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SubjectCFQ 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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