RFR (XS): 8150952: Net PLAB size is clipped to max PLAB size as a whole, not on a per thread basis

[Thomas Schatzl]

Hi all,

  could I have a "R"eviewer review for this change?

Thanks,
  Thomas

On Thu, 2016-03-03 at 15:07 +0100, Thomas Schatzl wrote:
> Hi all,
> 
>   can I have reviews for this change that fixes PLAB sizing clipping
> after we changed the desired PLAB size to be recorded without taking
> the number of threads to use into account?
> 
> The problem is that after calculating the net desired plab size, we
> clip it to min/max sizes. This means that, when handing out the
> desired
> PLAB sizes depending on the number of threads, if the number of
> threads
> is really large, the PLAB sizes get way too small.
> 
> I have seen some application logs where due to high number of
> threads,
> around 90%(!) of multiple GB of allocations during GC is direct
> -allocated. Instead of using PLABs...
> 
> This change fixes this issue.
> 
> CR:
> https://bugs.openjdk.java.net/browse/JDK-8150952
> Webrev:
> cr.openjdk.java.net/~tschatzl/8150952/webrev
> Testing:
> jprt, vm.gc, looking at several benchmarks to see whether the problem
> still occurs
> 
> Thanks,
>   Thomas