8354727: CompilationPolicy creates too many compiler threads when code cache space is scarce

[mhaessig]

Issue Summary

Running

java -XX:+SegmentedCodeCache -XX:ReservedCodeCacheSize=10M -XX:NonNMethodCodeHeapSize=6M \
     -XX:ProfiledCodeHeapSize=5M -XX:NonProfiledCodeHeapSize=5M -version

on a machine with more than 255 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 and causes a new check introduced in #17244 to fail.

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,NonNMethodHeapSizeis used as the maximum buffers size available for compilers buffers instead of ReservedCodeCacheSize if it was provided as a commandline flag.

It might be debatable if this is the correct fix, since the NonNMethodHeap can spill into the other heaps if it is too small. However, I am of the opinion that if the NonNMethodHeapSize is explicitly specified, then the compiler count should be calculated accordingly.

Testing

• GHA
• tier1 and tier2 plus Oracle internal testing on our supported platforms
• tier1 with a manually fixed core count of 288 (this reproduced the problem before the fix)

---

Progress

• Change must be properly reviewed (1 review required, with at least 1 Reviewer)
• Change must not contain extraneous whitespace
• Commit message must refer to an issue

Integration blocker

 ⚠️ Dependency #25791 must be integrated first

Issue

• JDK-8354727: CompilationPolicy creates too many compiler threads when code cache space is scarce (Bug - P4)

Reviewers

• Damon Fenacci (@dafedafe - Committer) 🔄 Re-review required (review was made when pull request targeted the master branch)

Contributors

• Aleksey Shipilev <shade@openjdk.org>

Reviewing

Using git

Checkout this PR locally:
$ git fetch https://git.openjdk.org/jdk.git pull/25770/head:pull/25770
$ git checkout pull/25770

Update a local copy of the PR:
$ git checkout pull/25770
$ git pull https://git.openjdk.org/jdk.git pull/25770/head

Using Skara CLI tools

Checkout this PR locally:
$ git pr checkout 25770

View PR using the GUI difftool:
$ git pr show -t 25770

Using diff file

Download this PR as a diff file:
https://git.openjdk.org/jdk/pull/25770.diff

Using Webrev

Link to Webrev Comment

[mhaessig]

/contributor add @shipilev

[bridgekeeper]

👋 Welcome back mhaessig! A progress list of the required criteria for merging this PR into master will be added to the body of your pull request. There are additional pull request commands available for use with this pull request.

[openjdk]

❗ This change is not yet ready to be integrated.
See the Progress checklist in the description for automated requirements.

[openjdk]

@mhaessig
Contributor Aleksey Shipilev <shade@openjdk.org> successfully added.

[openjdk]

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[mlbridge]

Webrevs

• 02: Full (98a8107d)
• 01: Full - Incremental (b62ef8ab)
• 00: Full (8d1b4f79)

[dafedafe]

Thanks for fixing this @mhaessig.
I guess the issue cannot happen if NonNMethodCodeHeapSize is not given as a flag as it will be dynamically adapted to the number of compiler threads created, right?

[mhaessig]

Thank you for having a look @dafedafe!

I guess the issue cannot happen if NonNMethodCodeHeapSize is not given as a flag as it will be dynamically adapted to the number of compiler threads created, right?

That is right. In this case, the heuristics in CodeCache::initialize_heaps() take over.

[dafedafe]

Looks good to me. Thanks @mhaessig.

[openjdk]

@mhaessig this pull request can not be integrated into pr/25791 due to one or more merge conflicts. To resolve these merge conflicts and update this pull request you can run the following commands in the local repository for your personal fork:

git checkout JDK-8354727-policy
git fetch https://git.openjdk.org/jdk.git pr/25791
git merge FETCH_HEAD
# resolve conflicts and follow the instructions given by git merge
git commit -m "Merge pr/25791"
git push

[openjdk]

@mhaessig Please do not rebase or force-push to an active PR as it invalidates existing review comments. Note for future reference, the bots always squash all changes into a single commit automatically as part of the integration. See OpenJDK Developers’ Guide for more information.

[mhaessig]

I made this dependent on #25791 and rebased onto it.