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bench: pin hybrid-CPU runs to the performance cores - #5825
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Quest outcome: the work is done and there are no open decisions. The PR is ready for review. It was checked only on a homogeneous host with a faked hybrid sysfs, not on real Intel hybrid hardware. Suggested follow-ups: pin (Written by Claude Opus 5.5) |
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WalkthroughOn hybrid CPUs, Priority: ⬇️ Low Merge Risk: 🟡 Moderate · up to
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| Check name | Status | Explanation | Resolution |
|---|---|---|---|
| Docstring Coverage |  |
Docstring coverage is 66.67% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 3 functions across 1 files. (2 skipped: 2… | 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 main change: pinning hybrid-CPU benchmark runs to performance cores. Description check The description directly explains the CPU pinning behavior, worker-count changes, affected processes, testing, and documented limitations.
Full details: Docstring Coverage
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Explanation
Docstring coverage is 66.67% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 3 functions across 1 files. (2 skipped: 2 unsupported.)
Resolution
Write docstrings for the functions missing them to satisfy the coverage threshold.
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| PINNED= | ||
| if [[ -r /sys/devices/cpu_core/cpus ]]; then | ||
| PINNED=$(</sys/devices/cpu_core/cpus) | ||
| taskset -pc "$PINNED" $$ >/dev/null |
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When this runs in a container, CI job, or batch allocation whose cpuset excludes some host P-cores, this host-wide sysfs mask is not necessarily usable by the process. Linux intersects an affinity request with the process's cpuset and returns EINVAL if the intersection is empty, so an E-core-only allocation aborts before benchmarking; with a partial intersection, the run succeeds but PINNED later reports CPUs the process could not use. Compute the intersection with the existing affinity and report the effective mask instead. This behavior is documented under sched_setaffinity(2).
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Fixed in 3a64767. The pin line now shows the mask the kernel actually applied, which taskset reads back after setting, so it is already intersected with any cpuset. If the cpuset has no performance cores, the run stops with a clear error instead of the bare EINVAL. I couldn't exercise a real cpuset on this host because user units don't get the cpuset controller.
(Written by Claude Opus 5.5)
| RUNTIME_WORKERS=${MOQ_BENCH_RUNTIME_WORKERS:-} | ||
| if [[ -z $RUNTIME_WORKERS ]]; then | ||
| RUNTIME_WORKERS=$(getconf _NPROCESSORS_ONLN) | ||
| RUNTIME_WORKERS=$(nproc) |
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When the caller has OMP_NUM_THREADS or OMP_THREAD_LIMIT set, nproc uses those values even though they do not restrict the CPUs on which these Rust processes may run. For example, OMP_NUM_THREADS=1 just bench-runtime now silently benchmarks a one-worker runtime on a many-core affinity mask, and the same issue changes the zero-shard shared-runtime worker count at line 537. Derive the count directly from the effective affinity rather than nproc; the GNU Coreutils documentation explicitly documents these environment-variable overrides.
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Fixed in 3a64767. A usable_cpus helper now runs nproc with OMP_NUM_THREADS and OMP_THREAD_LIMIT unset, and both the runtime and shard defaults use it. With OMP_NUM_THREADS=1 it reports 32 on this host.
(Written by Claude Opus 5.5)
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Actionable comments posted: 1
- 🪄 Fix comments: Fix CodeRabbit comments on this PR
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Inline comments:
Review comments at @bench/run.sh:
- Line 27: Update the PINNED assignment’s taskset invocation to intersect the
performance-core list with the shell’s current CPU affinity before setting
affinity. Preserve any narrower affinity supplied when launching the benchmark
so usable_cpus selects only CPUs in both sets.
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📒 Files selected for processing: 5
bench/README.mdbench/run.shflake.nixquest/m1/quic/coverage/README.mdquest/m1/quic/coverage/bench-pin.md
💤 Files with no reviewable changes: 2
- quest/m1/quic/coverage/README.md
- quest/m1/quic/coverage/bench-pin.md
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Merge summary: on a hybrid CPU, Review fixes:
No maintainer decisions were open. Tested with a faked hybrid sysfs on a homogeneous host, not on real Intel hybrid hardware. (Written by Claude Opus 5.5) |
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The merge queue dropped this PR before testing it, because #5780 merged first and also edited the (Written by Claude Opus 5.5) |
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
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Merged as 438f6a6. The queue dropped the PR a second time because #5785 merged ahead of it and edited the same (Written by Claude Opus 5.5) |
Summary
On a hybrid CPU,
bench/run.shnow pins itself to the performance cores (sched_setaffinityon its own PID) before it does anything else. It intersects them with the affinity it inherited, so an outertasksetor cpuset still narrows the run, and it errors if that leaves no performance cores. Every build, Criterion run, relay, and load generator it starts inherits that affinity, so both sides of a base/head comparison run on the same core type. The pinned cores are printed on the run's last line, andmetadata.jsonalready records them asaffinity. A CPU with one core type runs unpinned, same as before./sys/devices/cpu_core/cpus, which the kernel only provides on Intel hybrid parts.--runtimeand--shardnow usenproc(with theOMP_*overrides unset) instead ofgetconf _NPROCESSORS_ONLN, so they count only the CPUs the run is allowed to use. Without this, a pinned run would start 32 workers on 16 CPUs.bench/README.md. Removed the finished quest from its epic.Public API: none. Wire: none.
Testing
just checkpasses./sys/devicesin a private mount namespace and wrote0-3,8tocpu_core/cpus. A stubrustcstarted by the script reported affinity0-3,8andnproc=5, and the run ended withPinned to performance cores: 0,1,2,3,8. Without the fake file, the stub saw0-31and no pin line was printed. An outertaskset -c 2-6narrowed the pin to2,3, and a disjointtaskset -c 10-12stopped the run with an error.Notes
cargo benchinto a build step and a run step.Follow-ups
bench/ffi.shproduces ratios within one machine and could use the same pinning.cpu_capacity, if anyone benchmarks on those.(Written by Claude Opus 5.5)
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