RFR: 8261495: Shenandoah: reconsider update references memory ordering [v2]

[Aleksey Shipilev]

> Shenandoah update heap references code uses default Atomic::cmpxchg to avoid races with mutator updates. Hotspot's default for atomic operations is memory_order_conservative, which emits two-way memory fences around the CASes at least on AArch64 and PPC64.
> 
> This seems to be excessive for Shenandoah update references code, and "acq_rel" is enough. We do not seem to piggyback on update-references memory effects anywhere (in fact, if not for mutator, we would not even need a CAS). But, there is an interplay with concurrent evacuation and updates from self-healing.
> 
> Sample run with aggressive (back-to-back cycles) on SPECjvm2008:compiler.compiler on AArch64:
> 
> # Baseline
> [135.065s][info][gc,stats] Concurrent Update Refs         =   73.685 s (a =   295924 us) (n =   249) 
>     (lvls, us =      354,     3418,   349609,   564453,   715405)
> 
> # Patched
> [127.649s][info][gc,stats] Concurrent Update Refs         =   54.389 s (a =   169437 us) (n =   321)
>     (lvls, us =      324,     2188,   183594,   322266,   394495)
> 
> Average time goes down, the number of GC cycles go up, since the cycles are shorter.
> 
> Additional testing:
>  - [x] Linux x86_64 hotspot_gc_shenandoah
>  - [x] Linux AArch64 hotspot_gc_shenandoah
>  - [x] Linux AArch64 tier1 with Shenandoah

Aleksey Shipilev has updated the pull request incrementally with one additional commit since the last revision:

  Do acq_rel instead

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

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/2498/files
  - new: https://git.openjdk.java.net/jdk/pull/2498/files/d83b9af4..87a609f4

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=2498&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=2498&range=00-01

  Stats: 41 lines in 1 file changed: 38 ins; 0 del; 3 mod
  Patch: https://git.openjdk.java.net/jdk/pull/2498.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/2498/head:pull/2498

PR: https://git.openjdk.java.net/jdk/pull/2498