Re: Speed improvements in ZFS

On 8/22/23, Alexander Leidinger <Alexander@leidinger.net> wrote:
> Am 2023-08-21 10:53, schrieb Konstantin Belousov:
>> On Mon, Aug 21, 2023 at 08:19:28AM +0200, Alexander Leidinger wrote:
>>> Am 2023-08-20 23:17, schrieb Konstantin Belousov:
>>> > On Sun, Aug 20, 2023 at 11:07:08PM +0200, Mateusz Guzik wrote:
>>> > > On 8/20/23, Alexander Leidinger <Alexander@leidinger.net> wrote:
>>> > > > Am 2023-08-20 22:02, schrieb Mateusz Guzik:
>>> > > >> On 8/20/23, Alexander Leidinger <Alexander@leidinger.net> wrote:
>>> > > >>> Am 2023-08-20 19:10, schrieb Mateusz Guzik:
>>> > > >>>> On 8/18/23, Alexander Leidinger <Alexander@leidinger.net>
>>> > > >>>> wrote:
>>> > > >>>
>>> > > >>>>> I have a 51MB text file, compressed to about 1MB. Are you
>>> > > >>>>> interested
>>> > > >>>>> to
>>> > > >>>>> get it?
>>> > > >>>>>
>>> > > >>>>
>>> > > >>>> Your problem is not the vnode limit, but nullfs.
>>> > > >>>>
>>> > > >>>> https://people.freebsd.org/~mjg/netchild-periodic-find.svg
>>> > > >>>
>>> > > >>> 122 nullfs mounts on this system. And every jail I setup has
>>> > > >>> several
>>> > > >>> null mounts. One basesystem mounted into every jail, and then
>>> > > >>> shared
>>> > > >>> ports (packages/distfiles/ccache) across all of them.
>>> > > >>>
>>> > > >>>> First, some of the contention is notorious VI_LOCK in order to
>>> > > >>>> do
>>> > > >>>> anything.
>>> > > >>>>
>>> > > >>>> But more importantly the mind-boggling off-cpu time comes from
>>> > > >>>> exclusive locking which should not be there to begin with -- as
>>> > > >>>> in
>>> > > >>>> that xlock in stat should be a slock.
>>> > > >>>>
>>> > > >>>> Maybe I'm going to look into it later.
>>> > > >>>
>>> > > >>> That would be fantastic.
>>> > > >>>
>>> > > >>
>>> > > >> I did a quick test, things are shared locked as expected.
>>> > > >>
>>> > > >> However, I found the following:
>>> > > >>         if ((xmp->nullm_flags & NULLM_CACHE) != 0) {
>>> > > >>                 mp->mnt_kern_flag |=
>>> > > >> lowerrootvp->v_mount->mnt_kern_flag &
>>> > > >>                     (MNTK_SHARED_WRITES | MNTK_LOOKUP_SHARED |
>>> > > >>                     MNTK_EXTENDED_SHARED);
>>> > > >>         }
>>> > > >>
>>> > > >> are you using the "nocache" option? it has a side effect of
>>> > > >> xlocking
>>> > > >
>>> > > > I use noatime, noexec, nosuid, nfsv4acls. I do NOT use nocache.
>>> > > >
>>> > >
>>> > > If you don't have "nocache" on null mounts, then I don't see how
>>> > > this
>>> > > could happen.
>>> >
>>> > There is also MNTK_NULL_NOCACHE on lower fs, which is currently set
>>> > for
>>> > fuse and nfs at least.
>>>
>>> 11 of those 122 nullfs mounts are ZFS datasets which are also NFS
>>> exported.
>>> 6 of those nullfs mounts are also exported via Samba. The NFS exports
>>> shouldn't be needed anymore, I will remove them.
>> By nfs I meant nfs client, not nfs exports.
>
> No NFS client mounts anywhere on this system. So where is this exclusive
> lock coming from then...
> This is a ZFS system. 2 pools: one for the root, one for anything I need
> space for. Both pools reside on the same disks. The root pool is a 3-way
> mirror, the "space-pool" is a 5-disk raidz2. All jails are on the
> space-pool. The jails are all basejail-style jails.
>

While I don't see why xlocking happens, you should be able to dtrace
or printf your way into finding out.

-- 
Mateusz Guzik <mjguzik gmail.com>