RFR(S) 8189688: NMT: Report per-class load metadata information

[Thomas Stüfe]

On Mon, Oct 23, 2017 at 4:52 PM, Andrew Dinn <adinn at redhat.com> wrote:

> On 23/10/17 15:40, Thomas Stüfe wrote:
> > Okay. So this is important for understanding cases where we have a large
> > number of miniscule class loaders, each one only having a few numbers of
> > chunks. In this case, would it be useful to have a running total of
> "free"
> > and "waste", too? Or even better, also average and peak values of "free"
> > and "waste" (to tell apart cases where you have one outlier vs cases
> where
> > just all metaspaces waste a lot of memory).
> >
> > Just out of curiosity, I looked at
> > http://cr.openjdk.java.net/~zgu/cld_metaspace/wildfly.txt  and it seems
> > that "free" is the problem, not "waste"? So I guess what happens is that
> > all the little classloaders allocate just enough memory to push them over
> > "_small_chunk_limit=4", so they allocate the first medium chunk, from
> which
> > then they take a small bite and leave the rest laying around?
>
> Yes, that is pretty how it works. Only the problem is twice as bad when
> you have separate class metadata and non-class metadata (i.e. when using
> compressed class oops which is by far the most common case) since this
> invariably wastes more space.
>
>
:( In this case, is the performance gain of CompressedClassPointers even
worth it?


> Similar behaviour happens when an anonymous class is created because of
> a lambda, except that you only get one small allocation before a medium
> chunk is created. So, for small lambdas, you almost always end up using
> around 50% of 2 1MB chunks and, for more complex lambdas, around 50% of
> a 1MB chunk plus less than 50% of a 1MB + 4MB chunk. So, the space waste
> for lambdas is really quite spectacular.
>
>
Ouch.

I am confused about the sizes though . Are we talking about the chunk sizes
in metaspace.cpp? Because they are way smaller, with medium chunks being
32/64K? Or are these large chunks a RedHat addition?

Best Regards, Thomas


> regards,
>
>
> Andrew Dinn
> -----------
> Senior Principal Software Engineer
> Red Hat UK Ltd
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