Runtime.getRuntime().freeMemory() with ZGC

[Per Liden]

On 5/28/21 8:34 PM, Mike Rettig wrote:
> 
> 
> On Fri, May 28, 2021 at 1:16 PM Per Liden <per.liden at oracle.com 
> <mailto: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>
>      > <mailto: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?

Yes, the lower limit for uncommit is MinHeapSize, so it can uncommit 
below SoftMaxHeapSize. SoftMaxHeapSize is in this context just used as a 
way to make memory that it's strictly needed by the application become 
"aged" and thereby become eligible for uncommit with regards to 
ZUncommitDelay.

/Per