RFR (XXS): 8134857: Inconsistency in maximum TLAB/PLAB size and humonguous object size

[Thomas Schatzl]

Hi,

On Tue, 2015-09-01 at 18:07 +0200, Thomas Schatzl wrote:
> Hi all,
> 
>   can I have another review about PLAB argument processing? G1 now only
> allows a maximum TLAB/PLAB size of heap region size / 2 - 1 words
> (because with JDK-8078555 the bounds are actually checked).
> 
> This is wrong. Only heap region size / 2 + 1 objects are considered
> humongous (see G1CollectedHeap::is_humongous()). This bug disallows
> specification of Young- and OldPLABSizes of exactly heap region size /
> 2, which is inconvenient.
> 
> Finding out the exact value for 2^(heap region size words)/2-1 for some
> testing is much more work than just specifying the "round" value.
> 
> There is also no reason to disallow 2^(heap region size words) word
> sized T/PLABs.
> 
> CR:
> https://bugs.openjdk.java.net/browse/JDK-8134857
> Webrev:
> http://cr.openjdk.java.net/~tschatzl/8134857/webrev/
> Testing:
> jprt

  fyi, and to hopefully make reviewing easier, the change passed a vm.gc
testsuite run too.

Thanks,
  Thomas