Re: File System Performance results

[Chris Mason <chris.mason@oracle.com>]

On Mon, 2008-11-03 at 13:14 -0600, Steven Pratt wrote:
> Chris Mason wrote:
> > On Wed, 2008-10-22 at 15:06 -0500, Steven Pratt wrote:
> >   
> >> We have set up a new page which is intended mainly for tracking the 
> >> performance of BTRFS, but in doing so we are testing other filesystems 
> >> as well (ext3, ext4, xfs and jfs).  Thought some people here might find 
> >> the results useful.
> >>     
> >
> > I think I understand the bad read performance in btrfs.  I was forcing a
> > tiny max readahead size.
> >
> > The current git tree has fixes for it, along with a ton of new code.
> >   
> Results for the the new (Git pull on 10/29) on the raid system are 
> complete.  Sequential read with a small number of threads has increased 
> dramatically, however on large number of threads (128) we see a large 
> dropoff in performance from before, as well as a huge spike in CPU 
> utilization. A quick look at the oprofile reveals some new functions at 
> the top which seem really out of place on a read only workload.
> 
> samples  %        image name               app name                 symbol name
> 13752215 23.8658  btrfs.ko                 btrfs                    alloc_extent_state
> 12840571 22.2837  btrfs.ko                 btrfs                    free_extent_state

It took a while, but I think I've tracked this down.  Btrfs has some
debugging code to detect and cleanup leaks of the extent_state structs,
and this adds a lot of contention to the alloc_extent_state and
free_extent_state code.

I've pushed out fixes for this, along with some other important
optimizations that should improve btrfs scores in your benchmarks.

Could you please give things a try?

-chris