[jdk20] RFR: 8298215: gc/g1/TestVerifyGCType.java failed with "Missing expected verification pattern Verifying After GC for: Pause Young (Prepare Mixed): expected true, was false"

[Kim Barrett]

On Mon, 19 Dec 2022 11:42:54 GMT, Thomas Schatzl <tschatzl at openjdk.org> wrote:

>> Please review this fix to the calculation of the number of young regions to use.
>> 
>> That calculation was recently changed (JDK-8297186), but that change has a
>> problem.  It ensures the desired number of young regions calculated at the end
>> of a collection includes at least one eden region, to ensure progress is made.
>> It accomplished this by ensuring the minimum length was the number of already
>> allocated regions + 1 when doing the calculation at the end of a GC.  But for
>> a revision of the length during the mutator phase it intentionally didn't do
>> that, as we want to trigger a collection rather than forceably expand.
>> 
>> However, a revision may occur immediately after the GC (indeed that's fairly
>> likely after JDK-8137022).
>> 
>> A better calculation is to ensure the desired number of young regions is at
>> least the number of survivor regions + 1.  (That's effectively what the old
>> code was doing in the end-of-GC calculation, since at that time all allocated
>> regions are survivor regions.)  So that's the basic change.  This also removes
>> the need to distinguish between updating for GC or during the mutator phase
>> that was introduced by JDK-8297186 (the "after-gc" argument).
>> 
>> Also changed the test to also include the subprocess log in the error message
>> when the check for verification messages fails, to aid any future debugging.
>> 
>> Testing:
>> 
>> mach5 tier1-3.  This isn't all that revealing, since the test failure was very
>> intermittent.
>> 
>> Hand tested with my changes to address JDK-8293824.  With just those changes
>> the test was failing pretty reliably (instead of intermittently).  Adding the
>> changes proposed here lets that test (seemingly) reliably pass.  (There might
>> still be some work to do for JDK-8293824.)
>> 
>> There are some naming and description comment improvements around
>> update_young_length_bounds that I'd like to make, but I'm deferring those as a
>> separate RFE and keeping this bug fix fairly minimal.
>
> Marked as reviewed by tschatzl (Reviewer).

Thanks @tschatzl and @walulyai for reviews.

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PR: https://git.openjdk.org/jdk20/pull/52