Date: Wed, 20 Apr 2022 11:43:10 +0200 From: Mateusz Guzik <mjguzik@gmail.com> To: Doug Ambrisko <ambrisko@ambrisko.com> Cc: freebsd-current@freebsd.org Subject: Re: nullfs and ZFS issues Message-ID: <CAGudoHGv9Es4Qov9YSHQ=8jAOdiAmyWA0fCE1bgAStv%2Bn=J2dw@mail.gmail.com> In-Reply-To: <Yl7cP3RTWROiQLX5@ambrisko.com> References: <Yl31Frx6HyLVl4tE@ambrisko.com> <CAGudoHEqjs4QoAqvkvW5JdSOMZ_QNjd3XU65kULxgabsOva5Xw@mail.gmail.com> <CAGudoHGBfVFcsCbhC=MCRFPzCtVRYCa1pCU7cGuuJq1fOv6ttg@mail.gmail.com> <CAGudoHGP5MTaF_LKanCh88ufHM6mBdzicQg-KLdfw0xGA-AxJQ@mail.gmail.com> <Yl7cP3RTWROiQLX5@ambrisko.com>
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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. > > | 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>
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