Re: page fault scalability (ext3, ext4, xfs)

Andy Lutomirski
2013-08-17T00:20:54Z

On Fri, Aug 16, 2013 at 3:02 PM, J. Bruce Fields <bfields@fieldses.org> wrote:
> On Fri, Aug 16, 2013 at 07:37:25AM +1000, Dave Chinner wrote:
>> On Thu, Aug 15, 2013 at 08:17:18AM -0700, Andy Lutomirski wrote:
>> > On Thu, Aug 15, 2013 at 12:11 AM, Dave Chinner <david@fromorbit.com> wrote:
>> > > On Wed, Aug 14, 2013 at 11:14:37PM -0700, Andy Lutomirski wrote:
>> > >> On Wed, Aug 14, 2013 at 11:01 PM, Dave Chinner <david@fromorbit.com> wrote:
>> > >> > On Wed, Aug 14, 2013 at 09:32:13PM -0700, Andy Lutomirski wrote:
>> > >> >> On Wed, Aug 14, 2013 at 7:10 PM, Dave Chinner <david@fromorbit.com> wrote:
>> > >> >> > On Wed, Aug 14, 2013 at 09:11:01PM -0400, Theodore Ts'o wrote:
>> > >> >> >> On Wed, Aug 14, 2013 at 04:38:12PM -0700, Andy Lutomirski wrote:
>> > >> >> >> > > It would be better to write zeros to it, so we aren't measuring the
>> > >> >> >> > > cost of the unwritten->written conversion.
>> > >> >> >> >
>> > >> >> >> > At the risk of beating a dead horse, how hard would it be to defer
>> > >> >> >> > this part until writeback?
>> > >> >> >>
>> > >> >> >> Part of the work has to be done at write time because we need to
>> > >> >> >> update allocation statistics (i.e., so that we don't have ENOSPC
>> > >> >> >> problems). The unwritten->written conversion does happen at writeback
>> > >> >> >> (as does the actual block allocation if we are doing delayed
>> > >> >> >> allocation).
>> > >> >> >>
>> > >> >> >> The point is that if the goal is to measure page fault scalability, we
>> > >> >> >> shouldn't have this other stuff happening as the same time as the page
>> > >> >> >> fault workload.
>> > >> >> >
>> > >> >> > Sure, but the real problem is not the block mapping or allocation
>> > >> >> > path - even if the test is changed to take that out of the picture,
>> > >> >> > we still have timestamp updates being done on every single page
>> > >> >> > fault. ext4, XFS and btrfs all do transactional timestamp updates
>> > >> >> > and have nanosecond granularity, so every page fault is resulting in
>> > >> >> > a transaction to update the timestamp of the file being modified.
>> > >> >>
>> > >> >> I have (unmergeable) patches to fix this:
>> > >> >>
>> > >> >> http://comments.gmane.org/gmane.linux.kernel.mm/92476
>> > >> >
>> > >> > The big problem with this approach is that not doing the
>> > >> > timestamp update on page faults is going to break the inode change
>> > >> > version counting because for ext4, btrfs and XFS it takes a
>> > >> > transaction to bump that counter. NFS needs to know the moment a
>> > >> > file is changed in memory, not when it is written to disk. Also, NFS
>> > >> > requires the change to the counter to be persistent over server
>> > >> > failures, so it needs to be changed as part of a transaction....
>> > >>
>> > >> I've been running a kernel that has the file_update_time call
>> > >> commented out for over a year now, and the only problem I've seen is
>> > >> that the timestamp doesn't get updated :)
>> > >>
>> >
>> > [...]
>> >
>> > > If a filesystem is providing an i_version value, then NFS uses it to
>> > > determine whether client side caches are still consistent with the
>> > > server state. If the filesystem does not provide an i_version, then
>> > > NFS falls back to checking c/mtime for changes. If files on the
>> > > server are being modified without either the tiemstamps or i_version
>> > > changing, then it's likely that there will be problems with client
>> > > side cache consistency....
>> >
>> > I didn't think of that at all.
>> >
>> > If userspace does:
>> >
>> > ptr = mmap(...);
>> > ptr[0] = 1;
>> > sleep(1);
>> > ptr[0] = 2;
>> > sleep(1);
>> > munmap();
>> >
>> > Then current kernels will mark the inode changed on (only) the ptr[0]
>> > = 1 line. My patches will instead mark the inode changed when munmap
>> > is called (or after ptr[0] = 2 if writepages gets called for any
>> > reason).
>> >
>> > I'm not sure which is better. POSIX actually requires my behavior
>> > (which is most irrelevant).
>>
>> Not by my reading of it. Posix states that c/mtime needs to be
>> updated between the first access and the next msync() call. We
>> update mtime on the first access, and so therefore we conform to the
>> posix requirement....
>>
>> > My behavior also means that, if an NFS
>> > client reads and caches the file between the two writes, then it will
>> > eventually find out that the data is stale.
>>
>> "eventually" is very different behaviour to the current behaviour.
>>
>> My understanding is that NFS v4 delegations require the underlying
>> filesystem to bump the version count on *any* modification made to
>> the file so that delegations can be recalled appropriately.
>
> Delegations at least shouldn't be an issue here: they're recalled on the
> open.

Can you translate that into clueless-non-NFS-expert? :)

Anyway, I'm sending patches in a sec. Dave (Hansen), want to test? I
played with will-it-scale a bit, but I don't really know what I'm
doing.

--Andy