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

[John Cuthbertson]

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