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8284404: Too aggressive sweeping with Loom #8673

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@fisk fisk commented May 12, 2022

The normal sweeping heuristics trigger sweeping whenever 0.5% of the reserved code cache could have died. Normally that is fine, but with loom such sweeping requires a full GC cycle, as stacks can now be in the Java heap as well. In that context, 0.5% does seem to be a bit too trigger happy. So this patch adjusts that default when using loom to 10x higher.
If you run something like jython which spins up a lot of code, it unsurprisingly triggers a lot less GCs due to code cache pressure.


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👋 Welcome back eosterlund! 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 openjdk bot added the rfr Pull request is ready for review label May 12, 2022
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@fisk The following label will be automatically applied to this pull request:

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Webrevs

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Did you run our regular performance testing with loom to see how this change affect performance?
Why 10x and not other number?

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fisk commented May 12, 2022

Did you run our regular performance testing with loom to see how this change affect performance?

Why 10x and not other number?

I tried to find a problematic workload where tis is a real issue and and manually found that jython compiles a lot of methods yielding a lot of sweeper triggered GCs. This new threshold remedied the problem so that only a few were triggered.
I have however not run the wider perf suite. I can do that though.

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Thank you for explaining where the number came from.

I think SweeperThreshold should still be limited to some reasonable number. Otherwise this code may set it to 1000.0 (flag's max allowed value is 100.).

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fisk commented May 24, 2022

@vnkozlov Is your concern that a user explicitly overrides the default to a value that ends up not being good? If so, I'm not sure why we would be in the business of preventing the user from shooting itself in the foot and guessing what the user really wanted here. Maybe I missed something.

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@vnkozlov Is your concern that a user explicitly overrides the default to a value that ends up not being good? If so, I'm not sure why we would be in the business of preventing the user from shooting itself in the foot and guessing what the user really wanted here. Maybe I missed something.

Yes, it was my concern which was unjustifiable because I missed that this code is guarded by FLAG_IS_DEFAULT(SweeperThreshold). So you simply set SweeperThreshold to 5% (default is 0.5) which is fine.

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@fisk 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:

8284404: Too aggressive sweeping with Loom

Reviewed-by: kvn, thartmann

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fisk commented May 24, 2022

@vnkozlov Is your concern that a user explicitly overrides the default to a value that ends up not being good? If so, I'm not sure why we would be in the business of preventing the user from shooting itself in the foot and guessing what the user really wanted here. Maybe I missed something.

Yes, it was my concern which was unjustifiable because I missed that this code is guarded by FLAG_IS_DEFAULT(SweeperThreshold). So you simply set SweeperThreshold to 5% (default is 0.5) which is fine.

Okay great - thanks for the review!

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Looks good to me. A comment explaining the justification wouldn't hurt.

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TobiHartmann commented Jun 14, 2022

@fisk This PR needs to be re-submitted for the JDK 19 repository but given it was triaged as P4, it might be too late.

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fisk commented Jun 20, 2022

/integrate

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openjdk bot commented Jun 20, 2022

Going to push as commit 7d4df6a.
Since your change was applied there have been 629 commits pushed to the master branch:

Your commit was automatically rebased without conflicts.

@openjdk openjdk bot added the integrated Pull request has been integrated label Jun 20, 2022
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@openjdk openjdk bot removed ready Pull request is ready to be integrated rfr Pull request is ready for review labels Jun 20, 2022
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@fisk Pushed as commit 7d4df6a.

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