Re: zfs pool behavior - is it ever freed?

[David Brownlee <abs%absd.org@localhost>]

On Thu, 27 Jul 2023 at 13:24, Greg Troxel <gdt%lexort.com@localhost> wrote:
>
> Tobias Nygren <tnn%NetBSD.org@localhost> writes:
>
> > I use this patch on my RPi4, which I feel improves things.
> > People might find it helpful.
>
> That looks very helpful; I'll try it.
>
> > There ought to be writable sysctl knobs for some of the ZFS
> > tuneables, but looks like it isn't implemented in NetBSD yet.
>
> That seems not that hard -- it would be great if someone(tm) did that
> and mailed a patch.
>
> > --- external/cddl/osnet/dist/uts/common/fs/zfs/arc.c  3 Aug 2022 01:53:06 -0000       1.22
> > +++ external/cddl/osnet/dist/uts/common/fs/zfs/arc.c  27 Jul 2023 11:10:40 -0000
> > @@ -6100,6 +6100,10 @@ arc_init(void)
> >       else
> >               arc_c_max = arc_c_min;
> >       arc_c_max = MAX(arc_c * 5, arc_c_max);
> > +#if defined(__NetBSD__) && defined(_KERNEL)
> > +/* XXX prevent ARC from eating more than 12% of kmem */
> > +     arc_c_max = MIN(arc_c, vmem_size(heap_arena, VMEM_ALLOC | VMEM_FREE) / 8);
> > +#endif
> >
> >       /*
> >        * In userland, there's only the memory pressure that we artificially
>
> That seems eminently sensible and is sort of what I was thinking of
> heading to.  Interesting q about /8 vs /16, but it's a reasonable enough
> value to avoid lockups and that's 90% of the benefit.
>
> I wonder if we should commit that as obviously better than where we are
> now, where machines of <= 4G fail badly.
>
> It would be interesting for people with 8G and 16G machines to try this
> patch.  That will be somewhat less and maybe not less respectively.
>
> Also perhaps a dmesg printout of what arc_c_max is set to, to help in
> figuring things out.
>
> (I suppose one can gdb it, too, for testing.)

I would definitely like to see something like this in-tree soonest for
low memory (<6GB?) machines, but I'd prefer not to affect machines
with large amounts of memory used as dedicated ZFS fileservers (at
least not until its easily tunable)

Thanks

David