RFR (S): 8020155: PSR:PERF G1 not collecting old regions when humongous allocations interfer

[Bengt Rutisson]

Hi everyone,

Could I get a couple of reviews for this small change?

http://cr.openjdk.java.net/~brutisso/8020155/webrev.00/

When the marking phase finishes we signal that we should start doing 
mixed collections by setting _last_young_gc=true. The next GC will be a 
normal young GC but after that we will start doing mixed GCs.

G1CollectorPolicy::need_to_start_conc_mark() will return false if a 
marking cycle is already in progress or if we are doing mixed GCs. But 
it did not check the intermediate state represented by _last_young_gc. 
Instead this was explicitly checked by 
G1CollectorPolicy::record_collection_pause_end() for young GCs. However, 
need_to_start_conc_mark() is also used by humongous object allocations 
in two places. So, if a humongous allocation happens after marking has 
finished but before the following young GC has happened, we might 
trigger a new marking cycle and skip the mixed GCs.

The fix is to move the check of _last_young_gc in to 
need_to_start_conc_mark() to make sure that we don't skip mixed GCs.

Thanks,
Bengt