RFR: 8267703: runtime/cds/appcds/cacheObject/HeapFragmentationTest.java crashed with OutOfMemory

[Stefan Johansson]

On Thu, 27 May 2021 13:04:23 GMT, Stefan Johansson <sjohanss at openjdk.org> wrote:

> Please review this change to lower fragmentation when heap usage is low.
> 
> **Summary**
> The above test case fails because the G1 Full GC fails to compact the single used region below the needed threshold. In this case the region needs to be compacted below region index 400 to be able to fit the large array. The reason this fails is that the full GC uses a lot of parallel workers and if the system is under load it is possible that the worker finding the region to compact hasn't been able to claim any regions low enough in the heap to compact the live objects to.
> 
> To fix this we can add a third thing to consider in the code calculating the number of workers to use for a given compaction. So far we only look at keeping the waste down and using the default adaptive calculations. If we also consider how much is used we can get a lower worker count in cases like this and that will make it much more likely to succeed with the compaction. In this case it will guarantee it since there is a single region used, so there will be only one worker and then it will compact the region to the bottom of the heap.
> 
> **Testing**
> Manual verification that this will cause these collections to only use a single worker. Also currently running some performance regression testing to make sure this doesn't cause any big regressions.

As an additional note, this will cause a Full GC pause time increase for cases where few regions are used. So we might want to change the logic to require fewer regions per worker. The reason I used `HeapSizePerGCThread` was just because this is an existing flag that is already used for similar calculations. We could of course say that G1s policy in don't care about this and instead just makes sure to limit the number of workers to the number of used regions. This will lower the risk of getting big regressions.

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PR: https://git.openjdk.java.net/jdk/pull/4225