[8u, 11u] RFR: Shenandoah: enable low-frequency STW class unloading

[Thomas Stüfe]

Out of curiosity, the GCs induced by bumping against the Metaspace GC
threshold when loading classes would still work and unload stuff?

..Thomas

On Mon, Jul 20, 2020 at 4:21 PM Aleksey Shipilev <shade at redhat.com> wrote:

> Hi,
>
> I think we have gathered enough real world data to consider enabling STW
> class unloading by default,
> but on much lower frequency. The concurrent class unloading is enabled by
> default since JDK 14,
> because we have enough infrastructure there. In 8u and 11u, we don't have
> such luck, and so we can
> only do STW class unloading, which adds to Final Mark time when it
> happens. This is why it was
> disabled in 8u and 11u by default, asking users to explicitly enable it.
>
> There are still good reasons to do it, even with STW:
>   a) String Table cruft accumulates over time;
>   b) CLDG cruft accumulates over time;
>   c) The connection between the two above and the disabled -CUwCM is very
> opaque;
>
> The long-run performance stability would be improved if we accepted a
> longer Final Mark pause every
> once in a while. It seems doing this every 100th cycle is a good spot? In
> many cases I've seen, the
> GC runs a few times per minute, which means we would get the several class
> unloading cycles every
> hour. That should be enough to amortize costs, and also be frequent enough
> to see as regular outlier
> in users' monitoring, if they want to turn it off.
>
> 8u patch:
>
> https://cr.openjdk.java.net/~shade/shenandoah/regular-stw-cu/webrev.01.8u/
>   Passes: hotspot_gc_shenandoah, ad-hoc runs
>
> 11u patch:
>
> https://cr.openjdk.java.net/~shade/shenandoah/regular-stw-cu/webrev.01.11u/
>   Passes: hotspot_gc_shenandoah, tier{1,2} with Shenandoah
>
> Thoughts?
>
> --
> Thanks,
> -Aleksey
>
>