RFR: 8354727: CompilationPolicy creates too many compiler threads when code cache space is scarce [v4]

[Vladimir Kozlov]

On Thu, 19 Jun 2025 14:41:27 GMT, Manuel Hässig <mhaessig at openjdk.org> wrote:

>> Running
>> 
>> 
>> java -XX:+SegmentedCodeCache -XX:ReservedCodeCacheSize=10M -XX:NonNMethodCodeHeapSize=6M \
>>      -XX:ProfiledCodeHeapSize=5M -XX:NonProfiledCodeHeapSize=5M -version
>> 
>> 
>> on a machine with more than 285 cores, this would fail with the message that the specified `NonNMethodCodeHeapSize` is too small to fit all compiler buffers (instead of failing because the sum of the heaps is larger than the `ReservedCodeCacheSize`). Hence, the calculated compiler count is too high. This is due to CompilationPolicy::initialize() checking how many compiler buffers fit into the `ReservedCodeCacheSize`. However, in the case above, this is significantly larger than `NonNMethodCodeHeapSize` (especially on a debug build) and causes a check changed in #17244 to fail. That check was changed to check that all compiler buffers fit into the `NonNMethodCodeHeap` instead of the `NonNMethodCodeHeap` having at least a size of `CodeCacheMinimumUseSpace`.
>> 
>> # Changes
>> 
>> This PR fixes the calculation of the `CICompilerCount` ergonomic. Firstly, @shipilev kindly provided a fix for the compiler buffer size used in the calculation is also correct if we only have C2. Secondly, `NonNMethodHeapSize` is used as the maximum buffer size available for compilers buffers in the calculation of the maximum number of compiler threads instead of `ReservedCodeCacheSize`. Therefore, the check failing in the explanation above can never fail because we set the number of compiler threads only so high that they will always fit into the `NonNMethodCodeHeap`.
>> 
>> This change changes how many compiler threads are created by the `CICompilerCount` ergonomic. For the default value `NonNMethodCodeHeapSize=5M`this limit is 24 compiler threads on a system 285 cores or more for product builds and 20 threads for debug builds on a system with 145 cores or more.
>> 
>> # Testing
>> 
>> - [x] [Github Actions](https://github.com/mhaessig/jdk/actions/runs/15733154809)
>> - [x] tier1 through tier3 plus Oracle internal testing on our supported platforms
>
> Manuel Hässig has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Fix merge conflict resolution

Good.

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Marked as reviewed by kvn (Reviewer).

PR Review: https://git.openjdk.org/jdk/pull/25872#pullrequestreview-2943499388