Integrated: Concurrent remembered set scanning

[Kelvin Nilsen]

On Tue, 11 May 2021 14:40:15 GMT, Kelvin Nilsen <kdnilsen at openjdk.org> wrote:

> This pull request includes code to perform remembered set scanning concurrently with ongoing mutator execution.  As currently implemented, the remembered set is copied to a read-only remembered set during init-mark safepoint.  After copying, all cards in the remembered set are restored to a clean value.  Following the init-mark safepoint, mutator thread write barriers update the active remembered set copy every time a reference field is overwritten by dirtying the corresponding cards.
> 
> Concurrent remembered set scanning for the purpose of marking works with the read-only copy of the remembered set.  When concurrent remembered set scanning encounters an interesting pointer, the corresponding card of the active remembered set is overwritten dirty.
> 
> The remembered set is scanned a second time concurrently during the update-refs phase of gc.  The second scan uses the same read/write active copy of the remembered set that is being overwritten by the mutator write barrier.
> 
> The SATB barrier and load reference barriers are sufficient to resolve any races between setting of dirty flags and scanning of the remembered set.
> 
> A possible future enhancement involves replacing the remembered set copy (during init-mark safepoint) with a simple pointer swap operation to exchange the roles of a read-byte-map and a write-byte-map.  Making this change is deferred because it requires far-reaching changes to the implementations of mutator write barriers.  As currently implemented, these barriers hard-code the address of the card-table byte map.
> 
> This patch passes tier1 and hotspot_gc_shenandoah regression tests.

This pull request has now been integrated.

Changeset: d775516e
Author:    Kelvin Nilsen <kdnilsen at openjdk.org>
Committer: Roman Kennke <rkennke at openjdk.org>
URL:       https://git.openjdk.java.net/shenandoah/commit/d775516e9b2216735fa059f8e986c7a8d280c9f0
Stats:     1658 lines in 46 files changed: 1287 ins; 123 del; 248 mod

Concurrent remembered set scanning

Reviewed-by: rkennke

-------------

PR: https://git.openjdk.java.net/shenandoah/pull/37