RFR: 8372701: Randomized profile counters [v2]

[Andrew Haley]

On Thu, 27 Nov 2025 17:18:35 GMT, Andrew Haley <aph at openjdk.org> wrote:

>> Please use [this link](https://github.com/openjdk/jdk/pull/28541/files?w=1) to view the files changed.
>> 
>> Profile counters scale very badly.
>> 
>> The overhead for profiled code isn't too bad with one thread, but as the thread count increases, things go wrong very quickly.
>> 
>> For example, here's a benchmark from the OpenJDK test suite, run at TieredLevel 3 with one thread, then three threads:
>> 
>> 
>> Benchmark (randomized) Mode Cnt Score Error Units
>> InterfaceCalls.test2ndInt5Types false avgt 4 27.468 ± 2.631 ns/op
>> InterfaceCalls.test2ndInt5Types false avgt 4 240.010 ± 6.329 ns/op
>> 
>> 
>> This slowdown is caused by high memory contention on the profile counters. Not only is this slow, but it can also lose profile counts.
>> 
>> This patch is for C1 only. It'd be easy to randomize C1 counters as well in another PR, if anyone thinks it's worth doing.
>
> Andrew Haley has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains 52 commits:
> 
>  - Merge remote-tracking branch 'refs/remotes/origin/JDK-8134940' into JDK-8134940
>  - whitespace
>  - AArch64
>  - Minimize deltas to master
>  - Better
>  - Inter
>  - Cleanup
>  - Cleanup
>  - Merge master
>  - D'oh
>  - ... and 42 more: https://git.openjdk.org/jdk/compare/b2f97131...49d52d82

The inlined profile update code is moved to a stub, then in its place we put:


  ubfx x8, rng, #26, #6  // extract the top 6 bits of the random-number generator
  cbz x8, update         // if they are zero, jump to the stub that updates the profile counter
  next_random rng        // generate the next random number


At the moment, several C2 IR tests fail with randomized profile counters because they are acutely sensitive to small changes in profile counts. I think this can probably be fixed.

Also, I believe there are some kinds of event that should never be missed, even when subsampling profile counters in this way. I'd like people to advise me which events these are.

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PR Comment: https://git.openjdk.org/jdk/pull/28541#issuecomment-3586876351