RFR (XS): 7045751 G1: +ExplicitGCInvokesConcurrent causes excessive single region evacuation pauses

[Y. Srinivas Ramakrishna]

Looks good. One question: we return when the marking cycle is
complete, or when the marking cycle _and_ the succeeding
concurrent clean-up is also complete?

- ramki

On 6/13/2011 10:16 AM, John Cuthbertson wrote:
> Hi Everyone,
>
> Can I have a couple of volunteers look of the small fix for this CR? The webrev can be found at:
> http://cr.openjdk.java.net/~johnc/7045751/webrev.0/
>
> The issue here was that if we had multiple threads executing a System.gc() (with
> +ExplicitGCInvokesConcurrent) we would have a thundering herd trying to start an initial mark
> evacuation pause. One thread would win, do the pause, and start the marking cycle, and block until
> the cycle complete. The next thread in would see that a marking cycle was in progress and do a
> regular evacuation pause. If none of the mutators (that were not trying to do a System.gc() call)
> did not manage to run, the collection set for this second pause would comprise of only the survivors
> from the first pause. The second thread would then block until the marking cycle was complete. And
> so on for the remaining threads requesting a System.gc().
>
> The solution is to skip the evacuation pause if we fail to force it to be an initial mark pause and
> just block (as normal) until the existing cycle completes.
>
> Tested with: GC test suite with and without ExplicitGCInvokesConcurrent.
>
> Thanks,
>
> JohnC