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

[shipilev]

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:

• New regression test passes with the fix, fails without it
• GHA
• Linux AArch64 server fastdebug, all

---

Progress

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Issue

• JDK-8356000: C1/C2-only modes use 2 compiler threads on low CPU count machines (Bug - P4)

Reviewers

• Damon Fenacci (@dafedafe - Committer) Review applies to f8519b46
• Galder Zamarreño (@galderz - Author) Review applies to f8519b46
• Vladimir Kozlov (@vnkozlov - Reviewer)

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[openjdk]

@shipilev This change now passes all automated pre-integration checks.

ℹ️ This project also has non-automated pre-integration requirements. Please see the file CONTRIBUTING.md for details.

After integration, the commit message for the final commit will be:

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

Reviewed-by: kvn, dfenacci, galder

You can use pull request commands such as /summary, /contributor and /issue to adjust it as needed.

At the time when this comment was updated there had been 5 new commits pushed to the master branch:

• 0352477: 8357660: [JVMCI] Add support for retrieving all BootstrapMethodInvocations directly from ConstantPool
• a653ff4: 8358536: jdk/jfr/api/consumer/TestRecordingFileWrite.java times out
• 248341d: 8344942: Template-Based Testing Framework
• ... and 2 more: https://git.openjdk.org/jdk/compare/7838321b74276e45b92c54904ea31ef70ed9e33f...master

As there are no conflicts, your changes will automatically be rebased on top of these commits when integrating. If you prefer to avoid this automatic rebasing, please check the documentation for the /integrate command for further details.

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[mlbridge]

Webrevs

• 03: Full - Incremental (c43b18a6)
• 02: Full - Incremental (f8519b46)
• 01: Full - Incremental (e682962c)
• 00: Full (8e3366a1)

[dafedafe]

Thanks @shipilev for fixing this (it might be a minor bug but still... inconsistent 🙂)

[shipilev]

Update: Added the test that verifies heuristics performs as expected. This test also explores what happens with explicit CICompilerCount setting, varying the CPU sizes, etc. This allowed me to simplify the actual VM code: what we used to assert, we now directly test.

I also had to add CICompilerCount=0 for Xint mode, so that test is straightforward.

[shipilev]

Any further comments / testing? Thanks.

[galderz]

Thanks for the fix and expanding the test case.

[vnkozlov]

Looks fine. I submitted testing.

[vnkozlov]

Testing mostly passed. Only macOS-aarch64 left (linux-aarch64 passed). I think it is fine to integrate without waiting.

[vnkozlov]

All my testing passed.

[dafedafe]

Very thorough test! Thanks a lot @shipilev!

[shipilev]

During the final pre-integration checks, I noticed that this PR still has buffer adjustments for c2_only case. That adjustment is, strictly speaking, outside the scope of this improvement. So I reverted that hunk. Tests still pass.

I see JDK-8354727 was filed to figure out what happens when we are scarce on code cache, so I would feel better to put it on @mhaessig to mix c43b18a into his work :)

[mhaessig]

I see JDK-8354727 was filed to figure out what happens when we are scarce on code cache, so I would feel better to put it on @mhaessig to mix c43b18a into his work :)

Will do. Thanks for the heads up!

[vnkozlov]

Re-approved

[shipilev]

Excellent, thanks for reviews! Here goes.

/integrate

[openjdk]

Going to push as commit 4e314cb.
Since your change was applied there have been 7 commits pushed to the master branch:

• a2723d9: 8345347: Test runtime/cds/TestDefaultArchiveLoading.java should accept VM flags or be marked as flagless
• ae1892f: 8358003: KlassTrainingData initializer reads garbage holder
• 0352477: 8357660: [JVMCI] Add support for retrieving all BootstrapMethodInvocations directly from ConstantPool
• ... and 4 more: https://git.openjdk.org/jdk/compare/7838321b74276e45b92c54904ea31ef70ed9e33f...master

Your commit was automatically rebased without conflicts.

[openjdk]

@shipilev Pushed as commit 4e314cb.

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