RFR: 8356000: C1/C2-only modes use 2 compiler threads on low CPU count machines [v3]

[Damon Fenacci]

On Wed, 28 May 2025 18:05:12 GMT, Aleksey Shipilev <shade at openjdk.org> wrote:

>> There is an unfortunate limitation with default tiered policy that we would have at least 2 threads on 1 CPU machine: 1 thread for C1, and 1 thread for C2.
>> 
>> But if we select C1-only or C2-only modes, we _also_ get 2 compiler threads, for which we have no good reason. These threads would just step on each other toes. The fix changes the behavior for 1..3 CPU hosts in C1/C2-only configurations, by using 1 thread instead of 2 threads. The change for 1 CPU config is what we really need. The change in 2..3 CPU configs is an additional effect, but I think it is still good not to use 100%/66% of the CPUs in those configurations as well.
>> 
>> 
>> $ for I in `seq 1 8`; do build/linux-x86_64-server-release/images/jdk/bin/java \
>>   -XX:-TieredCompilation -XX:ActiveProcessorCount=${I} \
>>   -XX:+PrintFlagsFinal 2>&1 | grep "CICompilerCount "; done
>> 
>> # Before
>>      intx CICompilerCount                          = 2
>>      intx CICompilerCount                          = 2
>>      intx CICompilerCount                          = 2
>>      intx CICompilerCount                          = 3
>>      intx CICompilerCount                          = 3
>>      intx CICompilerCount                          = 3
>>      intx CICompilerCount                          = 3
>>      intx CICompilerCount                          = 4
>> 
>> # After
>>      intx CICompilerCount                          = 1
>>      intx CICompilerCount                          = 1
>>      intx CICompilerCount                          = 1
>>      intx CICompilerCount                          = 3
>>      intx CICompilerCount                          = 3
>>      intx CICompilerCount                          = 3
>>      intx CICompilerCount                          = 3
>>      intx CICompilerCount                          = 4
>> 
>> 
>> It is a minor bug in `CompilationPolicy::initialize`, but it gets in the way studying Leyden in tight CPU scenarios. 
>> 
>> Additional testing:
>>  - [x] New regression test passes with the fix, fails without it
>>  - [x] GHA
>>  - [x] Linux AArch64 server fastdebug, `all`
>
> Aleksey Shipilev has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains eight additional commits since the last revision:
> 
>  - Merge branch 'master' into JDK-8356000-c1-c2-compiler-count
>  - Better test, patch amendments
>  - Merge branch 'master' into JDK-8356000-c1-c2-compiler-count
>  - Merge branch 'master' into JDK-8356000-c1-c2-compiler-count
>  - Unnecessary arch limitation
>  - Simplify test
>  - Adjust test bound
>  - Fix

Very thorough test! Thanks a lot @shipilev!

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Marked as reviewed by dfenacci (Committer).

PR Review: https://git.openjdk.org/jdk/pull/24972#pullrequestreview-2895749955