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

[J. Bruce Fields]

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@xxxxxxxxxxxxx> 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@xxxxxxxxxxxxx> 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@xxxxxxxxxxxxx> 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.

--b.

> So not
> informing the filesystem that the file data has been changed is
> going to cause problems.
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