Min vs Max vs Soft Memory Limits

[Mike Rettig]

Thanks Per for your response. I added some additional questions below.

On Mon, May 10, 2021 at 3:04 AM Per Liden <per.liden at oracle.com> wrote:

> Hi,
>
> On 5/7/21 6:04 PM, Mike Rettig wrote:
> > I've been using ZGC with JDK 16 for a few months now and have been
> pleased
> > with the performance. I converted a few apps from G1 to ZGC so the heap
> > could be increased without increasing pause times. ZGC has performed well
> > in these scenarios.
>
> Good to hear!
>
> >
> > I've recently been modifying the ZGC options to optimize performance and
> > minimize the amount of vm option tweaking required for various
> > environments.  The apps are deployed to a large range of servers with
> > different amounts of memory.  The apps can be greedy about using memory
> so
> > I don't have to worry about uncommitting memory. However, I'd like the
> ZGC
> > collector to find an optimal state without giving it all the system
> memory
> > at startup.
>
> ZGC will typically try to use all the memory you give it, to space out
> GC cycles as much as possible, and thereby interfere as little as
> possible. So, ZGC will not really try to find the optimal state (in
> terms of memory usage) you're looking for here.
>

Will the jvm option ZAllocationSpikeTolerance influence the greediness of
ZGC?


> >
> > Here is an example config:
> >
> > -Xms1024m -XX:SoftMaxHeapSize=1024m -XX:-ZUncommit
> >
> > I know the app needs at least 1gb. I don't set the max heap because all
> the
> > servers are different. The jvm will default the max heap to 25% of the
> > system memory. I originally didn't set the soft max heap, but I was
> > surprised when it defaulted to the max heap. How does ZGC handle the soft
> > heap in this configuration? If the app starts at 1gb but soon needs 2gb
> > does the soft heap grow with it or does zgc still try to stay below 1gb
> > since that was the initial value?
>
> The SoftMaxHeapSize will stay at 1G, and if the heap grows to 2G, then
> ZGC will start doing GCs more often in order to try to get the heap
> usage down to 1G again.
>

Why doesn't it bump the soft heap to 2G when it is forced to grow and
uncommit is disabled? If ZGC generally performs better with a larger heap
and uncommitting memory is not an option, then why would it waste cycles
trying to keep memory usage below 1G? It seems keeping the soft heap at 1G
will cause ZGC to be less efficient.


>
> > Would it be better not to set the soft heap and let it default to the
> max heap?
>
> In your case, I would recommend that you set Xmx to more than what your
> app typically needs, and then set SoftMaxHeapSize to what that your app
> works well with, and so that GC cycles happen at some reasonable
> frequency. I would also recommend that you leave uncommit enabled. That
> way ZGC can grow the heap if there's a workload spike, but it will later
> return than memory once the spike is over.
>

It is very difficult to determine what the "app works well with" for every
environment. I'd prefer not to create custom gc settings for every server
or desktop configuration that I deploy to. I might be harming performance
with my memory configurations. With g1, I usually set the min/max memory
and the pause target.  That is sufficient for G1 to find a steady state
that is a good compromise that still meets the pause target. With G1, the
pause times increase linearly with the max heap so increasing the heap
isn't really an option to decrease pause times. However with ZGC I'd like
the heap to grow to avoid any and all allocation stalls. It seems the soft
heap and the allocation spike tolerance both factor into avoiding
allocation stalls. Is that true? Is there anything else I should look at to
avoid allocation stalls?

Would it be possible to specify the soft heap in a percentage of system
memory? That might make it easier in my scenario to find a good balance of
memory usage. Actually I think it would be helpful for all memory based jvm
options to allow for percentage based memory options (1g=1 gigabyte, 10per=
10 percent of available memory).


>
> > Does the jvm grow the heap
> > differently when the soft heap is set to the min heap instead of the max
> > heap when uncommit is disabled?
>
> The way the heap grows will not be affected, SoftMaxHeapSize only
> affects how often GCs happens.
>

> I wrote a blog post about SoftMaxHeapSize some time ago. I'm not sure
> the example I described there matches your use case, but you might find
> it useful nevertheless. https://malloc.se/blog/zgc-softmaxheapsize
>
> cheers,
> Per
>
> >
> > Mike
> >
>