RFR: 8236926: Concurrently uncommit memory in G1 [v2]

[Stefan Johansson]

On Thu, 12 Nov 2020 22:00:40 GMT, Albert Mingkun Yang <ayang at openjdk.org> wrote:

>> Stefan Johansson has updated the pull request incrementally with one additional commit since the last revision:
>> 
>>   Lock for small mapper and use BitMap parallel operations.
>
> The PR description provides a very good summary. Furthermore, it would be nice to have an overview of the algorithm: how the additional concurrency managed, what resources are protected by the lock, etc. Ideally, this should be in the comments, making references to corresponding classes/variables.
> 
>> I've also done a performance run and as expected there are not significant changes.
> 
> How about pauses? One of the motivations of this PR is to remove pauses introduced by synchronous uncommit, right?

Thanks @albertnetymk for your comments, this update does not include any updates for those. This instead fixes a race in the low level committing code for G1 where it was assumed only one thread can commit and uncommit regions at a time. This is no longer the case after this change. It is true for a single region, but adjacent regions are allowed to be committed/uncommitted in parallel. This can for example happen if a humongous allocation happens during concurrent uncommit. 

The fix is to use parallel versions of the bitmap operations and for the `G1RegionsSmallerThanCommitSizeMapper` add a lock to prevent parallel updates for this mapper. This is needed because multiple regions can share a single underlying OS page, so we need to make sure those updates are atomic on a page level.

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PR: https://git.openjdk.java.net/jdk/pull/1141