8260497: Shenandoah: Improve SATB flushing

[rkennke]

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

---

Progress

• Change must not contain extraneous whitespace
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Issue

• JDK-8260497: Shenandoah: Improve SATB flushing

Reviewers

• Aleksey Shipilev (@shipilev - Reviewer)
• Zhengyu Gu (@zhengyu123 - Reviewer)

Download

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

[bridgekeeper]

👋 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.

[openjdk]

⚠️ @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).

[openjdk]

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

• hotspot-gc
• shenandoah

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[shipilev]

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

[openjdk]

@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

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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

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[shipilev]

This looks good to me!

[mlbridge]

Webrevs

• 00: Full (8df11fc)

[rkennke]

/integrate

[openjdk]

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

• 11a70d1: 8260426: awt debug_mem.c DMem_AllocateBlock might leak memory
• 396a496: 8260467: Move well-known classes from systemDictionary.hpp to vmClasses.hpp
• 62ffe75: Merge
• e28e111: 8260370: C2: LoopLimit node is not eliminated
• 7030d2e: 8260200: G1: Remove unnecessary update in FreeRegionList::remove_starting_at
• c7661ae: 8260561: [doc] HexFormat has incorrect @SInCE tag
• c52212b: Merge
• 408772c: 8259025: Record compact constructor using Objects.requireNonNull
• 81e730e: 8259276: C2: Empty expression stack when reexecuting tableswitch/lookupswitch instructions after deoptimization
• c5ab7c3: 8260284: C2: assert(_base == Int) failed: Not an Int
• ... and 6 more: https://git.openjdk.java.net/jdk/compare/7ed591cc9a1c46f7c0b10a4c4ad3c181333bf5b6...master

Your commit was automatically rebased without conflicts.

Pushed as commit 316d52c.

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