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

[Kim Barrett]

> 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

-------------

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/2309/files
  - new: https://git.openjdk.java.net/jdk/pull/2309/files/12500d49..d67d5e20

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=2309&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=2309&range=00-01

  Stats: 14 lines in 1 file changed: 1 ins; 6 del; 7 mod
  Patch: https://git.openjdk.java.net/jdk/pull/2309.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/2309/head:pull/2309

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