Re: page fault scalability (ext3, ext4, xfs)

Dave Chinner
2013-08-15T00:25:12Z

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:
>
> > # 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