RFR(M): 8220089: G1 wastes a significant amount of space in survivor regions

sangheon.kim at oracle.com sangheon.kim at oracle.com
Thu Mar 28 19:51:52 UTC 2019


Hi all,

Please hold reviewing this.

During offline discussion with Thomas, he found a problem with 
MemoryMXBeans. As proposed patch is changing survivor region to mutator 
region, used bytes as survivor region will not count the retained 
survivor region. This problem will be addressed by JDK-8218049 but as 
this patch also would be affected, I propose to hold reviewing this.

Thanks,
Sangheon


On 3/27/19 10:49 PM, sangheon.kim at oracle.com wrote:
> Hi all,
>
> Can I have some reviews for the patch that changes to reuse survivor 
> region as mutator region?
>
> Currently survivor region is released at the end of gc and then added 
> to the next collection set. This means there would be waste if the 
> survivor region has enough free space. As the survivor regions are 
> converted to eden at the start of gc, reusing the *last* survivor 
> region as a mutator region would maximize the use of heap region. This 
> patch suggests to reuse the survivor region as described if the 
> remaining free space satisfies a minimum threshold(currently, larger 
> than or equal to minimum TLAB).
>
> Some benchmarks(specjbb2015, specjbb2005, specjvm2008) that I ran 
> didn't have noticeable improvement. However, I think this change would 
> benefit more with NUMA implementation because the NUMA implementation 
> manages survivor region per NUMA node. i.e. more wastes on multiple 
> node system.
>
> CR: https://bugs.openjdk.java.net/browse/JDK-8220089
> webrev: http://cr.openjdk.java.net/~sangheki/8220089/webrev.0
>              (based on JDK-8218668 webrev.1 which is under review now)
> Testing: hs-tier1 ~ 5
>
> Thanks,
> Sangheon



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