does UseParallelOldGC guarantee a better full gc performance

[Simone Bordet]

Hi,

On Wed, Apr 18, 2012 at 10:58, the.6th.month at gmail.com
<the.6th.month at gmail.com> wrote:
> Hi, Simon:
>
> this is the full gc log for your concern.
> 2012-04-18T16:47:24.824+0800: 988.392: [GC
> Desired survivor size 14876672 bytes, new threshold 1 (max 15)
>  [PSYoungGen: 236288K->8126K(247616K)] 4054802K->3830711K(4081472K),
> 0.0512250 secs] [Times: user=0.15 sys=0.00, real=0.05 secs]
>
> 2012-04-18T16:47:24.875+0800: 988.443: [Full GC [PSYoungGen:
> 8126K->0K(247616K)] [PSOldGen: 3822585K->1751429K(3833856K)]
> 3830711K->1751429K(4081472K) [PSPermGen: 81721K->81721K(262144K)], 6.6108630
> secs] [Times: user=6.62 sys=0.00, real=6.61 secs]
>
> the full gc time is almost unchanged since I enabled paralleloldgc.
>
> Do you have any recommendation for an appropriate young gen size?

Usually, applications generate a lot of short lived objects that can
be reclaimed very efficiently in the young generation.
If you have a small young generation, these objects will be promoted
in old generation, where their collection is usually more expensive.

It really depends what your application does, but I would remove the
-Xmn option for now (leaving the default of 1/3 of the total heap,
i.e. ~1.6 GiB), and see if you get benefits.

As for the times being unchanged, I do not know.
My experience is that UseParallelOldGC works as expected: I frequently
see 1.5-2x gains on 2 cores, and I have seen 6x gains on 8 cores.

Simon
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