RFR (XXS): 8202829: Implement CollectedHeap::get_safepoint_workers() for G1

[Erik Osterlund]

Hi Thomas,

A very beneficial one-liner indeed.

Looks good!

Thanks,
/Erik

> On 9 May 2018, at 17:15, Thomas Schatzl <thomas.schatzl at oracle.com> wrote:
> 
> Hi all,
> 
>  due to the recent RFR for ZGC I noticed that G1 would also benefit
> from implementing CollectedHeap::get_safepoint_workers().
> 
> It is a pretty trivial change for G1 as we already have/use a "hot"
> WorkGang that is always available during a safepoint.
> 
> CR:
> https://bugs.openjdk.java.net/browse/JDK-8202829
> Webrev:
> http://cr.openjdk.java.net/~tschatzl/8202829/webrev/
> Testing:
> hs-tier1-3
> 
> Results from random specjbb2015 runs:
> 
> Baseline for "safepoint cleanup tasks":
>              count: 1628
>                min: 7.5e-05
>                max: 0.0072682
>                sum: 4.76701
>                avg: 0.00292814 (+/-0.0016433)
>    95th percentile: 0.0054634
>    99th percentile: 0.0062773
>  99.9th percentile: 0.0072269
> 99.99th percentile: 0.0072682
> 
> With changes for "safepoint cleanup tasks":
>              count: 1772
>                min: 9.24e-05
>                max: 0.0014651
>                sum: 1.33761
>                avg: 0.000754861 (+/-0.000230524)
>    95th percentile: 0.0009985
>    99th percentile: 0.001097
>  99.9th percentile: 0.0012676
> 99.99th percentile: 0.0014651
> 
> So, some significant improvement here, although very small in total.
> Since it is such a simple change I guess we'll take it :)
> 
> I assume in the future the number of threads could be determined more
> cleverly depending on actual work.
> 
> Thanks,
>  Thomas
>