enhancement of cmpxchg and copy_to_survivor for ppc64

Christian Thalinger christian.thalinger at oracle.com
Mon Apr 11 17:59:42 UTC 2016

[This should be on hotspot-runtime-dev.  BCC’ing hotspot-compiler-dev.]

> On Apr 8, 2016, at 12:53 AM, Hiroshi H Horii <HORII at jp.ibm.com> wrote:
> Dear all:
> Can I please request reviews for the following change?
> This change was created for JDK 9 and ppc64.
> Description:
> This change adds options of compare-and-exchange for POWER architecture.
> As described in atomic_linux_ppc.inline.hpp, the current implementation of
> cmpxchg is fence_cmpxchg_acquire. This implementation is useful for
> general purposes because twice calls of sync before and after cmpxchg will
> keep consistency. However, they sometimes cause overheads because
> sync instructions are very expensive in the current POWER chip design.
> With this change, callers can explicitly specify to run fence and acquire with
> two additional bool parameters. Because their default values are "true",
> it is not necessary to modify existing cmpxchg calls. 
> In addition, with the new parameters of cmpxchg, this change improves
> performance of copy_to_survivor in the parallel GC. 
> copy_to_survivor changes forward pointers by using cmpxchg. This 
> operation doesn't require any sync instructions, in my understanding. 
> A pointer is changed at most once in a GC and when cmpxchg fails, 
> the latest pointer is available for the caller.
> When I evaluated SPECjbb2013 (slightly customized because obsolete grizzly
> doesn't support new version format of Java 9), pause time of young GC was
> reduced from 10% to 20%.
> Summary of source code changes:
> * src/share/vm/runtime/atomic.hpp
> * src/share/vm/runtime/atomic.cpp
> * src/os_cpu/linux_ppc/vm/atomic_linux_ppc.inline.hpp
>        - Add two arguments of fence and acquire to cmpxchg only for PPC64.
>          Though cmpxchg in atomic_linux_ppc.inline.hpp has some branches,
>          they are reduced while inlining to callers.
> * src/share/vm/oops/oop.inline.hpp
>       - Changed cas_set_mark to call cmpxchg without fence and acquire.
>          cas_set_mark is called only by cas_forward_to that is called only by
>          copy_to_survivor_space and oop_promotion_failed in 
>          psPromotionManager.
> Code change:
>    Please see an attached diff file that was generated with "hg diff -g" 
>    under the latest hotspot directory.
> Passed test:
>     SPECjbb2013 (customized)
> * I believe some other cmpxchg will be optimized by reducing fence 
>   or acquire because twice calls of sync are too conservative to implement
>   Java memory model.
> Regards,
> Hiroshi
> -----------------------
> Hiroshi Horii, Ph.D.
> IBM Research - Tokyo

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