Runtime.getRuntime().freeMemory() with ZGC

[Mike Rettig]

On Fri, May 28, 2021 at 1:16 PM Per Liden <per.liden at oracle.com> wrote:

>
>
> On 5/28/21 5:51 PM, Mike Rettig wrote:
> >
> >
> > On Fri, May 28, 2021 at 10:13 AM Per Liden <per.liden at oracle.com
> > <mailto:per.liden at oracle.com>> wrote:
> >
> >      > Here are the command line memory settings. Soft heap isn't set so
> >     I think
> >      > that means memory can't be uncommitted.
> >      >
> >      > -XX:InitialHeapSize=12884901888 -XX:MaxHeapSize=32210157568
> >      > -XX:MinHeapSize=12884901888
> >
> >     Memory will be uncommitted with this configuration, but only down to
> >     MinHeapSize and only memory that has been unused for ZUncommitDelay
> >     (which defaults to 300 seconds). So, if you application is filling up
> >     the heap regularly and no SoftMaxHeapSize is set, the likelihood of
> >     having some memory sit unused for 300s is low and nothing will be
> >     uncommitted. On the other hand, if your application were to idle for
> >     more than ZUncommitDelay, then unused memory would be uncommitted.
> >
> >
> > If SoftMaxHeapSize isn't set then it defaults to MaxHeapSize according
> > to PrintFlagsFinal. It doesn't default to MinHeap. Is the behavior
> > different from what I see by checking the values using
> -XX:PrintFlagsFinal?
>
> I'm not sure I understand the question, but yes, SoftMaxHeapSize
> defaults to MaxHeapSize.
>
>
In my configuration above, the SoftMaxHeapSize isn't set so it defaults to
MaxHeapSize. I'm confused by your statement "Memory will be uncommitted
with this configuration".

How is it possible for memory to be uncommitted down to MinHeapSize based
on the configuration I provided? Does ZGC uncommit memory below
SoftMaxHeapSize?

Mike