Request for review (S): 7163848: G1: Log GC Cause for a GC

[Bengt Rutisson]

Hi everyone,

Could I have a couple of reviews for this change?
http://cr.openjdk.java.net/~brutisso/7163848/webrev.00/

Background

Many times it is useful to know why a GC happened. The GC cause tells us 
that, but it has not been exposed in the log file before. The change 
adds that information to the log entries for PrintGC.

Here are some examples of what the logging looks like with this change:

[GC pause (young) (G1 Evacuation Pause) 149M->118M(244M), 0.2433432 secs]
[GC pause (young) (initial-mark) (G1 Evacuation Pause) 160M->128M(247M), 
0.1768634 secs]
[GC pause (young) (initial-mark) (G1 Humongous Allocation) 
32M->32M(100M), 0.0104583 secs]
[GC pause (mixed) (G1 Evacuation Pause) 128M->82M(256M), 0.5632070 secs]
[Full GC (System.gc()) 417K->321K(100M), 0.0324733 secs]
[Full GC (Permanent Generation Full) 285K->285K(7168K), 0.0270867 secs]
[Full GC (Last ditch collection) 285K->275K(7168K), 0.0268307 secs]
[Full GC (No GC)[GC concurrent-root-region-scan-end, 0.0007064]

We might want to re-phrase the GC cause names, but that should probably 
be a separate CR since it will affect jstat and others that use the 
GCCause::to_string() method.

Thanks,
Bengt