linux-kernel - Re: [cgroup] a0f9ec1f181: -4.3% will-it-scale.per_thread

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Date:	Thu, 15 May 2014 16:16:05 +0800
From:	Fengguang Wu <fengguang.wu@...el.com>
To:	Tejun Heo <tj@...nel.org>
Cc:	Jet Chen <jet.chen@...el.com>, LKML <linux-kernel@...r.kernel.org>,
	lkp@...org
Subject: Re: [cgroup] a0f9ec1f181: -4.3% will-it-scale.per_thread_ops

On Thu, May 15, 2014 at 02:14:22AM -0400, Tejun Heo wrote:
> Hello, Fengguang.
> 
> On Thu, May 15, 2014 at 02:00:26PM +0800, Fengguang Wu wrote:
> > > > 2074b6e38668e62  a0f9ec1f181534694cb5bf40b
> > > > ---------------  -------------------------
> > 
> > 2074b6e38668e62 is the base of comparison. So "-4.3% will-it-scale.per_thread_ops"
> > in the below line means a0f9ec1f18 has lower will-it-scale throughput.
> > 
> > > >    1027273 ~ 0%      -4.3%     982732 ~ 0%  TOTAL will-it-scale.per_thread_ops
> > > >        136 ~ 3%     -43.1%         77 ~43%  TOTAL proc-vmstat.nr_dirtied
> > > >       0.51 ~ 3%     +98.0%       1.01 ~ 4%  TOTAL perf-profile.cpu-cycles.shmem_write_end.generic_perform_write.__generic_file_aio_write.generic_file_aio_write.do_sync_write
> > > >       1078 ~ 9%     -16.3%        903 ~11%  TOTAL numa-meminfo.node0.Unevictable
> > > >        269 ~ 9%     -16.2%        225 ~11%  TOTAL numa-vmstat.node0.nr_unevictable
> > > >       1.64 ~ 1%     -14.3%       1.41 ~ 4%  TOTAL perf-profile.cpu-cycles.find_lock_entry.shmem_getpage_gfp.shmem_write_begin.generic_perform_write.__generic_file_aio_write
> > > >       1.62 ~ 2%     +14.1%       1.84 ~ 1%  TOTAL perf-profile.cpu-cycles.lseek64
> > 
> > The perf-profile.cpu-cycles.* lines are from "perf record/report".
> > 
> > The last line shows that lseek64() takes 1.62% CPU cycles for
> > commit 2074b6e38668e62 and that percent increased by +14.1% on
> > a0f9ec1f181. One of the raw perf record output is
> > 
> >      1.84%  writeseek_proce  libc-2.17.so         [.] lseek64                               
> >             |
> >             --- lseek64
> > 
> > There are 5 runs and 1.62% is the average value.
> > 
> > > I have no idea how to read the above.  Which direction is plus and
> > > which is minus? Are they counting cpu cycles?  Which files is the
> > > test seeking?
> > 
> > It's tmpfs files. Because the will-it-scale test case is mean to
> > measure scalability of syscalls. We do not use HDD/SSD etc. storage
> > devices when running it.
> 
> Hmmm... I'm completely stumped.  The commit in question has nothing to
> do with tmpfs.  It only affects three cgroup files - "tasks",
> "cgroup.procs" and "release_agent".  It can't possibly have any effect
> on tmpfs operation.  Maybe random effect through code alignment?  Even
> that is highly unlikely.  I'll look into it tomorrow but can you
> please try to repeat the test?  It really doesn't make any sense to
> me.

Yes, sorry! Even though the "first bad" commit a0f9ec1f1 and its
parent commit 2074b6e38 has clear and stable performance changes:

5 runs of a0f9ec1f1:

  "will-it-scale.per_thread_ops": [
    983098,
    985112,
    982690,
    976157,
    986606
  ],

5 runs of 2074b6e38:

  "will-it-scale.per_thread_ops": [
    1027667,
    1029414,
    1026736,
    1025678,
    1026871
  ],

Comparing the bisect-good and bisect-bad *kernels*, you'll find the
performance changes are not as stable:

                             will-it-scale.per_thread_ops
  
  1.14e+06 ++---------------------------------------------------------------+
  1.12e+06 ++                               *..                             |
           |                               :   *                            |
   1.1e+06 ++                              :    :                           |
  1.08e+06 ++                             :     :                           |
           |                              :      :                          |
  1.06e+06 ++                            :       :                          |
  1.04e+06 *+.*...*..*..*..*...*..*..    :        :    ..*..*..             |
  1.02e+06 ++              O         *..*         *..*.        *..*...*..*..*
           |                   O                  O                         |
     1e+06 O+ O   O  O  O                      O                            |
    980000 ++                           O   O        O   O  O  O  O         |
           |                                                                |
    960000 ++                     O  O                                      |
    940000 ++---------------------------------------------------------------+

        [*] bisect-good sample
        [O] bisect-bad  sample

So it might be some subtle data padding/alignment issue.

Thanks,
Fengguang
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