linux-kernel - Re: Latest vfs scalability patch

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Date:	Tue, 6 Oct 2009 12:26:05 +0200
From:	Jens Axboe <jens.axboe@...cle.com>
To:	Nick Piggin <npiggin@...e.de>
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>
Subject: Re: Latest vfs scalability patch

On Tue, Oct 06 2009, Jens Axboe wrote:
> On Tue, Oct 06 2009, Nick Piggin wrote:
> > Hi,
> > 
> > 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.
> 
> I gave this a quick spin on the 64-thread nehalem. Just a simple dbench
> with 64 clients on tmpfs. The results are below. While running perf top
> -a in mainline, the top 5 entries are:
> 
>           2086691.00 - 96.6% : _spin_lock
>             14866.00 -  0.7% : copy_user_generic_string
>              5710.00 -  0.3% : mutex_spin_on_owner
>              2837.00 -  0.1% : _atomic_dec_and_lock
>              2274.00 -  0.1% : __d_lookup
> 
> Uhm auch... It doesn't look much prettier for the patch kernel, though:
> 
>           9396422.00 - 95.7% : _spin_lock
>             66978.00 -  0.7% : copy_user_generic_string
>             43775.00 -  0.4% : dput
>             23946.00 -  0.2% : __link_path_walk
>             17699.00 -  0.2% : path_init
>             15046.00 -  0.2% : do_lookup

I did a quick perf analysis on that, but only on 8 clients (64 clients
basically causes perf to shit itself, it's just not functional). So for
a 8 client 60s dbench run, we're already into ~75% spinlock time. The
components are:

dput (44%)
        path_put
                path_walk
                        do_path_lookup
path_get (44%)
        path_init
                do_path_lookup
__d_path (7%)

vfsmount_read_lock (3%)

This is for the patched kernel.

-- 
Jens Axboe

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