Re: Latest vfs scalability patch

[Nick Piggin <npiggin@suse.de>]

On Tue, Oct 06, 2009 at 12:14:14PM +0200, 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

Yep, this is the problem of the common-path lookup. Every dentry
element in the path has its d_lock taken for every path lookup,
so cwd dentry lock bounces a lot for dbench.

I'm working on doing path traversal without any locks or stores
to the dentries in the common cases, so that should basically
be the last bit of the puzzle for vfs locking (although it can be
considered a different type of problem than the global lock
removal, but RCU-freed struct inode is important for the approach
I'm taking, so I'm basing it on top of these patches).
 
It's a copout, but you could try running multiple dbenches under
different working directories (or actually, IIRC dbench does root
based path lookups so maybe that won't help you much).


> Anyway, below are the results. Seem very stable.
> 
>                                 throughput
> ------------------------------------------------
> 2.6.32-rc3-git          |      561.218 MB/sec
> 2.6.32-rc3-git+patch    |      627.022 MB/sec

Well it's good to see you got some improvement.