linux-kernel - [sched] 23f0d2093c: -12.6% regression on sparse file copy

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Date:	Sun, 5 Jan 2014 17:04:56 +0800
From:	fengguang.wu@...el.com
To:	Joonsoo Kim <iamjoonsoo.kim@....com>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...nel.org>,
	LKML <linux-kernel@...r.kernel.org>, lkp@...ux.intel.com
Subject: [sched] 23f0d2093c: -12.6% regression on sparse file copy

Hi Joonsoo,

We noticed the below changes for commit 23f0d2093c ("sched: Factor out
code to should_we_balance()") in test vm-scalability/300s-lru-file-readtwice

        95a79b805b935f4  23f0d2093c789e612185180c4
        ---------------  -------------------------
==>           4.45 ~ 5%   +1777.7%      83.60 ~ 5%  vm-scalability.stddev
==>       14966511 ~ 0%     -12.6%   13084545 ~ 2%  vm-scalability.throughput
                38 ~ 9%    +406.3%        193 ~ 7%  proc-vmstat.kswapd_low_wmark_hit_quickly
            610823 ~ 0%     -41.4%     357990 ~ 0%  softirqs.SCHED
         5.424e+08 ~ 0%     -38.5%  3.338e+08 ~ 6%  proc-vmstat.pgdeactivate
          4.68e+08 ~ 0%     -37.5%  2.924e+08 ~ 6%  proc-vmstat.pgrefill_normal
         5.549e+08 ~ 0%     -37.1%  3.491e+08 ~ 6%  proc-vmstat.pgactivate
          14938509 ~ 1%     +27.0%   18974176 ~ 1%  vmstat.memory.free
            978771 ~ 1%     +23.9%    1212704 ~ 3%  numa-vmstat.node2.nr_free_pages
           3747434 ~ 0%     +21.7%    4560196 ~ 2%  proc-vmstat.nr_free_pages
==>      1.353e+08 ~ 0%     +18.8%  1.607e+08 ~ 0%  proc-vmstat.numa_foreign
         1.353e+08 ~ 0%     +18.8%  1.607e+08 ~ 0%  proc-vmstat.numa_miss
         1.353e+08 ~ 0%     +18.8%  1.607e+08 ~ 0%  proc-vmstat.numa_other
           3936842 ~ 1%     +22.2%    4812045 ~ 4%  numa-meminfo.node2.MemFree
          21803812 ~ 0%     +17.7%   25661536 ~ 4%  numa-vmstat.node3.numa_foreign
          73701524 ~ 0%     +15.0%   84769542 ~ 0%  proc-vmstat.pgscan_direct_dma32
          73700683 ~ 0%     +15.0%   84768687 ~ 0%  proc-vmstat.pgsteal_direct_dma32
         3.101e+08 ~ 0%     +11.2%  3.448e+08 ~ 0%  proc-vmstat.pgsteal_direct_normal
         3.103e+08 ~ 0%     +11.2%  3.449e+08 ~ 0%  proc-vmstat.pgscan_direct_normal
          45613907 ~ 0%     +12.6%   51342974 ~ 3%  numa-vmstat.node0.numa_other
            795639 ~ 0%     -48.6%     409113 ~13%  time.voluntary_context_switches
               375 ~ 0%      +6.1%        398 ~ 0%  time.elapsed_time
              9427 ~ 0%      -5.8%       8880 ~ 0%  time.percent_of_cpu_this_job_got

The test case basically does

for i in `seq 1 $nr_cpu`
do      
        create_sparse_file huge-$i
        dd if=huge-$i of=/dev/null &
        dd if=huge-$i of=/dev/null &
done

where nr_cpu=120 (test box is a 4-socket ivybridge system).

The change looks stable, each point below is a sample run:

                               vm-scalability.stddev

   120 ++-------------------------------------------------------------------+
       |                                                                    |
   100 ++             *            *                                        |
       | *.***        :    **      :     *    *  *     *     *              |
       **     * *.** * :  *  :*.*  :: .* :   : * :*   * :  .* :   .*   * .**|
    80 ++      *    *  *. :  *   *: ** : ::  :  *  :.*  * *   * ** :   :*   *
       |                 *        *     : ***      *     *     *    :**     |
    60 ++                               *                           *       |
       |                                                                    |
    40 ++                                                                   |
       |                                                                    |
       |                                                                    |
    20 ++                                                                   |
       | O  OO OO OOO  O OO  O                                              |
     0 OO--O--O------OO----OO-----------------------------------------------+

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