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