RFR: 8256298: Shenandoah: Enable concurrent stack processing [v6]

[Aleksey Shipilev]

On Mon, 25 Jan 2021 18:27:02 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:
> 
>   Reverted override

Good job, I have only a few minor comments.

src/hotspot/share/gc/shenandoah/shenandoahPhaseTimings.hpp line 56:

> 54:   f(init_manage_tlabs,                              "  Manage TLABs")                  \
> 55:   f(init_update_region_states,                      "  Update Region States")          \
> 56:   f(scan_roots,                                     "  Scan Roots")                    \

Is `scan_roots` used anywhere still?

src/hotspot/share/gc/shenandoah/shenandoahPhaseTimings.hpp line 59:

> 57:   SHENANDOAH_PAR_PHASE_DO(scan_,                    "    S: ", f)                      \
> 58:                                                                                        \
> 59:   f(conc_mark_roots,                                "Concurrent Mark Roots ")          \

This seems like fixing the regression from JDK-8255765, is it not? Should be a separate issue?

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Changes requested by shade (Reviewer).

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