linux-kernel - CFQ is worse than other IO schedulers in some cases

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Date:	Wed, 18 Feb 2009 14:00:51 +0800
From:	Shan Wei <shanwei@...fujitsu.com>
To:	jens.axboe@...cle.com
CC:	linux-kernel@...r.kernel.org
Subject: CFQ is worse than other IO schedulers in some cases

I found that CFQ's performance is worse than other IO scheduer in some cases
I confirmed its phenomenon when I executed dump command and sysbench on 2.6.28.


In dump(version:dump-0.4b41-2.fc6), I confirmed 
the speed under CFQ is slower than other IO schedulers.


The Test Result(dump):
   UNIT:Mb/sec
    _______________________
    |   IO       |        | 
    | scheduler  |  Speed |
    +------------|--------|
    |cfq         | 24.310 |  
    |noop        | 36.885 |  
    |anticipatory| 34.956 |  
    |deadline    | 36.758 |  
    +----------------------


Steps to reproduce(dump):
  #dump -0uf /dev/null /dev/sda6

  #df -h /dev/sda6
   Filesystem            Size  Used Avail Use% Mounted on
   /dev/sda6              19G   10G  7.6G  57% /mnt



In sysbench(version:sysbench-0.4.10), I confirmed followings.
  - CFQ's performance is worse than other IO schedulers when only multiple
    threads test.
    (There is no difference under single thread test.)
  - It is worse than other IO scheduler when
    I used read mode. (No regression in write mode).
  - There is no difference among other IO schedulers. (e.g noop deadline)


The Test Result(sysbench):
   UNIT:Mb/sec
    __________________________________________________
    |   IO       |      thread  number               |  
    | scheduler  |-----------------------------------|
    |            |  1   |  3    |  5   |   7  |   9  |
    +------------|------|-------|------|------|------|
    |cfq         | 77.8 |  32.4 | 43.3 | 55.8 | 58.5 | 
    |noop        | 78.2 |  79.0 | 78.2 | 77.2 | 77.0 |
    |anticipatory| 78.2 |  78.6 | 78.4 | 77.8 | 78.1 |
    |deadline    | 76.9 |  78.4 | 77.0 | 78.4 | 77.9 |
    +------------------------------------------------+


Steps to reproduce(sysbench):

  (1)#echo cfq > /sys/block/sda/queue/scheduler 

  (2)#sysbench --test=fileio --num-threads=1 --file-total-size=10G --file-test-mode=seqrd prepare

  (3)#sysbench --test=fileio --num-threads=1 --file-total-size=10G --file-test-mode=seqrd run
      [snip]
      Operations performed:  655360 Read, 0 Write, 0 Other = 655360 Total
      Read 10Gb  Written 0b  Total transferred 10Gb  (77.835Mb/sec)
      4981.44 Requests/sec executed                   ~~~~~~~~~~~
  (4)#sysbench --test=fileio --num-threads=1 --file-total-size=10G --file-test-mode=seqrd cleanup

  (5)#sysbench --test=fileio --num-threads=5 --file-total-size=10G --file-test-mode=seqrd prepare
  (6)#sysbench --test=fileio --num-threads=5 --file-total-size=10G --file-test-mode=seqrd run
      [snip]
      Operations performed:  655360 Read, 0 Write, 0 Other = 655360 Total
      Read 10Gb  Written 0b  Total transferred 10Gb  (43.396Mb/sec)
      2777.35 Requests/sec executed                   ~~~~~~~~~~~~
  (7)#sysbench --test=fileio --num-threads=5 --file-total-size=10G --file-test-mode=seqrd cleanup

when doing step 2 or 5, sysbench creats 128 files, and 80M each one. 
when doing step 4 or 7, sysbench deletes the files. 
when doing step 3 or 6, thread reads these files continuously and 
reads file-block-size(default:16Kbyte) at once, just like :

       t_0   t_0   t_0   t_0   t_0   t_0   t_0
        ^     ^     ^     ^     ^     ^     ^
     ---|-----|-----|-----|-----|-----|-----|--------
file | 16k | 16k | 16k | 16k | 16k | 16k | 16k | ... 
     ------------------------------------------------ 
                  (num-threads=1)

(t_0 stand for the first thread) 

       t_0   t_1   t_2   t_3   t_4   t_0   t_1
        ^     ^     ^     ^     ^     ^     ^
     ---|-----|-----|-----|-----|-----|-----|--------
file | 16k | 16k | 16k | 16k | 16k | 16k | 16k | ... 
     ------------------------------------------------ 
                  (num-threads=5)

(the executed threads are decide by the thread scheduler)


The Hardware Infos:
Arch    :x86_64
CPU     :4cpu; GenuineIntel 3325.087 MHz
MEMORY  :4044128kB

---- 
Shan Wei

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