CMS cycles triggered by Perm almost being full

[Srinivas Ramakrishna]

Thomas, my recollection is from a while back and I haven't looked at the
code recently, but
CMS does increase perm size as the heap fills up. However, as you may be
implying, the heap size increase is not tied to the CMS perm trigger
setting.
Thus, it's possible that the occupancy of the perm gen is such that it
exceeds
the perm trigger ratio, but is not sufficiently large that the perm gen
will be increased
in size.

As to Kirk's original question, well, it seems strange that the Perm
trigger ratio is being used
as a CMS trigger when CMS class unloading is disabled. I thought that the
CMS initiating
trigger was tied to the occupancy of the perm only when class unloading was
enabled,
and was otherwise only affected by the occupancy of the old generation. You
can enable
a flag that makes CMS more talkative about why it's starting a CMS
collection and that
might provide a clue. Perhaps there's a bug in the trigger.

-- ramki


On Wed, Oct 2, 2013 at 12:12 AM, Thomas Viessmann <
thomas.viessmann at oracle.com> wrote:

>  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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