On Wed, Aug 14, 2013 at 10:10:07AM -0700, Dave Hansen wrote:

We talked a little about this issue in this thread:

	http://marc.info/?l=linux-mm&m=137573185419275&w=2

but I figured I'd follow up with a full comparison.  ext4 is about 20%
slower in handling write page faults than ext3.  xfs is about 30% slower
than ext3.  I'm running on an 8-socket / 80-core / 160-thread system.
Test case is this:

	https://github.com/antonblanchard/will-it-scale/blob/master/tests/page_fault3.c

So, it writes a 128MB file sequentially via mmap page faults. This
isn't a page fault benchmark, as such...

It's a little easier to look at the trends as you grow the number of
processes:

	http://www.sr71.net/~dave/intel/page-fault-exts/cmp.html?1=ext3&2=ext4&3=xfs&hide=linear,threads,threads_idle,processes_idle&rollPeriod=16

I recorded and diff'd some perf data (I've still got the raw data if
anyone wants it), and the main culprit of the ext4/xfs delta looks to be
spinlock contention (or at least bouncing) in xfs_log_commit_cil().
This looks to be a known problem:

	http://oss.sgi.com/archives/xfs/2013-07/msg00110.html

Yup, apparently they've been pulled into the xfsdev tree, but i
haven't seen it updated since they were pulled in so the linux-next
builds aren't picking up the fixes yet.

Here's a brief snippet of the ext4->xfs 'perf diff'.  Note that things
like page_fault() go down in the profile because we are doing _fewer_ of
them, not because it got faster:

quoted

# Baseline    Delta          Shared Object                                          Symbol
# ........  .......  .....................  ..............................................
#
    22.04%   -4.07%  [kernel.kallsyms]      [k] page_fault                                
     2.93%  +12.49%  [kernel.kallsyms]      [k] _raw_spin_lock                            
     8.21%   -0.58%  page_fault3_processes  [.] testcase                                  
     4.87%   -0.34%  [kernel.kallsyms]      [k] __set_page_dirty_buffers                  
     4.07%   -0.58%  [kernel.kallsyms]      [k] mem_cgroup_update_page_stat               
     4.10%   -0.61%  [kernel.kallsyms]      [k] __block_write_begin                       
     3.69%   -0.57%  [kernel.kallsyms]      [k] find_get_page                             

It's a bit of a bummer that things are so much less scalable on the
newer filesystems.

Sorry, what? What filesystems are you comparing here? XFS is
anything but new...

I expected xfs to do a _lot_ better than it did.

perf diff doesn't tell me anything about how you should expect the
workload to scale.

This workload appears to be a concurrent write workload using
mmap(), so performance is going to be determined by filesystem
configuration, storage capability and the CPU overhead of the
page_mkwrite() path through the filesystem. It's not a page fault
benchmark at all - it's simply a filesystem write bandwidth
benchmark.

So, perhaps you could describe the storage you are using, as that
would shed more light on your results. A good summary of what
information is useful to us is here:

http://xfs.org/index.php/XFS_FAQ#Q:_What_information_should_I_include_when_reporting_a_problem.3F

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....

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com

_______________________________________________
xfs mailing list
xfs@oss.sgi.com
http://oss.sgi.com/mailman/listinfo/xfs