G1 log output interpretation

[Simone Bordet]

Hi,

I am using 1.7.0_09 with:

-XX:InitialHeapSize=1073741824
-XX:MaxHeapSize=1073741824
-XX:MaxPermSize=268435456
-XX:ReservedCodeCacheSize=67108864
-XX:InitiatingHeapOccupancyPercent=60
-XX:MaxGCPauseMillis=200
-XX:+PrintAdaptiveSizePolicy
-XX:+PrintCommandLineFlags
-XX:+PrintGCDateStamps
-XX:+PrintGCDetails
-XX:+UseCompressedOops
-XX:+UseG1GC

and I get this log, from time to time:

2012-11-05T12:09:02.422+0100: [GC pause (young) 2341.660: [G1Ergonomics
(CSet Construction) start choosing CSet, predicted base time: 65.42 ms,
remaining time: 134.58 ms, target pause time: 200.00 ms]
 2341.660: [G1Ergonomics (CSet Construction) add young regions to CSet,
eden: 198 regions, survivors: 6 regions, predicted young region time: 27.03
ms]
 2341.660: [G1Ergonomics (CSet Construction) finish choosing CSet, eden:
198 regions, survivors: 6 regions, old: 0 regions, predicted pause time:
92.45 ms, target pause time: 200.00 ms]
 2341.739: [G1Ergonomics (Heap Sizing) attempt heap expansion, reason:
recent GC overhead higher than threshold after GC, recent GC overhead:
11.81 %, threshold: 10.00 %, uncommitted: 0 bytes, calculated expansion
amount: 0 bytes (20.00 %)]
, 0.07907900 secs]

The relevant section is the last line, marked as G1Ergonomics (Heap Sizing).

Can someone shed a light on what it means ? Seems an attempt to expand, but
calculates to 0 ?
I remember G1 has some "extra" allocation space that can be used beyond
-Xmx, do I remember well ?

Thanks !

Simon
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