RFR (S): 8067339: PLAB reallocation might result in failure to allocate object in that recently allocated PLAB

[Thomas Schatzl]

Hi all,

  can I have reviews for the following small fix: if G1 reallocates a
PLAB for an object of size X, it allocates a new PLAB of size Y. 

PLABs are required to contain enough space for the object plus some
alignment reserve; so if X >= Y - alignment-reserve, G1 failed to
allocate this object it just allocated the PLAB for into that PLAB,
resulting in an evacuation failure although there is actually enough
space.

As far as I know this has been a rare occurrence until now because
current PLAB sizing tended to maximize PLAB sizes, and it is rare to
have such large objects, but with future changes that actually try to
size the PLABs so that the expected waste is kept, I noticed a few cases
of that issue occurring. So this needs to be fixed.

The fix is to consider the required alignment reserve in the PLAB
allocation request.

This is a day one G1 bug.

CR:
https://bugs.openjdk.java.net/browse/JDK-8067339
Webrev:
http://cr.openjdk.java.net/~tschatzl/8067339/webrev/
Testing:
JPRT, vm.gc testlist, lots of testing.

Thanks,
  Thomas