Re: [PATCH 2/6] xfs: don't serialise adjacent concurrent direct IO appending writes

[Dave Chinner <david@fromorbit.com>]

On Thu, Aug 25, 2011 at 04:08:03PM -0500, Alex Elder wrote:
> On Thu, 2011-08-25 at 17:17 +1000, Dave Chinner wrote:
> > For append write workloads, extending the file requires a certain
> > amount of exclusive locking to be done up front to ensure sanity in
> > things like ensuring that we've zeroed any allocated regions
> > between the old EOF and the start of the new IO.
> > 
> > For single threads, this typically isn't a problem, and for large
> > IOs we don't serialise enough for it to be a problem for two
> > threads on really fast block devices. However for smaller IO and
> > larger thread counts we have a problem.
> > 
> > Take 4 concurrent sequential, single block sized and aligned IOs.
> > After the first IO is submitted but before it completes, we end up
> > with this state:
> > 
> >         IO 1    IO 2    IO 3    IO 4
> >       +-------+-------+-------+-------+
> >       ^       ^
> >       |       |
> >       |       |
> >       |       |
> >       |       \- ip->i_new_size
> >       \- ip->i_size
> > 
> > And the IO is done without exclusive locking because offset <=
> > ip->i_size. When we submit IO 2, we see offset > ip->i_size, and
> > grab the IO lock exclusive, because there is a chance we need to do
> > EOF zeroing. However, there is already an IO in progress that avoids
> > the need for IO zeroing because offset <= ip->i_new_size. hence we
> > could avoid holding the IO lock exlcusive for this. Hence after
> > submission of the second IO, we'd end up this state:
> > 
> >         IO 1    IO 2    IO 3    IO 4
> >       +-------+-------+-------+-------+
> >       ^               ^
> >       |               |
> >       |               |
> >       |               |
> >       |               \- ip->i_new_size
> >       \- ip->i_size
> > 
> > There is no need to grab the i_mutex of the IO lock in exclusive
> > mode if we don't need to invalidate the page cache. Taking these
> > locks on every direct IO effective serialises them as taking the IO
> > lock in exclusive mode has to wait for all shared holders to drop
> > the lock. That only happens when IO is complete, so effective it
> > prevents dispatch of concurrent direct IO writes to the same inode.
> > 
> > And so you can see that for the third concurrent IO, we'd avoid
> > exclusive locking for the same reason we avoided the exclusive lock
> > for the second IO.
> > 
> > Fixing this is a bit more complex than that, because we need to hold
> > a write-submission local value of ip->i_new_size to that clearing
> > the value is only done if no other thread has updated it before our
> > IO completes.....
> > 
> > Signed-off-by: Dave Chinner <dchinner@redhat.com>
> 
> This looks good.  What did you do with the little
> "If the IO is clearly not beyond the on-disk inode size,
> return before we take locks" optimization in xfs_setfilesize()
> from the last time you posted this?

That's take care of in Christoph's recent patch set.

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com

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