RFR: 8255019: Shenandoah: Split STW and concurrent mark into separate classes [v29]

[Aleksey Shipilev]

On Wed, 13 Jan 2021 13:09:18 GMT, Zhengyu Gu <zgu at openjdk.org> wrote:

>> This is the first part of refactoring, that aims to isolate three Shenandoah GC modes (concurrent, degenerated and full gc).
>> 
>> Shenandoah started with two GC modes, concurrent and full gc, with minimal shared code, mainly in mark phase. After introducing degenerated GC, it shared quite large portion of code with concurrent GC, with the concept that degenerated GC can simply pick up remaining work of concurrent GC in STW mode.
>> 
>> It was not a big problem at that time, since concurrent GC also processed roots STW. Since Shenandoah gradually moved root processing into concurrent phase, code started to diverge, that made code hard to reason and maintain.
>> 
>> First step, I would like to split STW and concurrent mark, so that:
>> 1) Code has to special case for STW and concurrent mark.
>> 2) STW mark does not need to rendezvous workers between root mark and the rest of mark
>> 3) STW mark does not need to activate SATB barrier and drain SATB buffers.
>> 4) STW mark does not need to remark some of roots.
>> 
>> The patch mainly just shuffles code.  Creates a base class ShenandoahMark, and moved shared code (from current shenandoahConcurrentMark) into this base class. I did 'git mv shenandoahConcurrentMark.inline.hpp  shenandoahMark.inline.hpp, but git does not seem to reflect that.
>> 
>> A few changes:
>> 1) Moved task queue set from ShenandoahConcurrentMark to ShenandoahHeap. ShenandoahMark and its subclasses are stateless. Instead, mark states are maintained in task queue, mark bitmap and SATB buffers, so that they can be created on demand.
>> 2) Split ShenandoahConcurrentRootScanner template to ShenandoahConcurrentRootScanner and ShenandoahSTWRootScanner
>> 3) Split code inside op_final_mark code into finish_mark and prepare_evacuation helper functions.
>> 4) Made ShenandoahMarkCompact stack allocated (as well as ShenandoahConcurrentGC and ShenandoahDegeneratedGC in upcoming refactoring)
>
> Zhengyu Gu has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains 36 commits:
> 
>  - Merge branch 'master' into JDK-8255019-sh-mark
>  - Fixes based on shade's comments
>  - Merge branch 'master' into JDK-8255019-sh-mark
>  - Silent MacOSX build
>  - @shade's comments
>  - Merge
>  - Update copyright years
>  - Merge
>  - Merge branch 'master' into JDK-8255019-sh-mark
>  - Concurrent mark does not expect forwarded objects
>  - ... and 26 more: https://git.openjdk.java.net/jdk/compare/ce945120...4fbbfd27

All right, fine, let's do it.

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Marked as reviewed by shade (Reviewer).

PR: https://git.openjdk.java.net/jdk/pull/1009