RFC: Draft JEP: Improved Metaspace allocator

[Thomas Stüfe]

Hi Thomas,

On Wed, Mar 27, 2019 at 2:57 PM Thomas Schatzl <thomas.schatzl at oracle.com>
wrote:

> On Wed, 2019-03-20 at 20:19 +0100, Thomas Stüfe wrote:
> > Dear all,
> >
> > I would like to hear what you think about the following proposal:
> >
> > https://bugs.openjdk.java.net/browse/JDK-8221173
> >
> > Do you consider this worthwhile?
>
> Yes, definitely!
>

Great :)


>
> At the beginning of this month we were internally looking at a
> metaspace stress application, and it took some time to nail the problem
> on metaspace fragmentation. Improving fragmentation, the visibility of
> such problems (fragmentation statistics) and potential options to e.g.
> favor compactness vs. performance.
>

Just to be sure, are you aware of the jcmd "VM.metaspace" command we
(RedHat & SAP) added for jdk11? It can be used to analyze Metaspace in
quite some detail. We did not make a big fuss over it, but it is there and
quite useful. It may of course be missing functionality; if yes, please
tell us.

About your stress test, I'd be interested in a reproduction case if that is
possible. I am always collecting pathological cases to play with.


>
> > I debated with myself whether a JEP is the correct umbrella for it.
> > In the past I did larger changes like this without JEP but in this
> > case I feel better with a confirmed and sponsored JEP behind me -
> > from a certain size on it just feels more appropriate.
> >
> > As usual, I am not sure where to put Metaspace-related proposals to
> > catch all interested people, so I start with hs-dev. Please feel free
> > to redirect if needed.
>
> You are right that a JEP is the only way to get guaranteed visibility
> to people. JEPs are mostly the only source for third party "what's new
> in JDK XX" write-ups.
> So if you want to reach a wide (non-hotspot-dev) audience, JEPs are
> basically the only way. I could kind of see that this might be the
> case.
>

For me it is more of a question of "I don't want to make a fuss" vs "I
don't want to sneak large changes in behind everyone's back". In this case,
I side with the latter.

As additional advantage, a JEP gives a notion of progress to the outside
world. And I have the feeling it activates more Reviewer muscle than
large-non-JEP patches.

Thanks, Thomas


> Thanks,
>   Thomas
>
>
>