Re: nullfs and ZFS issues
- In reply to: Mateusz Guzik : "Re: nullfs and ZFS issues"
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Date: Wed, 20 Apr 2022 16:11:03 UTC
On Wed, Apr 20, 2022 at 11:43:10AM +0200, Mateusz Guzik wrote:
| On 4/19/22, Doug Ambrisko <ambrisko@ambrisko.com> wrote:
| > On Tue, Apr 19, 2022 at 11:47:22AM +0200, Mateusz Guzik wrote:
| > | Try this: https://people.freebsd.org/~mjg/vnlru_free_pick.diff
| > |
| > | this is not committable but should validate whether it works fine
| >
| > As a POC it's working. I see the vnode count for the nullfs and
| > ZFS go up. The ARC cache also goes up until it exceeds the ARC max.
| > size tten the vnodes for nullfs and ZFS goes down. The ARC cache goes
| > down as well. This all repeats over and over. The systems seems
| > healthy. No excessive running of arc_prune or arc_evict.
| >
| > My only comment is that the vnode freeing seems a bit agressive.
| > Going from ~15,000 to ~200 vnode for nullfs and the same for ZFS.
| > The ARC drops from 70M to 7M (max is set at 64M) for this unit
| > test.
| >
|
| Can you check what kind of shrinking is requested by arc to begin
| with? I imagine encountering a nullfs vnode may end up recycling 2
| instead of 1, but even repeated a lot it does not explain the above.
I dug it into a bit more and think there could be a bug in:
module/zfs/arc.c
arc_evict_meta_balanced(uint64_t meta_used)
prune += zfs_arc_meta_prune;
//arc_prune_async(prune);
arc_prune_async(zfs_arc_meta_prune);
Since arc_prune_async, is queuing up a run of arc_prune_task for each
call it is actually already accumulating the zfs_arc_meta_prune
amount. It makes the count to vnlru_free_impl get really big quickly
since it is looping via restart.
1 HELLO arc_prune_task 164 ticks 2147465958 count 20480000
dmesg | grep arc_prune_task | uniq -c
14 HELLO arc_prune_task 164 ticks -2147343772 count 100
50 HELLO arc_prune_task 164 ticks -2147343771 count 100
46 HELLO arc_prune_task 164 ticks -2147343770 count 100
49 HELLO arc_prune_task 164 ticks -2147343769 count 100
44 HELLO arc_prune_task 164 ticks -2147343768 count 100
116 HELLO arc_prune_task 164 ticks -2147343767 count 100
1541 HELLO arc_prune_task 164 ticks -2147343766 count 100
53 HELLO arc_prune_task 164 ticks -2147343101 count 100
100 HELLO arc_prune_task 164 ticks -2147343100 count 100
75 HELLO arc_prune_task 164 ticks -2147343099 count 100
52 HELLO arc_prune_task 164 ticks -2147343098 count 100
50 HELLO arc_prune_task 164 ticks -2147343097 count 100
51 HELLO arc_prune_task 164 ticks -2147343096 count 100
783 HELLO arc_prune_task 164 ticks -2147343095 count 100
884 HELLO arc_prune_task 164 ticks -2147343094 count 100
Note I shrunk vfs.zfs.arc.meta_prune=100 to see how that might
help. Changing it to 1, helps more! I see less agressive
swings.
I added
printf("HELLO %s %d ticks %d count %ld\n",__FUNCTION__,__LINE__,ticks,nr_scan);
to arc_prune_task.
Adjusting both
sysctl vfs.zfs.arc.meta_adjust_restarts=1
sysctl vfs.zfs.arc.meta_prune=100
without changing arc_prune_async(prune) helps avoid excessive swings.
Thanks,
Doug A.
| > | On 4/19/22, Mateusz Guzik <mjguzik@gmail.com> wrote:
| > | > On 4/19/22, Mateusz Guzik <mjguzik@gmail.com> wrote:
| > | >> On 4/19/22, Doug Ambrisko <ambrisko@ambrisko.com> wrote:
| > | >>> I've switched my laptop to use nullfs and ZFS. Previously, I used
| > | >>> localhost NFS mounts instead of nullfs when nullfs would complain
| > | >>> that it couldn't mount. Since that check has been removed, I've
| > | >>> switched to nullfs only. However, every so often my laptop would
| > | >>> get slow and the the ARC evict and prune thread would consume two
| > | >>> cores 100% until I rebooted. I had a 1G max. ARC and have increased
| > | >>> it to 2G now. Looking into this has uncovered some issues:
| > | >>> - nullfs would prevent vnlru_free_vfsops from doing anything
| > | >>> when called from ZFS arc_prune_task
| > | >>> - nullfs would hang onto a bunch of vnodes unless mounted with
| > | >>> nocache
| > | >>> - nullfs and nocache would break untar. This has been fixed
| > now.
| > | >>>
| > | >>> With nullfs, nocache and settings max vnodes to a low number I can
| > | >>> keep the ARC around the max. without evict and prune consuming
| > | >>> 100% of 2 cores. This doesn't seem like the best solution but it
| > | >>> better then when the ARC starts spinning.
| > | >>>
| > | >>> Looking into this issue with bhyve and a md drive for testing I
| > create
| > | >>> a brand new zpool mounted as /test and then nullfs mount /test to
| > /mnt.
| > | >>> I loop through untaring the Linux kernel into the nullfs mount, rm
| > -rf
| > | >>> it
| > | >>> and repeat. I set the ARC to the smallest value I can. Untarring
| > the
| > | >>> Linux kernel was enough to get the ARC evict and prune to spin since
| > | >>> they couldn't evict/prune anything.
| > | >>>
| > | >>> Looking at vnlru_free_vfsops called from ZFS arc_prune_task I see it
| > | >>> static int
| > | >>> vnlru_free_impl(int count, struct vfsops *mnt_op, struct vnode
| > *mvp)
| > | >>> {
| > | >>> ...
| > | >>>
| > | >>> for (;;) {
| > | >>> ...
| > | >>> vp = TAILQ_NEXT(vp, v_vnodelist);
| > | >>> ...
| > | >>>
| > | >>> /*
| > | >>> * Don't recycle if our vnode is from different type
| > | >>> * of mount point. Note that mp is type-safe, the
| > | >>> * check does not reach unmapped address even if
| > | >>> * vnode is reclaimed.
| > | >>> */
| > | >>> if (mnt_op != NULL && (mp = vp->v_mount) != NULL &&
| > | >>> mp->mnt_op != mnt_op) {
| > | >>> continue;
| > | >>> }
| > | >>> ...
| > | >>>
| > | >>> The vp ends up being the nulfs mount and then hits the continue
| > | >>> even though the passed in mvp is on ZFS. If I do a hack to
| > | >>> comment out the continue then I see the ARC, nullfs vnodes and
| > | >>> ZFS vnodes grow. When the ARC calls arc_prune_task that calls
| > | >>> vnlru_free_vfsops and now the vnodes go down for nullfs and ZFS.
| > | >>> The ARC cache usage also goes down. Then they increase again until
| > | >>> the ARC gets full and then they go down again. So with this hack
| > | >>> I don't need nocache passed to nullfs and I don't need to limit
| > | >>> the max vnodes. Doing multiple untars in parallel over and over
| > | >>> doesn't seem to cause any issues for this test. I'm not saying
| > | >>> commenting out continue is the fix but a simple POC test.
| > | >>>
| > | >>
| > | >> I don't see an easy way to say "this is a nullfs vnode holding onto a
| > | >> zfs vnode". Perhaps the routine can be extrended with issuing a nullfs
| > | >> callback, if the module is loaded.
| > | >>
| > | >> In the meantime I think a good enough(tm) fix would be to check that
| > | >> nothing was freed and fallback to good old regular clean up without
| > | >> filtering by vfsops. This would be very similar to what you are doing
| > | >> with your hack.
| > | >>
| > | >
| > | > Now that I wrote this perhaps an acceptable hack would be to extend
| > | > struct mount with a pointer to "lower layer" mount (if any) and patch
| > | > the vfsops check to also look there.
| > | >
| > | >>
| > | >>> It appears that when ZFS is asking for cached vnodes to be
| > | >>> free'd nullfs also needs to free some up as well so that
| > | >>> they are free'd on the VFS level. It seems that vnlru_free_impl
| > | >>> should allow some of the related nullfs vnodes to be free'd so
| > | >>> the ZFS ones can be free'd and reduce the size of the ARC.
| > | >>>
| > | >>> BTW, I also hacked the kernel and mount to show the vnodes used
| > | >>> per mount ie. mount -v:
| > | >>> test on /test (zfs, NFS exported, local, nfsv4acls, fsid
| > | >>> 2b23b2a1de21ed66,
| > | >>> vnodes: count 13846 lazy 0)
| > | >>> /test on /mnt (nullfs, NFS exported, local, nfsv4acls, fsid
| > | >>> 11ff002929000000, vnodes: count 13846 lazy 0)
| > | >>>
| > | >>> Now I can easily see how the vnodes are used without going into ddb.
| > | >>> On my laptop I have various vnet jails and nullfs mount my homedir
| > into
| > | >>> them so pretty much everything goes through nullfs to ZFS. I'm
| > limping
| > | >>> along with the nullfs nocache and small number of vnodes but it would
| > be
| > | >>> nice to not need that.
| > | >>>
| > | >>> Thanks,
| > | >>>
| > | >>> Doug A.
| > | >>>
| > | >>>
| > | >>
| > | >>
| > | >> --
| > | >> Mateusz Guzik <mjguzik gmail.com>
| > | >>
| > | >
| > | >
| > | > --
| > | > Mateusz Guzik <mjguzik gmail.com>
| > | >
| > |
| > |
| > | --
| > | Mateusz Guzik <mjguzik gmail.com>
| >
|
|
| --
| Mateusz Guzik <mjguzik gmail.com>