CMS cycles triggered by Perm almost being full

[Thomas Viessmann]

Hi Kirk,

Disclaimer: I'm in support and all I can share is my experience:

You always need a full GC if heap resizing is needed.
CMS  AFAIHS cannot initiate an increase of perm.
Sooner or later it will bail out.
I always start with the heap sizing first. Making sure that
all areas have sufficient capacity and a fixed size in order
to avoid constant CMS runs or even bail outs.

Thanks and Regards

Thomas


On 10/02/13 04:38, Kirk Pepperdine wrote:
> Hi,
>
> I've just witnessed in 1.7.0_17-b02 (Solaris AMD) CMS cycles being initiated every 7.390 seconds. The system is idle and there are no foreground (ParNew) collections running. Perm occupancy is quite close to it's configured size so it's quite likely that the cause of the CMS cycle is this. However, Class unloading is not enabled and thus the CMS cycle doesn't "fix" the trigger by cleaning out perm or being able to enlarge it (configured size < max size) and there isn't any pressure for a Full collection (CMF??). Triggering a collection (System.gc()) of course "fixes" the problem (facilitates a perm space expansion).
>
> Ok, so there are work arounds for this but it really confused the person who contacted me with the problem and he's no slouch when it comes to GC. I've advised him to turn on perm space sweeping with class unloading. That said, it seems that CMS should know that it's not going to be able to fix the problem that triggered to to run and it should degrade into a CMF, reason perm space needs resizing. My questions are, have I missed something? Should this be filed as a bug? Or, is this as intended?
>
> On a side note I found the 7.390 second period to be an interesting distraction.
>
> Regards,
> Kirk

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