RFR: 8260044: Parallel GC: Concurrent allocation after heap expansion may cause unnecessary full gc [v2]

[Stefan Johansson]

On Sat, 30 Jan 2021 06:09:01 GMT, Kim Barrett <kbarrett at openjdk.org> wrote:

>> Please review this change to ParallelGC to avoid unnecessary full GCs when
>> concurrent threads attempt oldgen allocations during evacuation.
>> 
>> When a GC thread fails an oldgen allocation it expands the heap and retries
>> the allocation.  If the second allocation attempt fails then allocation
>> failure is reported to the caller, which can lead to a full GC.  But the
>> retried allocation could fail because, after expansion, some other thread
>> allocated enough of the available space that the retry fails.  This can
>> happen even though there is plenty of space available, if only that retry
>> were to perform another expansion.
>> 
>> Rather than trying to combine the allocation retry with the expansion (it's
>> not clear there's a way to do so without breaking invariants), we instead
>> simply loop on the allocation attempt + expand, until either the allocation
>> succeeds or the expand fails.  If some other thread "steals" space from the
>> expanding thread and causes its next allocation attempt to fail and do
>> another expansion, that's functionally no different from the expanding
>> thread succeeding and causing the other thread to fail allocation and do the
>> expand instead.
>> 
>> This change includes modifying PSOldGen::expand_to_reserved to return false
>> when there is no space available, where it previously returned true.  It's
>> not clear why it returned true; that seems wrong, but was harmless.  But it
>> must not do so with the new looping behavior for allocation, else it would
>> never terminate.
>> 
>> Testing:
>> mach5 tier1-3, tier5 (tier2-3, 5 do a lot of ParallelGC testing)
>
> Kim Barrett has updated the pull request incrementally with one additional commit since the last revision:
> 
>   require non-zero expand size

Looks good!

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Marked as reviewed by sjohanss (Reviewer).

PR: https://git.openjdk.java.net/jdk/pull/2309