RFR: 8261495: Shenandoah: reconsider update references memory ordering

[Zhengyu Gu]

On Wed, 10 Feb 2021 09:52:11 GMT, Aleksey Shipilev <shade at openjdk.org> wrote:

> 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 "relaxed" 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).
> 
> 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

src/hotspot/share/gc/shenandoah/shenandoahHeap.inline.hpp line 149:

> 147:   assert(is_aligned(addr, sizeof(narrowOop)), "Address should be aligned: " PTR_FORMAT, p2i(addr));
> 148:   narrowOop val = CompressedOops::encode(n);
> 149:   return CompressedOops::decode(Atomic::cmpxchg(addr, c, val, memory_order_relaxed));

Are you sure it is sufficient? I would think it needs acq/rel pair, otherwise, read side can see incomplete oop ...

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