linux-kernel - Re: Latest vfs scalability patch

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Date:	Thu, 15 Oct 2009 12:39:58 +0200
From:	Nick Piggin <npiggin@...e.de>
To:	Anton Blanchard <anton@...ba.org>
Cc:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	linux-fsdevel@...r.kernel.org,
	Ravikiran G Thirumalai <kiran@...lex86.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Jens Axboe <axboe@...nel.dk>
Subject: Re: Latest vfs scalability patch

Hi Anton,

On Thu, Oct 15, 2009 at 09:08:54PM +1100, Anton Blanchard wrote:
> 
> Hi Nick,
> 
> > Several people have been interested to test my vfs patches, so rather
> > than resend patches I have uploaded a rollup against Linus's current
> > head.
> > 
> > ftp://ftp.kernel.org/pub/linux/kernel/people/npiggin/patches/fs-scale/
> > 
> > I have used ext2,ext3,autofs4,nfs as well as in-memory filesystems
> > OK (although this doesn't mean there are no bugs!). Otherwise, if your
> > filesystem compiles, then there is a reasonable chance of it working,
> > or ask me and I can try updating it for the new locking.
> > 
> > I would be interested in seeing any numbers people might come up with,
> > including single-threaded performance.
> 
> Thanks for doing a rollup patch, it made it easy to test. I gave it a spin on
> a 64 core (128 thread) POWER5+ box. I started simple by looking at open/close
> performance, eg:
> 
> void testcase(void)
> {
> 	char tmpfile[] = "/tmp/testfile.XXXXXX";
> 
> 	mkstemp(tmpfile);
> 
> 	while (1) {
> 		int fd = open(tmpfile, O_RDWR);
> 		close(fd);
> 	}
> }
> 
> At first the results were 10x slower. I took a look and it appears the
> MNT_MOUNTED flag is getting cleared by a remount (I'm testing on the root
> filesystem). This fixed it:

Oh dear, thanks for that. That bugfix is needed for the patchset
I just sent to be merged.

 
> --- fs/namespace.c~	2009-10-15 04:34:02.000000000 -0500
> +++ fs/namespace.c	2009-10-15 04:35:00.000000000 -0500
> @@ -1711,7 +1711,8 @@ static int do_remount(struct path *path,
>  	else
>  		err = do_remount_sb(sb, flags, data, 0);
>  	if (!err)
> -		path->mnt->mnt_flags = mnt_flags;
> +		path->mnt->mnt_flags = mnt_flags |
> +			(path->mnt->mnt_flags & MNT_MOUNTED);
>  	up_write(&sb->s_umount);
>  	if (!err) {
>  		security_sb_post_remount(path->mnt, flags, data);
> 
> Attached is a before and after graph. Single thread performance is 20%
> faster, and we go from hitting a wall at 2 cores to scaling all the way
> to 64 cores. Nice work!!!

Nice looking graph, thanks! Did you use the rcu patch there as
well? (my open-close testing chokes RCU but your system might
be managing to keep outstanding callbacks below the threshold)
 
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