RFR (S): 6490394: G1: Allow heap shrinking / memory unmapping after reclaiming regions during Remark

[Thomas Schatzl]

Hi Ruslan,

On Mon, 2018-12-10 at 18:43 +0100, Ruslan Synytsky wrote:
> 
> On Mon, 10 Dec 2018 at 16:48, Thomas Schatzl <
> thomas.schatzl at oracle.com> wrote:
> > Hi,
> > 
> >   see inline.
> > 
> > On Mon, 2018-12-10 at 16:22 +0100, Ruslan Synytsky wrote:
> > > Dear Thomas and Sangheon, hope you are doing great. 
> > > 
> > > I just noticed an important improvement related to vertical
> > > scaling and would like to clarify details mentioned in the 
> > > release notes 
> > > 
>   [...]
> > > > This behavior may be disabled by changing minimum Java heap 
> > > > size via the -Xms option.
> > > At the same time if a user adjusts Xms option then he/she does 
> > > not get this "smart behaviour" at all.
> > > 
> > 
> > This sentence should indicate that you can disable giving back
> > memory to the OS by modifying the Xms value. In particular, setting
> > the Xms to the Xmx value - which is a rule that is part of the
> > default heap sizing policy.
> 
> Then I think the wording in the release notes is confusing as there
> is only one case when changing Xms disables giving memory back,
> specifically when Xms = Xmx.

The behavior with Xms = Xmx corresponds to normal heap sizing
heuristics.

What would you prefer? Something like:

"This behavior may be disabled in accordance with default heap sizing
policies by setting minimum Java heap size to maximum Java heap size
via the -Xms option."

Or just delete this sentence, as this behavior is part of default Java
heap sizing policies anyway?

> > > Are these rephrased statements correct?
> > 
> > Almost :) Corrections above.
> > 
> > Note that the implementation of JEP 346 has been pushed today, so
> > it will be JDK12 no matter what - there is only some post-push JEP
> > work to do.
> 
> It's great! I will run some tests on my side later this week. 

Looking forward to your results.

Thomas