Why does G1GC do Full GC when it only needs 40 bytes?

[YU ZHANG]

Martin,

The log shows full gc happened when it tries to satisfy a 40 byte 
allocation request.
-XX:InitiatingHeapOccupancyPercent=0 should trigger concurrent gcs too 
much.

What is the jdk version?   Can you share the gc log?

Thanks,
Jenny

On 6/7/2014 1:03 AM, Martin Makundi wrote:
> Why does G1GC do Full GC when it only needs only 40 bytes?
>
> Is there a way to tune this so that it would try to free up "some" 
> chunk of memory and escape the full gc when enough memory has been 
> freed? This  would lower the freeze time?
>
> See logs:
>
>  [Times: user=12.89 sys=0.16, real=1.25 secs]
> 2014-06-06T17:11:48.727+0300: 34022.631: [GC 
> concurrent-root-region-scan-start]
> 2014-06-06T17:11:48.727+0300: 34022.631: [GC 
> concurrent-root-region-scan-end, 0.0000180 secs]
> 2014-06-06T17:11:48.727+0300: 34022.631: [GC concurrent-mark-start]
>  34022.632: [G1Ergonomics (Heap Sizing) attempt heap expansion, 
> reason: allocation request failed, allocation request: 40 bytes]
>  34022.632: [G1Ergonomics (Heap Sizing) expand the heap, requested 
> expansion amount: 8388608 bytes, attempted expansion amount: 8388608 
> bytes]
>  34022.632: [G1Ergonomics (Heap Sizing) did not expand the heap, 
> reason: heap expansion operation failed]
> {Heap before GC invocations=1867 (full 1):
>  garbage-first heap   total 31457280K, used 31453781K 
> [0x00007f1724000000, 0x00007f1ea4000000, 0x00007f1ea4000000)
>   region size 8192K, 0 young (0K), 0 survivors (0K)
>  compacting perm gen  total 524288K, used 166271K [0x00007f1ea4000000, 
> 0x00007f1ec4000000, 0x00007f1ec4000000)
>    the space 524288K,  31% used [0x00007f1ea4000000, 
> 0x00007f1eae25fdc8, 0x00007f1eae25fe00, 0x00007f1ec4000000)
> No shared spaces configured.
> 2014-06-06T17:11:48.728+0300: 34022.632: [Full GC 29G->13G(30G), 
> 47.7918670 secs]
>
>
> Settings:
>
> -server -XX:InitiatingHeapOccupancyPercent=0 -Xss4096k 
> -XX:MaxPermSize=512m -XX:PermSize=512m -Xms20G -Xmx30G -Xnoclassgc 
> -XX:-OmitStackTraceInFastThrow -XX:+UseNUMA 
> -XX:+UseFastAccessorMethods -XX:ReservedCodeCacheSize=128m 
> -XX:-UseStringCache -XX:+UseGCOverheadLimit -Duser.timezone=EET 
> -XX:+UseCompressedOops -XX:+DisableExplicitGC -XX:+AggressiveOpts 
> -XX:CMSInitiatingOccupancyFraction=70 -XX:+ParallelRefProcEnabled 
> -XX:+UseAdaptiveSizePolicy -XX:MaxGCPauseMillis=500 -XX:+UseG1GC 
> -XX:G1HeapRegionSize=8M -XX:GCPauseIntervalMillis=10000 
> -XX:+PrintGCDetails -XX:+PrintHeapAtGC -XX:+PrintAdaptiveSizePolicy 
> -XX:+PrintGCDateStamps -XX:+PrintGC -Xloggc:gc.log
>
>
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