RFR: 8256298: Shenandoah: Enable concurrent stack processing [v2]

[Roman Kennke]

On Sun, 24 Jan 2021 01:46:56 GMT, Zhengyu Gu <zgu at openjdk.org> wrote:

>> Please review this patch that enables concurrent stack processing for Shenandoah GC.
>> 
>> After this patch, all root processing is done concurrently for concurrent GC.
>> 
>> Test:
>> - [x] hotspot_gc_shenandoah Linux x86_64 and x86_32
>> - [x] Nightly
>> - [x] tier1 with -XX:+UseShenandoahGC on Linux x86_32
>
> Zhengyu Gu has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Minor cleanup

Wow that looks very nice and clean! I only have a few minor comments.

src/hotspot/share/gc/shenandoah/shenandoahHeap.hpp line 512:

> 510: //
> 511: public:
> 512:   bool uses_stack_watermark_barrier() const { return true; }

This overrides a CollectedHeap method. I wonder if we should start adding 'override' keywords to overridden methods. However, this requires >=C++11, and might disturb backporting. Not sure about it.

src/hotspot/share/gc/shenandoah/shenandoahStackWatermark.cpp line 2:

> 1: /*
> 2:  * Copyright (c) 2020, Red Hat, Inc. All rights reserved.

This is structurally similar to zStackWatermark.cpp, and perhaps 'inspired' by it. Consider adding Oracle copyright there in addition to the Red Hat copyright. We did the same in e.g. shenandoahReferenceProcessor.hpp/cpp. Also, bump the copyright year to 2021 maybe?

src/hotspot/share/gc/shenandoah/shenandoahStackWatermark.hpp line 2:

> 1: /*
> 2:  * Copyright (c) 2020, Red Hat, Inc. All rights reserved.

Same comment as in shenandoahStackWatermark.cpp above.

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Changes requested by rkennke (Reviewer).

PR: https://git.openjdk.java.net/jdk/pull/2185