Heap dump is always around 8G on process with 80G heap

[Sundara Mohan M]

Hi Per,
 "ZGC only ever dumps reachable objects"
Does that mean we can never dump unreachable objects in ZGC or there is
some options that can be passed to get it?

I will try to see if the dump from other GC has unreachable object which is
showing as large file.

Thanks
Sundar

On Fri, Nov 22, 2019 at 12:31 AM Per Liden <per.liden at oracle.com> wrote:

> On 11/22/19 12:37 AM, Sundara Mohan M wrote:
> > Hi,
> >     I am trying to take a heap dump of java process with 80G heap with
> ZGC,
> > this is always giving me around 8G dump file.
> > Same application with ParallelGC running with 48G heap i am getting
> around
> > 30G dump file.
> > I am using following command on both process and verified both process
> has
> > same no of request processed and Used memory from gc log is similar
> >     jcmd <pid> GC.heap_dump <filename>
> >
> > 1. Why is ZGC heap dump always less compared to process running with
> > ParallelGC?
> > 2. Is there something i am missing?
> >
>
> There are various reasons why a heap dump from one GC is larger or
> smaller compared to another GC. For example, ZGC only ever dumps
> reachable objects, while PrallelGC can also dump unreachable objects
> under some conditions (even though you didn't ask for them).
>
> It's hard to tell where the difference comes from in your case, without
> further inspection/debugging.
>
> /Per
>