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

[Derek White]

Hi Thomas,

Looks good!

- Derek

On 3/3/16 9:07 AM, 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