |
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When eu-stack returns the accepted partial-backtrace status for only one worker, another worker's resolved MoQ frame satisfies this process-wide any, so the unresolved worker remains in the result and all its samples are classified as non-mutex waits. That silently underreports contention instead of refusing the corrupted profile; require at least one resolved MoQ/kio frame for each worker TID before reducing its samples.
AGENTS.md reference: AGENTS.md:L17-L17
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Agreed and fixed in 1b9ad55: each worker now needs at least one resolved MoQ/kio frame across its own samples, with a regression test where a resolved worker sits beside an unresolved one.
(Written by Claude Opus 5.5)
One worker's resolved stack no longer vouches for another whose unwinding failed, which would have silently reported zero contention for it. Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Replace the eu-stack sampler, whose ptrace stops likely inflated the waits it measured, with perf: sched:sched_switch stacks plus switch-in events give each worker an exact off-CPU interval, classified by std's Mutex::lock_contended futex path. User stacks unwind by DWARF because glibc's frameless syscall wrapper hides lock_contended from frame-pointer unwinding. The recipe refuses with the exact sysctl and tracefs commands until the kernel allows the capture; nightly sets them with sudo. Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
kixelated
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Sample the context-switches software event with a period of 1 instead of the sched:sched_switch tracepoint. It records the same switch-out stacks but needs only perf_event_paranoid <= 1, not a tracefs remount or kptr_restrict=0. DWARF unwinding makes the frame-pointer build flag unnecessary. Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
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WalkthroughThe change adds Linux perf-based lock-wait profiling for relay benchmarks. The profiler parses switch events, pairs worker switch-outs with switch-ins, classifies waits, and writes worker and mutex-stack results. The benchmark runner adds a configurable lock-profiling mode with control runs. Documentation, tests, development-shell packages, and a nightly smoke test are also updated. Priority: ⬇️ Low Merge Risk: 🟡 Moderate · up to
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| Check name | Status | Explanation | Resolution |
|---|---|---|---|
| Docstring Coverage |  |
Docstring coverage is 26.92% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 26 functions across 5 files. (7 skipped: … | Write docstrings for the functions missing them to satisfy the coverage threshold. |
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Check name Status Explanation Linked Issues check Check skipped because no linked issues were found for this pull request. Out of Scope Changes check Check skipped because no linked issues were found for this pull request. Title check The title clearly and concisely describes the primary change: recording relay worker lock waits with perf. Description check The description directly explains the profiling problem, implementation, validation, outputs, and impact of the changeset.
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Docstring coverage is 26.92% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 26 functions across 5 files. (7 skipped: 7 unsupported.)
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| script = subprocess.run(['perf', 'script', '-i', str(data), '--show-switch-events', '--inline', '--ns', | ||
| '-F', 'tid,time,event,ip,sym,dso'], capture_output=True, text=True, check=True, | ||
| env=dict(os.environ, DEBUGINFOD_URLS='')) |
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When the period-1 DWARF samples overflow perf's ring buffer, especially at the default 16-worker setting, this invocation neither exposes nor rejects PERF_RECORD_LOST; it also captures and ignores diagnostics, so reduce can publish incomplete wait totals as valid results. The upstream perf-script documentation states that --show-lost-events displays these records and that debug mode checks for lost events, so the capture must detect them and fail instead of silently underreporting contention.
AGENTS.md reference: AGENTS.md:L17-L17
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Agreed, and the default 16-worker setting really did drop events: about 2% of samples on fanout-heavy with the default 512 KiB buffer. Fixed in 3f02d0a:
perf scriptnow runs with--show-lost-events, and anyPERF_RECORD_LOST*record refuses the result. A regression test covers it.-m 2Mstops the drops at 16 workers. That exceeds the unprivileged locked-memory budget, so the setup check now requiresperf_event_paranoid=-1.
(Written by Claude Opus 5.5)
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| def waits(records): | ||
| """Pair each switch-out stack with the thread's next switch-in for an exact off-CPU duration.""" | ||
| end = max((record[1] for record in records), default=0) |
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When a worker switches out near the end of the capture and does not switch back in, end is merely the timestamp of the last recorded event, not the end of the five-second window. If that switch-out is the final event, its duration becomes zero; if another event occurs later, only the interval up to that unrelated event is counted. This makes the supposedly exact off-CPU totals silently omit the tail of open mutex waits, so the actual recording boundary must be carried into waits.
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Agreed, fixed in 3f02d0a. perf now records with -k monotonic, and waits takes the window end, which is read from the same clock right after perf record exits, at the same moment as the closing schedstat read. An open wait now closes there, even when its switch-out is the last event. The regression test covers that case.
(Written by Claude Opus 5.5)
| if [[ -n $PROFILE_PID ]]; then | ||
| kill "$PROFILE_PID" 2>/dev/null || true | ||
| wait "$PROFILE_PID" 2>/dev/null || true |
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When the benchmark is cancelled or another child fails while profiling is active, PROFILE_PID identifies only the Python wrapper. Killing it does not terminate the perf record process started by subprocess.run or perf's sleep workload, so those processes remain orphaned and can continue writing the retained artifact after cleanup reports completion. Start the profiler in an owned process group or make the Python wrapper terminate and reap its child on signals.
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Agreed, fixed in 3f02d0a. relay.sh starts the profiler with setsid, and cleanup kills the whole group with kill -- -$PROFILE_PID. A background job of a non-interactive script is not a group leader, so setsid execs without forking and the PID is the group ID. I checked locally that the group kill also takes down the wrapper's grandchild.
(Written by Claude Opus 5.5)
…lved Address review: refuse a recording with PERF_RECORD_LOST, close open waits at the window end on a monotonic clock, and run the profiler in its own process group so cleanup stops perf. A 2 MiB ring buffer stops drops at 16 workers, which needs perf_event_paranoid=-1. Restore frame pointers: without them perf's DWARF unwinder stops at the first relay frame. Report workers asleep for the whole window as idle instead of refusing. Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Merge summaryRecords exact per-worker relay off-CPU time with Decisions (maintainer-confirmed)
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Review comments at @bench/profile.py:
- Line 120: Synchronize the schedstat reads in the profiling flow with the perf
capture window: use perf’s control interface to start capture before taking the
initial CPU counters and stop capture before taking the final counters. Keep the
before/after measurements aligned so CPU activity outside the recording is
excluded.
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| raise ValueError(f'PID {pid} has no moq-uring-N or moq-quic-N workers') | ||
| time.sleep(delay) | ||
| data = directory / 'perf.data' | ||
| before, start = schedstat(pid, names), time.monotonic_ns() |
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🎯 Functional Correctness | 🟠 Major | 🏗️ Heavy lift
Align CPU counters with the perf capture window.
Line 120 reads schedstat before perf record starts; Line 128 reads it after perf record exits. If a worker runs during perf setup and then sleeps throughout capture, reduce() rejects a valid recording because its CPU counter advanced but it has no recorded switch-out. The extra CPU time also lowers the reported wait/CPU ratio. Synchronize counter reads with capture start and stop; perf provides a control interface for coordinated measurement. (man7.org)
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Review comment at @bench/profile.py at line 120:
Synchronize the schedstat reads in the profiling flow with the perf capture
window: use perf’s control interface to start capture before taking the initial
CPU counters and stop capture before taking the final counters. Keep the
before/after measurements aligned so CPU activity outside the recording is
excluded.
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review. Visit https://docs.coderabbit.ai/cli?utm_source=ghpr
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This is real, and the PR already lists it as a follow-up: perf records no switch-out samples for its first ~115 ms, while schedstat brackets the whole perf run. That biases wait/CPU about 2% low on the 5 s window.
I'm leaving it out of this PR. The planned fix is to derive CPU from the recorded switch-in/out intervals and drop schedstat entirely, rather than add perf's control FIFO to gate both reads. That removes the second clock and also closes the edge case you describe.
That edge case makes the harness refuse ("ran but never switched out") rather than publish a wrong number. It also can't hit the nightly fanout-heavy 4 run, where every worker is busy throughout.
(Written by Claude Opus 5.5)
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@kixelated Thanks for the clarification. Deriving CPU time from recorded switch-in/out intervals avoids the separate schedstat measurement window.
The edge case causes a refusal, not an incorrect report. Your explanation also limits the risk for nightly’s busy fanout-heavy 4 run. Deferring the fix is reasonable.
Would you like me to open a follow-up issue for the interval-based CPU calculation?
✏️ Learnings added
Learnt from: kixelated
Repo: moq-dev/moq PR: 5277
File: bench/profile.py:120-120
Timestamp: 2026-10-13T04:29:18.394Z
Learning: For Linux relay lock-wait profiling in bench/profile.py, the maintainer prefers deriving worker CPU time from recorded perf switch-in/out intervals and removing schedstat, rather than adding perf control FIFOs to coordinate schedstat reads.
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Problem
Relay lock-wait profiling needs exact per-worker off-CPU time and resolved stacks, so the kio channel contention quest can name the contended channels. Earlier drafts did not provide this. The first only probed bpftrace and perf. The second sampled with eu-stack under ptrace, and those stops likely inflated the waits it measured: it read 0.15 wait/CPU on fanout-heavy at 4 workers, while exact intervals read 0.0013.
Approach
just bench-lock [WORKLOAD=fanout] [WORKERS="1 4 16"](bench/run.sh --lock) records with perf, as the maintainer chose.profile.py checkruns before the build. It refuses unlesskernel.perf_event_paranoid=-1, and prints the fix:sudo sysctl -w kernel.perf_event_paranoid=-1. There is no fallback.[profile.profiling]and frame pointers, and runs through the existingrelay.shlifecycle on Tokio workers. Each worker count gets an unprofiled control run, then a profiled run.perf record -e context-switches -c 1 --call-graph dwarf --switch-events -k monotonic -m 2M -p RELAY -- sleep 5:--switch-eventssupplies the switch-in. The sample fires in the outgoing thread, so-pnever sees it otherwise./procschedstat supplies CPU.perf script --inline --ns --show-lost-eventssymbolizes the recording, demangling Rust v0 and expanding inlined frames. Any dropped event refuses the result.DEBUGINFOD_URLSis cleared because perf otherwise blocks on debuginfod lookups.Mutex::lock_contended. Allocator (__lll_lock_wait_private), condvar and parking waits are counted separately, and spinning is excluded.lock/lock.json(per-workermutex_wait_ns,other_futex_wait_ns,other_off_cpu_ns,cpu_ns,mutex_wait_per_cpu, plusidle),lock/mutex.folded(flamegraph folded format, weighted by µs) andperf.data.setsid), so cleanup also stops perf and its workload.PR_SET_PTRACERplumbing are deleted.pkgs.perf(Linux only) replaceselfutilsin the dev shell.just bench-lock fanout-heavy 4. It was not dispatched on this branch.bench/profile.test.pyhas 10 unit tests againstbench/profile.test.txt, a recordedperf scriptexcerpt (an epoll idle, a std mutex wait and a malloc arena wait). They cover parsing, interval pairing, open waits, lost events, classification, reduction, idle and silent workers, and the setup check.Decisions
--call-graph dwarf), rather than frame pointers alone. In a frame-pointer (-g) recording,lock_contendedappeared in 1 stack, a thread preempted while spinning. A DWARF recording of the same workload shows it in 578 frames. glibc'ssyscallwrapper keeps no frame pointer, so the unwinder skips its caller and mutex waits classify as zero. The cost is a largerperf.data: about 130 MB at 4 workers and 570 MB at 16 on fanout-heavy.Driver::turn), and almost no stack reached MoQ code. With it, stacks are 14 to 28 frames deep.context-switchessoftware event with a period of 1, not thesched:sched_switchtracepoint. Both fire on every switch-out in the outgoing thread with the same stacks. The tracepoint also needs a readable/sys/kernel/tracing, which is mode 0700 root by default and needs a remount.kptr_restrictno longer matters either, because kernel frames are discarded. Sampling with:u(user only) records nothing, since the switch happens in kernel context.kptr_restrict=0as well.perf_event_paranoid=-1, with a 2 MiB ring buffer. The default 512 KiB buffer dropped about 2% of samples at 16 workers on fanout-heavy. 2 MiB per CPU drops none, but exceeds the unprivileged locked-memory budget (perf_event_mlock_kb× CPUs +RLIMIT_MEMLOCK), which perf enforces unless paranoid is -1. Kernel samples alone would only need 1.-k monotonicagainst the same clock that brackets schedstat, instead of at the last recorded event (Codex review).fanoutdefault, one worker had no connections and slept through the window, so the default recipe failed.Local verification
This host has
perf_event_paranoid=-1.just bench-lock(defaults) with the committed code:just bench-lock fanout-heavy 16gave 175.8 ms of mutex wait (0.020 wait/CPU) with no dropped events. The top blocking call sites are allkio::Lock: the transportSessionpoll,TrackStatefrompoll_drift,RequestServeandpoll_stale. The idle-worker path was hit once onfanoutat 16 workers and is covered by a unit test.Impact
just bench-lock(new on main). The dev shell addsperfon Linux.Follow-ups
relay.shpicks a free port, then binds it later. On a busy host another process took it once, the relay exited with "Address already in use", and the harness only noticed when the load generator received zero groups. This is on main and not caused by this PR.🤖 Generated with Claude Code
(Written by Claude Opus 5.5)