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testing: parallel benchmark results are poorly documented #31884
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When many UUIDs are being generated concurrently, contention on the atomic counter can slow things down. There might be ways to speed this up, but for now, just add a parallel benchmark so we can measure the baseline. Initial results with `go test -bench . -cpu 1,2` on my machine (two physical cores): BenchmarkContended 56413502 18.3 ns/op BenchmarkContended-2 82533951 33.5 ns/op Note that the 33.5ns/op measure is worse than it appears, because that's wall ns/op, not cpu-ns/op (see golang/go#31884), so the time is actually 67.0ns/op when contended.
Looking at improving this documentation now. Considering where best to document - thinking this detail would be best added in a little of the documentation for Would anyone recommend any additional areas to document this? |
Change https://go.dev/cl/447136 mentions this issue: |
I just submitted https://go-review.googlesource.com/c/go/+/447136 for improving the documentation, PTAL.
That's unfortunate. I think fixing the value would be the best solution here. This doesn't really fall under the Go 1 compatibility promise, does it?
I can send a patch for this. Maybe It also seems like it's possible to overwrite the reported "ns/op" with a better value like this:
Perhaps instead of reporting a new metric, we could add a convenience API, e.g. But again, that's also ugly. I'd prefer if the current behavior could be fixed. Regardless of further action, I hope the documentation patch makes sense and can be applied in the meantime. |
I'd suggest adding a |
Updates #31884 Change-Id: Ibad3d31038a8426c0bce61c1726392880f934865 Reviewed-on: https://go-review.googlesource.com/c/go/+/447136 Reviewed-by: Bryan Mills <bcmills@google.com> Run-TryBot: Bryan Mills <bcmills@google.com> Reviewed-by: Benny Siegert <bsiegert@gmail.com> Auto-Submit: Bryan Mills <bcmills@google.com> TryBot-Result: Gopher Robot <gobot@golang.org>
The way parallel benchmark results are reported is easy to misinterpret, and the correct way to interpret them is not documented. Typically this makes perfectly-scalable benchmarks appear to perform much worse at low parallelism, and makes not-at-all-scalable benchmarks appear to be doing fine. For example, this came up in #31820 (comment)
Specifically, the "ns/op" reported is not CPU-ns/op, it's wall-ns/op. For example, suppose each op takes exactly 100 ns, regardless of parallelism. If the single-threaded benchmark runs for 1 sec, it will execute 10,000,000 ops, so ns/op = 1s/10,000,000 ops = 100 ns/op. But if the same benchmark runs 4-way parallel for 1 sec, it will execute 40,000,000 ops, so ns/op = 1s/40,000,000 = 25 ns/op. (I really wish it didn't work this way...)
Interpreting the results of CPU-bound parallel benchmarks is further complicated by hyper-threading (though this isn't the fault of the testing package).
I don't think we can change the reported ns/op at this point. We could perhaps introduce a new metric for parallel benchmarks. At the very least, we should document this.
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