Integrated: 8261493: Shenandoah: reconsider bitmap access memory ordering

[Aleksey Shipilev]

On Wed, 10 Feb 2021 09:32:18 GMT, Aleksey Shipilev <shade at openjdk.org> wrote:

> Shenandoah currently uses its own marking bitmap (added by JDK-8254315). It accesses the marking bitmap with "acquire" for reads and "conservative" for 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.
> 
> I think both are actually excessive for marking bitmap accesses: we do not piggyback object updates on it, the atomics there are only to guarantee the access atomicity and CAS updates to bits.  It seems "relaxed" is enough for marking bitmap accesses.
> 
> Sample run with "compact" (frequent GC cycles) on SPECjvm2008:compiler.sunflow on AArch64:
> 
> # Baseline
> # Baseline
> [146.028s][info][gc,stats] Concurrent Marking =   50.315 s (a =   258024 us) (n =   195) (lvls, us =    31836,   230469,   273438,   306641,   464255)
> [141.458s][info][gc,stats] Concurrent Marking =   47.819 s (a =   242737 us) (n =   197) (lvls, us =    42773,   197266,   267578,   287109,   433948)
> [144.108s][info][gc,stats] Concurrent Marking =   49.806 s (a =   250283 us) (n =   199) (lvls, us =    32227,   201172,   267578,   296875,   448549)
> 
> # Patched
> [144.238s][info][gc,stats] Concurrent Marking =   46.627 s (a =   220981 us) (n =   211) (lvls, us =    24414,   197266,   238281,   259766,   345112)
> [138.406s][info][gc,stats] Concurrent Marking =   45.022 s (a =   227383 us) (n =   198) (lvls, us =    20508,   205078,   244141,   271484,   427658)
> [140.950s][info][gc,stats] Concurrent Marking =   45.073 s (a =   222036 us) (n =   203) (lvls, us =    21680,   181641,   240234,   265625,   375750)
> 
> Average time goes down, total marking time goes down.
> 
> Additional testing:
>  - [x] Linux x86_64 `hotspot_gc_shenandoah`
>  - [x] Linux AArch64 `hotspot_gc_shenandoah`
>  - [x] Linux AArch64 `tier1` with Shenandoah

This pull request has now been integrated.

Changeset: 745c0b91
Author:    Aleksey Shipilev <shade at openjdk.org>
URL:       https://git.openjdk.java.net/jdk/commit/745c0b91
Stats:     18 lines in 2 files changed: 0 ins; 14 del; 4 mod

8261493: Shenandoah: reconsider bitmap access memory ordering

Reviewed-by: rkennke, zgu

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

PR: https://git.openjdk.java.net/jdk/pull/2497