8368152: Shenandoah: Incorrect behavior at end of degenerated cycle

[earthling-amzn]

There are several issues addressed in this PR:

• Shenandoah always ran a full GC after any degenerated cycle
• The number of consecutive degenerated GCs with bad progress was reset for every degenerated cycle
• Good progress was reported in generational mode even when no progress is made

---

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Issue

• JDK-8368152: Shenandoah: Incorrect behavior at end of degenerated cycle (Bug - P4)

Reviewers

• Kelvin Nilsen (@kdnilsen - Committer)
• Y. Srinivas Ramakrishna (@ysramakrishna - Reviewer)

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8368152: Shenandoah: Incorrect behavior at end of degenerated cycle

Reviewed-by: kdnilsen, ysr

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At the time when this comment was updated there had been 24 new commits pushed to the master branch:

• f68cba3: 8367611: Enable vblendvp[sd] on Future ECore
• f1ee1b4: 8366844: Update and automate MouseDraggedOriginatedByScrollBarTest.java
• 218e82c: 8368192: Test java/lang/ProcessBuilder/Basic.java#id0 fails with Exception: Stack trace
• ... and 21 more: https://git.openjdk.org/jdk/compare/4882559ae34e3fee2fd1fd14cb9617df68664281...master

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[mlbridge]

Webrevs

• 03: Full - Incremental (4c36c6f3)
• 02: Full - Incremental (2324450a)
• 01: Full - Incremental (c962f198)
• 00: Full (f5aa7b13)

[kdnilsen]

A common scenario that I have seen in various GC logs is the following:

1. Concurrent GC experiences allocation failure and degenerates
2. Degenerated GC fails to make progress because lots of garbage was allocated after the start of concurrent marking (ie. all garbage is floating)
3. We escalate to Full GC, resulting in a very long STW delay (The Full GC delay plus the delay for degen that preceded Full GC)
4. It is often the case that better throughput and better latency is realized if we run a concurrent GC which may once again degenerate, rather than escalating from the first degen to Full GC. All of the floating garbage that was locked up during the previous GC should be reclaimed by the second concurrent (possibly degenerated) GC, so the second cycle should have "good progress" even if the first did not.
5. I believe the solution you've implemented here will first try a second concurrent/degenerated cycle before escalating to Full GC. Will you confirm that?

Should we have some comments in this code to explain why CONSECUTIVE_BAD_DEGEN_PROGRESS_THRESHOLD is 2 and not 1?

[earthling-amzn]

There is a comment in shDegeneratedGC explaining this scenario. I'll move it to shCollectorPolicy. I didn't quite understand why single-gen mode would not benefit equally from this reasoning, but I tried to preserve the original intended behavior.

[ysramakrishna]

Looks good; left some documentation nits, but approved even if the documentation suggestions aren't taken.

🚢

[ysramakrishna]

Also, independently, I think the notion of progress should be clarified based on current implementation in ShenandoahMetricsSnapshot::is_good_progress() (which declaration should itself have a brief comment):

   Compare before and after snapshots to determine if GC
   made sufficient progress. The notion of progress is based on space free after,
   change in space freed, or changes in internal and/or external fragmentation.

I found it slightly confusing that even though a Generation argument is passed into the method, it's really not used directly as far as I could tell in the notion of progress. The metrics all appear to be heap-wide, rather than generation-wide. There is a comment about the collection set being proportional to the generation in question which of course makes sense, but I'd much rather we didn't pass in an argument that isn't used there, but rather address it directly in the documentation.

May be I am missing something?

[earthling-amzn]

You're right that the generation argument to is_good_progress is unused. I think it makes sense to instead pass this to the shMetrics constructor and have it evaluate used from the generation. I will experiment with this, but I think it's outside the scope of this PR.

[earthling-amzn]

/integrate

[openjdk]

Going to push as commit f36c33c.
Since your change was applied there have been 24 commits pushed to the master branch:

• f68cba3: 8367611: Enable vblendvp[sd] on Future ECore
• f1ee1b4: 8366844: Update and automate MouseDraggedOriginatedByScrollBarTest.java
• 218e82c: 8368192: Test java/lang/ProcessBuilder/Basic.java#id0 fails with Exception: Stack trace
• ... and 21 more: https://git.openjdk.org/jdk/compare/4882559ae34e3fee2fd1fd14cb9617df68664281...master

Your commit was automatically rebased without conflicts.

[openjdk]

@earthling-amzn Pushed as commit f36c33c.

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