RFR: 8069273: Decrease Hot Card Cache Lock contention

[Claes Redestad]

Hi all,

updated webrev: http://cr.openjdk.java.net/~redestad/8069273/webrev.02/

Changes: after some helpful suggestions offline from Stefan Karlsson I've
replaced the cmpxchg loop on _hot_cache_idx with use of Atomic::add.
This is arguably cleaner and also improves performance dramatically on
architectures with support for atomic adds.

/Claes

On 01/22/2015 05:27 PM, Claes Redestad wrote:
> Hi all,
>
> please review this patch which replaces the use of mutex-based locking in
> G1HotCardCache with a non-blockingCAS loop.
>
> webrev: http://cr.openjdk.java.net/~redestad/8069273/webrev.01/
> bug: https://bugs.openjdk.java.net/browse/JDK-8069273
>
> This improves performance in a microbenchmark designed to indirectly
> stress the "card is hot" branch of G1HotCardCache::insert[1]. More 
> thorough
> performance testing is in progress.
>
> An initial prototype did not isolate the _hot_cache_idx, which introduced
> catastrophic levels of false sharing on nearby fields, ending up in
> odd results and no real effect on total throughput. The padding 
> introduced
> around_hot_cache_idx and _hot_cache_par_claimed_idx was necessary to
> address this.
>
> Testing: JPRT -testset hotspot
>
> /Claes
>
> [1] http://cr.openjdk.java.net/~redestad/8069273/G1HotCardBench.java