G1 feedback: frantic GC cycles

[Tony Printezis]

Simone,

On 5/12/2011 11:08 AM, Simone Bordet wrote:
>
>>>> -XX:InitiatingHeapOccupancyPercent=60
>>> Tony, would you mind to explain this parameter ?
>> It tells G1 to start a marking cycle when the heap is 60% full after a GC.
> This is related to the full heap ?
> E.g. if young size == old size, and old is 35% full of live objects,
> does that mean that a concurrent GC starts when young is only 25% full
> ?

It refers to the full heap. The lines between young and old gen in G1 
are blurred compared to the other GCs (i.e., we have no physically 
separated spaces for young and old gens compared to our other GCs). So, 
it's understandable (IMHO at least) to define the initiating occupancy 
as a percentage of the overall heap.

> I am trying to compare this to CMSInitiatingOccupancyFraction, which -
> if I am right - refers only to old generation occupation percentage.

This is correct. But, given that CMS uses fixed young and old gen sizes, 
you can easily translate the old percentage to the heap percentage.

> I had the chance to run with -XX:-UseAdaptiveSizePolicy, although I
> have not used the application as heavily as in previous days when I
> reported the logs, and I have not experienced the frantic cycles
> anymore.
> I see very regular logs where the young GC starts when I have 512
> blocks full and ends with 5-15 blocks full, for example:
>
> {Heap before GC invocations=309 (full 48):
>   garbage-first heap   total 1048576K, used 868128K
> [0x00000000b0000000, 0x00000000f0000000, 0x00000000f0000000)
>    region size 1024K, 512 young (524288K), 4 survivors (4096K)
>   compacting perm gen  total 137216K, used 137168K [0x00000000f0000000,
> 0x00000000f8600000, 0x0000000100000000)
>     the space 137216K,  99% used [0x00000000f0000000,
> 0x00000000f85f40b8, 0x00000000f85f4200, 0x00000000f8600000)
> No shared spaces configured.
> 2011-05-12T16:53:26.011+0200: [GC pause (young), 0.11289800 secs]
> ...
> Heap after GC invocations=310 (full 48):
>   garbage-first heap   total 1048576K, used 348126K
> [0x00000000b0000000, 0x00000000f0000000, 0x00000000f0000000)
>    region size 1024K, 4 young (4096K), 4 survivors (4096K)
>   compacting perm gen  total 137216K, used 137168K [0x00000000f0000000,
> 0x00000000f8600000, 0x0000000100000000)
>     the space 137216K,  99% used [0x00000000f0000000,
> 0x00000000f85f40b8, 0x00000000f85f4200, 0x00000000f8600000)
> No shared spaces configured.
> }
>
> and the next young GC again has 512 blocks full.
> I'll confirm this when I have the chance to run the application more
> heavily in the next days, but so far looks good.

Great! Thanks for the feedback.

Tony

> Thanks !
>
> Simon