Date:	Sun, 18 Aug 2013 16:17:06 -0400
From:	"J. Bruce Fields" <bfields@...ldses.org>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	Dave Chinner <david@...morbit.com>, Theodore Ts'o <tytso@....edu>,
	Dave Hansen <dave.hansen@...el.com>,
	Dave Hansen <dave.hansen@...ux.intel.com>,
	Linux FS Devel <linux-fsdevel@...r.kernel.org>,
	xfs@....sgi.com,
	"linux-ext4@...r.kernel.org" <linux-ext4@...r.kernel.org>,
	Jan Kara <jack@...e.cz>, LKML <linux-kernel@...r.kernel.org>,
	Tim Chen <tim.c.chen@...ux.intel.com>,
	Andi Kleen <ak@...ux.intel.com>
Subject: Re: page fault scalability (ext3, ext4, xfs)

On Fri, Aug 16, 2013 at 04:18:33PM -0700, Andy Lutomirski wrote:
> On Fri, Aug 16, 2013 at 3:02 PM, J. Bruce Fields <bfields@...ldses.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@...morbit.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@...morbit.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@...morbit.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? :)

An NFS "delegation" is roughly the same thing as what's called a "lease"
by the linux vfs or an "OpLock" in SMB.  It's a lock that is recalled
from the holder on certain conflicting operations.  (Basically a way to
tell a client "you're the only one using this file, feel free to cache
it until I tell you otherwise".)

Delegations are recalled on conflicting opens, so by the time you get to
IO there shouldn't be any.  I don't think they're really relevant to
this discussion.

--b.

> 
> 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
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