Runtime.getRuntime().freeMemory() with ZGC

[Per Liden]

On 5/28/21 10:21 PM, Mike Rettig wrote:
> 
> On Fri, May 28, 2021 at 1:50 PM Per Liden <per.liden at oracle.com 
> <mailto:per.liden at oracle.com>> wrote:
> 
>      > 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
> 
> 
> For my purposes, I can set min heap equal to max heap then it will work 
> as I intended. I don't want memory uncommitted once it has been claimed 
> by the jvm.

Ok, then you want to use -XX:-ZUncommit, that will stop ZGC from ever 
uncommitting any part of the heap.

> 
> However, I don't feel that I fully understand the relationships between 
> the settings. In your article, what is the value of the MinHeapSize? 
> Does MinHeapSize affect the behavior as described in the article? Will 
> the behavior change if the min heap is equal to the max heap?

I see that my blog post wasn't very clear on that, sorry. In the example 
I gave there it's assumed that MinHeapSize is set to something lower or 
equal to SoftMaxHeapSize. So, MinHeapSize controls the amount of memory 
that will never be uncommitted.

> 
> https://malloc.se/blog/zgc-softmaxheapsize 
> <https://malloc.se/blog/zgc-softmaxheapsize>
> 
> As I've said in prior posts, I think ZGC (and other gc's) could be 
> easier to use if the jvm options captured the goals of the users. 
> Because ZGC lacks a good tuning option, I've spent a lot time 
> experimenting with options trying to get the behavior I want. I never 
> had to do this with G1. I just set MaxGCPauseMillis and things worked as 
> I expected. The final ZGC settings that I'm using are not ideal. I've 
> decided to just give ZGC more memory than I think it will ever need 
> since I don't have an easy way to specify the balance of memory vs pause 
> time that I would consider healthy.

Thanks for the feedback. What option would you have liked to have? 
Something like G1's MaxGCPauseMillis wouldn't make sense for ZGC, since 
pauses are O(1).

/Per