linux-kernel - Re: Slow file transfer speeds with CFQ IO scheduler in some cases

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Date:	Tue, 11 Nov 2008 14:36:07 -0500
From:	Jeff Moyer <jmoyer@...hat.com>
To:	Jens Axboe <jens.axboe@...cle.com>
Cc:	"Vitaly V. Bursov" <vitalyb@...enet.dn.ua>,
	linux-kernel@...r.kernel.org, jlayton@...hat.com
Subject: Re: Slow file transfer speeds with CFQ IO scheduler in some cases

Jens Axboe <jens.axboe@...cle.com> writes:

> OK, that looks better. Can I talk you into just trying this little
> patch, just to see what kind of performance that yields? Remove the cfq
> patch first. I would have patched nfsd only, but this is just a quick'n
> dirty.

I went ahead and gave it a shot.  The updated CFQ patch with no I/O
context sharing does about 40MB/s reading a 1GB file.  Backing that
patch out, and then adding the patch to share io_context's between
kthreads yields 45MB/s.

By the way, in looking at the copy_io function, I noticed what appears
to be a (minor) bug:

        if (clone_flags & CLONE_IO) {
                tsk->io_context = ioc_task_link(ioc);
                if (unlikely(!tsk->io_context))
                        return -ENOMEM;

According to comments in ioc_task_link, tsk->io_context == NULL means:
        /*
         * if ref count is zero, don't allow sharing (ioc is going away, it's
         * a race).
         */

It seems more appropriate to just create a new I/O context at this
point, don't you think?  (Sorry, I know it's off-topic!)

Cheers,

Jeff

diff --git a/kernel/fork.c b/kernel/fork.c
index f608356..483d95c 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -723,10 +723,17 @@ static int copy_io(unsigned long clone_flags, struct task_struct *tsk)
 	 * Share io context with parent, if CLONE_IO is set
 	 */
 	if (clone_flags & CLONE_IO) {
+		/*
+		 * If ioc_task_link fails, it just means that we raced
+		 * with io context cleanup.  Continue on to allocate
+		 * a new context in this case.
+		 */
 		tsk->io_context = ioc_task_link(ioc);
-		if (unlikely(!tsk->io_context))
-			return -ENOMEM;
-	} else if (ioprio_valid(ioc->ioprio)) {
+		if (likely(tsk->io_context))
+			return 0;
+	}
+
+	if (ioprio_valid(ioc->ioprio)) {
 		tsk->io_context = alloc_io_context(GFP_KERNEL, -1);
 		if (unlikely(!tsk->io_context))
 			return -ENOMEM;
--
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