Re: ZFS perfomance regression in FreeBSD 12 APLHA3->ALPHA4

On Wed, Sep 5, 2018 at 5:58 PM Allan Jude <allanjude_at_freebsd.org> wrote:
>
> On 2018-09-05 10:04, Subbsd wrote:
> > Hi,
> >
> > I'm seeing a huge loss in performance ZFS after upgrading FreeBSD 12
> > to latest revision (r338466 the moment) and related to ARC.
> >
> > I can not say which revision was before except that the newver.sh
> > pointed to ALPHA3.
> >
> > Problems are observed if you try to limit ARC. In my case:
> >
> > vfs.zfs.arc_max="128M"
> >
> > I know that this is very small. However, for two years with this there
> > were no problems.
> >
> > When i send SIGINFO to process which is currently working with ZFS, i
> > see "arc_reclaim_waiters_cv":
> >
> > e.g when i type:
> >
> > /bin/csh
> >
> > I have time (~5 seconds) to press several times 'ctrl+t' before csh is executed:
> >
> > load: 0.70  cmd: csh 5935 [arc_reclaim_waiters_cv] 1.41r 0.00u 0.00s 0% 3512k
> > load: 0.70  cmd: csh 5935 [zio->io_cv] 1.69r 0.00u 0.00s 0% 3512k
> > load: 0.70  cmd: csh 5935 [arc_reclaim_waiters_cv] 1.98r 0.00u 0.01s 0% 3512k
> > load: 0.73  cmd: csh 5935 [arc_reclaim_waiters_cv] 2.19r 0.00u 0.01s 0% 4156k
> >
> > same story with find or any other commans:
> >
> > load: 0.34  cmd: find 5993 [zio->io_cv] 0.99r 0.00u 0.00s 0% 2676k
> > load: 0.34  cmd: find 5993 [arc_reclaim_waiters_cv] 1.13r 0.00u 0.00s 0% 2676k
> > load: 0.34  cmd: find 5993 [arc_reclaim_waiters_cv] 1.25r 0.00u 0.00s 0% 2680k
> > load: 0.34  cmd: find 5993 [arc_reclaim_waiters_cv] 1.38r 0.00u 0.00s 0% 2684k
> > load: 0.34  cmd: find 5993 [arc_reclaim_waiters_cv] 1.51r 0.00u 0.00s 0% 2704k
> > load: 0.34  cmd: find 5993 [arc_reclaim_waiters_cv] 1.64r 0.00u 0.00s 0% 2716k
> > load: 0.34  cmd: find 5993 [arc_reclaim_waiters_cv] 1.78r 0.00u 0.00s 0% 2760k
> >
> > this problem goes away after increasing vfs.zfs.arc_max
> > _______________________________________________
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> >
>
> Previously, ZFS was not actually able to evict enough dnodes to keep
> your arc_max under 128MB, it would have been much higher based on the
> number of open files you had. A recent improvement from upstream ZFS
> (r337653 and r337660) was pulled in that fixed this, so setting an
> arc_max of 128MB is much more effective now, and that is causing the
> side effect of "actually doing what you asked it to do", in this case,
> what you are asking is a bit silly. If you have a working set that is
> greater than 128MB, and you ask ZFS to use less than that, it'll have to
> constantly try to reclaim memory to keep under that very low bar.
>
> --
> Allan Jude
>


Hi,

Thanks for comments. Mark was right when he pointed to r338416 (
https://svnweb.freebsd.org/base/head/sys/cddl/contrib/opensolaris/uts/common/fs/zfs/arc.c?r1=338416&r2=338415&pathrev=338416
). Commenting aggsum_value returns normal speed regardless of the rest
of the new code from upstream.
I would like to repeat that the speed with these two lines is not just
slow, but _INCREDIBLY_ slow! Probably, this should be written in the
relevant documentation for FreeBSD 12+