page fault scalability (ext3, ext4, xfs)

[Dave Chinner]

On Wed, Aug 14, 2013 at 07:24:01PM -0700, Andi Kleen wrote:
> > And FWIW, it's no secret that XFS has more per-operation overhead
> > than ext4 through the write path when it comes to allocation, so
> > it's no surprise that on a workload that is highly dependent on
> > allocation overhead that ext4 is a bit faster....
> 
> This cannot explain a worse scaling curve though?

The scaling curve is pretty much identical. The difference in
performance will be the overhead of timestamp updates through
the transaction subsystems of the filesystems.

> w-i-s is all about scaling.

Sure, but scaling *what*? It's spending all it's time in the
filesystem through the .page_mkwrite path. It's not a page fault
scaling test - it's a filesystem overwrite test that uses mmap.
Indeed, I bet if you replace the mmap() with a write(fd, buf, 4096)
loop, you'd get almost identical behaviour from the filesystems.

Cheers,

Dave.
-- 
Dave Chinner
david at fromorbit.com