RFR (S): 8022872: G1: Use correct GC cause for young GC triggered by humongous allocations

[Bengt Rutisson]

Hi All,

Could I have a couple for reviews for this small change?

http://cr.openjdk.java.net/~brutisso/8022872/webrev.00/

This change makes sure that we report "humongous allocation" as the GC 
cause when we trigger a young collection because we are attempting a 
humongous allocation.

Before the change a log entry for such a young GC looked like this:

[GC pause (G1 Evacuation Pause) (young) 1364M->1364M(2048M), 0.0004200 secs]

After the change it looks like this:

[GC pause (G1 Humongous Allocation) (young) 1364M->1364M(2047M), 
0.0541270 secs]

While doing this change I realized that the GC cause 
_g1_inc_collection_pause was only used in one more place. The slow path 
for allocations. There is already a cause called _allocation_failure, so 
I suggest that we use that one instead. This will eliminate the need for 
_g1_inc_collection_pause.

In that case the webrev will look like this:

http://cr.openjdk.java.net/~brutisso/8022872/webrev_incCause/

I prefer this cleanup, but I'm fine with just doing the first one if we 
think _g1_inc_collection_pause cause to keep for some reason.

Thanks,
Bengt
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