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Re: concurrent direct IO write in xfs

To: Dave Chinner <david@xxxxxxxxxxxxx>
Subject: Re: concurrent direct IO write in xfs
From: Zheng Da <zhengda1936@xxxxxxxxx>
Date: Wed, 25 Jan 2012 16:20:12 -0500
Cc: xfs@xxxxxxxxxxx
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Hello Dave, 

On Mon, Jan 23, 2012 at 10:54 PM, Dave Chinner <david@xxxxxxxxxxxxx> wrote:
> >> > So the test case is pretty simple and I think it's easy to reproduce it.
> >> > It'll be great if you can try the test case.
> >>
> >> Can you post your test code so I know what I test is exactly what
> >> you are running?
> >>
> > I can do that. My test code gets very complicated now. I need to simplify
> > it.
> >
> Here is the code. It's still a bit long. I hope it's OK.
> You can run the code like "rand-read file option=direct pages=1048576
> threads=8 access=write/read".

With 262144 pages on a 2Gb ramdisk, the results I get on 3.2.0 are

Threads         Read    Write
   1           0.92s   1.49s
   2           0.51s   1.20s
   4           0.31s   1.34s
   8           0.22s   1.59s
  16           0.23s   2.24s

the contention is on the ip->i_ilock, and the newsize update is one
of the offenders It probably needs this change to
xfs_aio_write_newsize_update():

-        if (new_size == ip->i_new_size) {
+        if (new_size && new_size == ip->i_new_size) {

to avoid the lock being taken here.

But all that newsize crap is gone in the current git Linus tree,
so how much would that gains us:

Threads         Read    Write
   1           0.88s   0.85s
   2           0.54s   1.20s
   4           0.31s   1.23s
   8           0.27s   1.40s
  16           0.25s   2.36s

Pretty much nothing. IOWs, it's just like I suspected - you are
doing so many write IOs that you are serialising on the extent
lookup and write checks which use exclusive locking..

Given that it is 2 lock traversals per write IO, we're limiting at
about 4-500,000 exclusive lock grabs per second and decreasing as
contention goes up.

For reads, we are doing 2 shared (nested) lookups per read IO, we
appear to be limiting at around 2,000,000 shared lock grabs per
second. Ahmdals law is kicking in here, but it means if we could
make the writes to use a shared lock, it would at least scale like
the reads for this "no metadata modification except for mtime"
overwrite case.

I don't think that the generic write checks absolutely need
exclusive locking - we probably could get away with a shared lock
and only fall back to exclusive when we need to do EOF zeroing.
Similarly, for the block mapping code if we don't need to do
allocation, a shared lock is all we need. So maybe in that case for
direct IO when create == 1, we can do a read lookup first and only
grab the lock exclusively if that falls in a hole and requires
allocation.....

Do you think if you will provide a patch for the changes?

Thanks,
Da
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