ZGC and 1G pages?

[Florian Weimer]

* Thomas Stüfe:

> ZGC, on Linux, seems only compatible with 2M pages. Seeing that ZGC is
> often used with very large heaps, is support for 1GB pages planned?
>
> Especially if one disregards uncommitting (-ZUncommit), 1G pages could
> be a speed boost for customers with gigantic heaps, as well as reduce
> their number of VMAs.

Is the number of VMAs really tied to hugepage support?  I think ZGC
could keep the number of VMAs down simply by processing mappings at
larger granularity.

There is a Fedora discussion under way to eliminate the kernel VMA limit
completely, but the kernel OOM handler isn't really compatible with
that.  The current heuristics do not seem to pick the most appropriate
process if the kernel ends up with too much (unswappable?) memory used
due to an excessive count of VMAs, so I'm not sure that we're going to
change the default.

  F39 proposal: Increase vm.max_map_count value (System-Wide Change proposal)
  <https://lists.fedoraproject.org/archives/list/devel@lists.fedoraproject.org/thread/WVWHTLXSGZN4QMAE577ZFZX4ZI6YZF3A/>

Thanks,
Florian