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Hi Jaromir,</div>
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That sounds interesting :), as long as we are confident that your observation is part of the user ABI. Feel free to submit a PR and I will happily review it. Also add a link or reasoning to confirm that it is part of the user ABI.</div>
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Thank you,</div>
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Jonas</div>
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<b>From: </b>hotspot-runtime-dev <hotspot-runtime-dev-retn@openjdk.org> on behalf of Jaromir Hamala <jaromir.hamala@gmail.com><br>
<b>Date: </b>Tuesday, 23 December 2025 at 14:36<br>
<b>To: </b>Kevin Walls <kevinw@openjdk.org><br>
<b>Cc: </b>hotspot-runtime-dev@openjdk.org <hotspot-runtime-dev@openjdk.org><br>
<b>Subject: </b>Re: RFR: 8372584: [Linux]: Replace reading proc to get thread user CPU time with clock_gettime [v7]<br>
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<div dir="ltr" class="gmail_attr">On Wed, Dec 3, 2025 at 10:35 AM Kevin Walls <<a href="mailto:kevinw@openjdk.org" data-outlook-id="4db8ad15-d1b2-4b1e-8ffe-f0c4651dadd6">kevinw@openjdk.org</a>> wrote:</div>
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<div dir="ltr" class="gmail_quote">On Tue, 2 Dec 2025 20:59:41 GMT, Jonas Norlinder <<a href="mailto:jnorlinder@openjdk.org" target="_blank" data-outlook-id="5c24a8a2-13e2-442a-af60-73fdf06f8f6f">jnorlinder@openjdk.org</a>> wrote:<br>
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>> Since kernel v2.6.12 the Linux ABI have had support for encoding the clock types in the last three bits. Setting bit to 001 (CPUCLOCK_VIRT) will result in the kernel returning only user time. POSIX compliant implementations of pthread_getcpuclockid for the
Linux kernel defaults to construct a clockid that with 010 (CPUCLOCK_SCHED) set, which return system+user time, which is what the POSIX standard mandates, see POSIX.1-2024/IEEE Std 1003.1-2024 §3.90. This patch joins the family of glibc, musl etc. that utilities
this bit pattern.<br>
>><br>
>> This PR also results in improved performance and thus a reduced observer effect, especially for the 100th percentile (max).<br>
>><br>
>> Before patch:<br>
>><br>
>> Benchmark Mode Cnt Score Error Units<br>
>> CPUTime.execute sample 7506555 0.008 ± 0.001 ms/op<br>
>> CPUTime.execute:p0.00 sample 0.008 ms/op<br>
>> CPUTime.execute:p0.50 sample 0.008 ms/op<br>
>> CPUTime.execute:p0.90 sample 0.008 ms/op<br>
>> CPUTime.execute:p0.95 sample 0.008 ms/op<br>
>> CPUTime.execute:p0.99 sample 0.012 ms/op<br>
>> CPUTime.execute:p0.999 sample 0.015 ms/op<br>
>> CPUTime.execute:p0.9999 sample 0.021 ms/op<br>
>> CPUTime.execute:p1.00 sample 1.030 ms/op<br>
>><br>
>><br>
>> After patch:<br>
>><br>
>> Benchmark Mode Cnt Score Error Units<br>
>> CPUTime.execute sample 8984189 ≈ 10⁻³ ms/op<br>
>> CPUTime.execute:p0.00 sample ≈ 10⁻³ ms/op<br>
>> CPUTime.execute:p0.50 sample ≈ 10⁻³ ms/op<br>
>> CPUTime.execute:p0.90 sample ≈ 10⁻³ ms/op<br>
>> CPUTime.execute:p0.95 sample ≈ 10⁻³ ms/op<br>
>> CPUTime.execute:p0.99 sample 0.001 ms/op<br>
>> CPUTime.execute:p0.999 sample 0.001 ms/op<br>
>> CPUTime.execute:p0.9999 sample 0.006 ms/op<br>
>> CPUTime.execute:p1.00 sample 0.054 ms/op<br>
>><br>
>><br>
>> Testing: `java/lang/management/ThreadMXBean/ThreadUserTime.java` and the added microbenchmark.<br>
><br>
> Jonas Norlinder has updated the pull request incrementally with one additional commit since the last revision:<br>
><br>
> Align signature to standard<br>
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Looks good - I remember that fix for parsing the program binary name containing brackets, good to have it gone.<br>
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Marked as reviewed by kevinw (Reviewer).<br>
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PR Review: <a href="https://git.openjdk.org/jdk/pull/28556#pullrequestreview-3534064399" target="_blank" rel="noreferrer" data-outlook-id="4b515724-a903-4c15-a5ae-336f2a6b97c3">
https://git.openjdk.org/jdk/pull/28556#pullrequestreview-3534064399</a></div>
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<div dir="ltr" class="ms-outlook-mobile-reference-message skipProofing">Apologies for reviving an old treat. I was experimenting with this change, and I believe there is a further optimisation opportunity: When clockid has TID set to 0, then the kernel treats
it as 'the current task' (=which is what getCurrentThreadUserTime() requires) and avoids a radix lookup required for an arbitrary TID.</div>
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<div dir="ltr" class="ms-outlook-mobile-reference-message skipProofing">The change:
<a href="https://github.com/jerrinot/jdk/compare/master...jerrinot:jdk:jh_faster_getCurrentThreadUserTime" data-outlook-id="511df8ef-760a-49d7-9468-cfc72930f97b">
https://github.com/jerrinot/jdk/compare/master...jerrinot:jdk:jh_faster_getCurrentThreadUserTime</a></div>
<div dir="ltr" class="ms-outlook-mobile-reference-message skipProofing">The benchmark from
<a href="https://github.com/openjdk/jdk/pull/28556" data-outlook-id="7318077b-c6fa-4493-9ba6-e69483208b9b">
https://github.com/openjdk/jdk/pull/28556</a> (switched to nanos + more iterations + fork count):</div>
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<div dir="ltr" class="ms-outlook-mobile-reference-message skipProofing">Before:</div>
<div dir="ltr" class="ms-outlook-mobile-reference-message skipProofing">Benchmark Mode Cnt Score Error Units<br>
ThreadMXBeanBench.getCurrentThreadUserTime sample 4347067 81.746 ± 0.510 ns/op<br>
ThreadMXBeanBench.getCurrentThreadUserTime:p0.00 sample 69.000 ns/op<br>
ThreadMXBeanBench.getCurrentThreadUserTime:p0.50 sample 80.000 ns/op<br>
ThreadMXBeanBench.getCurrentThreadUserTime:p0.90 sample 90.000 ns/op<br>
ThreadMXBeanBench.getCurrentThreadUserTime:p0.95 sample 90.000 ns/op<br>
ThreadMXBeanBench.getCurrentThreadUserTime:p0.99 sample 90.000 ns/op<br>
ThreadMXBeanBench.getCurrentThreadUserTime:p0.999 sample 230.000 ns/op<br>
ThreadMXBeanBench.getCurrentThreadUserTime:p0.9999 sample 1980.000 ns/op<br>
ThreadMXBeanBench.getCurrentThreadUserTime:p1.00 sample 653312.000 ns/op<br>
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After:<br>
Benchmark Mode Cnt Score Error Units<br>
ThreadMXBeanBench.getCurrentThreadUserTime sample 5081223 70.813 ± 0.325 ns/op<br>
ThreadMXBeanBench.getCurrentThreadUserTime:p0.00 sample 59.000 ns/op<br>
ThreadMXBeanBench.getCurrentThreadUserTime:p0.50 sample 70.000 ns/op<br>
ThreadMXBeanBench.getCurrentThreadUserTime:p0.90 sample 70.000 ns/op<br>
ThreadMXBeanBench.getCurrentThreadUserTime:p0.95 sample 70.000 ns/op<br>
ThreadMXBeanBench.getCurrentThreadUserTime:p0.99 sample 80.000 ns/op<br>
ThreadMXBeanBench.getCurrentThreadUserTime:p0.999 sample 170.000 ns/op<br>
ThreadMXBeanBench.getCurrentThreadUserTime:p0.9999 sample 1830.000 ns/op<br>
ThreadMXBeanBench.getCurrentThreadUserTime:p1.00 sample 425472.000 ns/op</div>
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<div dir="ltr" class="ms-outlook-mobile-reference-message skipProofing">There is around 13% latency improvement on average.</div>
<div dir="ltr" class="ms-outlook-mobile-reference-message skipProofing">It increases coupling to kernel internals a bit further, but the original patch already does that by poking the lower bits + Linux has a strong policy on ABI stability.</div>
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<div dir="ltr" class="ms-outlook-mobile-reference-message skipProofing">Would you be interested in merging a similar patch?</div>
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<div dir="ltr" class="ms-outlook-mobile-reference-message skipProofing">Cheers,</div>
<div dir="ltr" class="ms-outlook-mobile-reference-message skipProofing">Jaromir Hamala</div>
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<div dir="ltr" class="ms-outlook-mobile-reference-message skipProofing">--</div>
<div dir="ltr" class="gmail_signature">“Perfection is achieved, not when there is nothing more to add, but when there is nothing left to take away.”<br>
Antoine de Saint Exupéry</div>
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