ParNew - how does it decide if Full GC is needed

[Jon Masamitsu]

On 05/07/2014 05:55 PM, Vitaly Davidovich wrote:
>
> Yes, I know :) This is some cruft that needs to be cleaned up.
>
> So my suspicion is that full gc is triggered precisely because old gen 
> occupancy is almost 100%, but I'd appreciate confirmation on that.  
> What's surprising is that even though old gen is almost full, young 
> gen has lots of room now. In fact, this system is restarted daily so 
> we never see another young gc before the restart.
>
> The other odd observation is that survivor spaces are completely empty 
> after this full gc despite tenuring threshold not being adjusted.
>

The full gc algorithm used compacts everything (old gen and young gen) into
the old gen unless it does not all fit.   If the old gen overflows, the 
young gen
is compacted into itself.  Live in the young gen is compacted into eden 
first and
then into the survivor spaces.
>
> My intuitive thinking is that there was no real reason for the full gc 
> to occur; whatever allocation failed in young could now succeed and 
> whatever was tenured fit, albeit very tightly.
>

Still puzzling about the full GC.  Are you using CMS?  If you have 
PrintGCDetails output,
that might help.

Jon
>
> Sent from my phone
>
> On May 7, 2014 8:40 PM, "Bernd Eckenfels" <bernd-2014 at eckenfels.net 
> <mailto:bernd-2014 at eckenfels.net>> wrote:
>
>     Am Wed, 7 May 2014 19:34:20 -0400
>     schrieb Vitaly Davidovich <vitalyd at gmail.com
>     <mailto:vitalyd at gmail.com>>:
>
>     > The vm args are:
>     >
>     > -Xms16384m -Xmx16384m -Xmn16384m -XX:NewSize=12288m
>     > -XX:MaxNewSize=12288m -XX:SurvivorRatio=10
>
>     Hmm... you have confliciting arguments here, MaxNewSize overwrites
>     Xmn.
>     You will get 16384-12288=4gb old size, thats quite low. As you can see
>     in your FullGC the steady state after FullGC has filled it nearly
>     completely.
>
>     Gruss
>     Bernd
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