Re: Time sliced CFQ io scheduler

[Nick Piggin <nickpiggin@yahoo.com.au>]

Jens Axboe wrote:
> On Thu, Dec 02 2004, Andrew Morton wrote:
> 
>>Jens Axboe <axboe@suse.de> wrote:
>>
>>>>So what are you doing different?
>>>
>>>Doing sync io, most likely. My results above are 64k O_DIRECT reads and
>>>writes, see the mention of the test cases in the first mail.
>>
>>OK.
>>
>>Writer:
>>
>>	while true
>>	do
>>	write-and-fsync -o -m 100 -c 65536 foo 
>>	done
>>
>>Reader:
>>
>>	time-read -o -b 65536 -n 256 x      (This is O_DIRECT)
>>or:	time-read -b 65536 -n 256 x	    (This is buffered)
>>
>>`vmstat 1':
>>
>>procs -----------memory---------- ---swap-- -----io---- --system-- ----cpu----
>> r  b   swpd   free   buff  cache   si   so    bi    bo   in    cs us sy id wa
>> 1  1   1032 137412   4276  84388   32    0 15456 25344 1659  1538  0  3 50 47
>> 0  1   1032 137468   4276  84388    0    0     0 32128 1521  1027  0  2 51 48
>> 0  1   1032 137476   4276  84388    0    0     0 32064 1519  1026  0  1 50 49
>> 0  1   1032 137476   4276  84388    0    0     0 33920 1556  1102  0  2 50 49
>> 0  1   1032 137476   4276  84388    0    0     0 33088 1541  1074  0  1 50 49
>> 0  2   1032 135676   4284  85944    0    0  1656 29732 1868  2506  0  3 49 47
>> 1  1   1032  96532   4292 125172    0    0 39220   128 10813 39313  0 31 35 34
>> 0  2   1032  57724   4332 163892    0    0 38828   128 10716 38907  0 28 38 35
>> 0  2   1032  18860   4368 202684    0    0 38768   128 10701 38845  1 28 38 35
>> 0  2   1032   3672   4248 217764    0    0 39188   128 10803 39327  0 28 37 34
>> 0  1   1032   2832   4260 218840    0    0 16812 17932 5504 17457  0 14 46 40
> 
> 
> Well there you go, exactly what I saw. The writer(s) basically make no
> progress as long as the reader is going. Since 'as' treats the sync
> writes like reads internally and given the really bad fairness problems
> demonstrated for same direction clients, that might be the same problem.
> 
> 
>>Ugly.
>>
>>(write-and-fsync and time-read are from
>>http://www.zip.com.au/~akpm/linux/patches/stuff/ext3-tools.tar.gz)
> 
> 
> I'll try and post my cruddy test programs tomorrow as well. Pretty handy
> for getting a good feel for N client read/write performance.
> 


OK, sorry for not jumping in earlier. Yes, it will be synch IO that
is your problem.

I'll see if I can try improving things there for AS. I see (from your
first results in this thread) that CFQ does quite nicely here, better
than deadline.