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8260497: Shenandoah: Improve SATB flushing #2254

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@rkennke rkennke commented Jan 27, 2021

Currently, we periodically force flushing of SATB queues. This works by activating a flag every 100ms in every thread, which causes that thread to enqueue its SATB buffer the next time it overflows, even if it doesn't meet its threshold after filtering. This is somewhat problematic when a thread does not actually overflow its SATB queue in time. The whole point of the exercise is to try and avoid having too much left-over work when we reach final-mark.

We can do better than that: when concurrent mark is done we can handshake all threads, and let them flush their respective SATB queues, and re-enter concurrent mark loop again, until flushing yields no more work. Experiments show that it usually takes 1-3 flushes to clean out leftover work properly.

I ran benchmarks, 3 high-pressure preset runs of SPECjbb2015, 10 minutes each:

baseline:
Finish Mark                  =    0,251 s (a =      688 us) (n =   364) (lvls, us =      125,      486,      621,      824,     4156)
Finish Mark                  =    0,338 s (a =      922 us) (n =   366) (lvls, us =      131,      494,      652,      852,    72948)
Finish Mark                  =    0,257 s (a =      699 us) (n =   368) (lvls, us =      111,      492,      645,      826,     4447)

patched:
Finish Mark                  =    0,112 s (a =      301 us) (n =   370) (lvls, us =      115,      207,      250,      281,     3709)
Finish Mark                  =    0,107 s (a =      292 us) (n =   368) (lvls, us =      107,      209,      248,      287,     3329)
Finish Mark                  =    0,114 s (a =      310 us) (n =   367) (lvls, us =      115,      211,      254,      285,     3819)

It reliably lowers all timings for finish-mark. It also doesn't cause any regressions in throughput.

Testing:

  • hotspot_gc_shenandoah
  • benchmarks

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$ git fetch https://git.openjdk.java.net/jdk pull/2254/head:pull/2254
$ git checkout pull/2254

@rkennke rkennke marked this pull request as draft Jan 27, 2021
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@bridgekeeper bridgekeeper bot commented Jan 27, 2021

👋 Welcome back rkennke! 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.

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@openjdk openjdk bot commented Jan 27, 2021

⚠️ @rkennke This pull request contains merges that bring in commits not present in the target repository. Since this is not a "merge style" pull request, these changes will be squashed when this pull request in integrated. If this is your intention, then please ignore this message. If you want to preserve the commit structure, you must change the title of this pull request to Merge <project>:<branch> where <project> is the name of another project in the OpenJDK organization (for example Merge jdk:master).

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@openjdk openjdk bot commented Jan 27, 2021

@rkennke The following labels will be automatically applied to this pull request:

  • hotspot-gc
  • shenandoah

When this pull request is ready to be reviewed, an "RFR" email will be sent to the corresponding mailing lists. If you would like to change these labels, use the /label pull request command.

@openjdk openjdk bot added hotspot-gc shenandoah labels Jan 27, 2021
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@shipilev shipilev left a comment

This looks nice. Although you want to fix some of the build falures: https://github.com/rkennke/jdk/runs/1776231024?check_suite_focus=true

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@openjdk openjdk bot commented Jan 27, 2021

@rkennke 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:

8260497: Shenandoah: Improve SATB flushing

Reviewed-by: shade, zgu

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 3 new commits pushed to the master branch:

  • 311a0a9: 8259808: Add JFR event to detect GC locker stall
  • f353fcf: 8258894: C2: Forbid GCM to move stores into loops
  • ac276bb: 8257074: Update the ByteBuffers micro benchmark

Please see this link for an up-to-date comparison between the source branch of this pull request and the master branch.
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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@openjdk openjdk bot added the ready label Jan 27, 2021
@openjdk openjdk bot removed the ready label Jan 27, 2021
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@shipilev shipilev left a comment

This looks good to me!

@openjdk openjdk bot added the ready label Jan 27, 2021
@rkennke rkennke marked this pull request as ready for review Jan 27, 2021
@openjdk openjdk bot added the rfr label Jan 27, 2021
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@mlbridge mlbridge bot commented Jan 27, 2021

Webrevs

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@rkennke rkennke commented Jan 28, 2021

/integrate

@openjdk openjdk bot closed this Jan 28, 2021
@openjdk openjdk bot added integrated and removed ready rfr labels Jan 28, 2021
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@openjdk openjdk bot commented Jan 28, 2021

@rkennke Since your change was applied there have been 16 commits pushed to the master branch:

Your commit was automatically rebased without conflicts.

Pushed as commit 316d52c.

💡 You may see a message that your pull request was closed with unmerged commits. This can be safely ignored.

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