[Devel] Re: [RFC] IO scheduler based io controller (V5)

[Jeff Moyer]

Vivek Goyal <vgoyal at redhat.com> writes:

> On Mon, Jun 22, 2009 at 11:40:42AM -0400, Jeff Moyer wrote:
>> Vivek Goyal <vgoyal at redhat.com> writes:
>> 
>> > On Sun, Jun 21, 2009 at 08:51:16PM +0530, Balbir Singh wrote:
>> >> * Vivek Goyal <vgoyal at redhat.com> [2009-06-19 16:37:18]:
>> >> 
>> >> > 
>> >> > Hi All,
>> >> > 
>> >> > Here is the V5 of the IO controller patches generated on top of 2.6.30.
>> >> [snip]
>> >> 
>> >> > Testing
>> >> > =======
>> >> >
>> >> 
>> >> [snip]
>> >> 
>> >> I've not been reading through the discussions in complete detail, but
>> >> I see no reference to async reads or aio. In the case of aio, aio
>> >> presumes the context of the user space process. Could you elaborate on
>> >> any testing you've done with these cases? 
>> >> 
>> >
>> > Hi Balbir,
>> >
>> > So far I had not done any testing with AIO. I have done some just now.
>> > Here are the results.
>> >
>> > Test1 (AIO reads)
>> > ================
>> > Set up two fio, AIO read jobs in two cgroup with weight 1000 and 500
>> > respectively. I am using cfq scheduler. Following are some lines from my test
>> > script.
>> >
>> > ===================================================================
>> > fio_args="--ioengine=libaio --rw=read --size=512M"
>> 
>> AIO doesn't make sense without O_DIRECT.
>> 
>
> Ok, here are the read results with --direct=1 for reads. In previous posting,
> writes were already direct.
>
> test1 statistics: time=8 16 20796   sectors=8 16 1049648
> test2 statistics: time=8 16 10551   sectors=8 16 581160
>
>
> Not sure why reads are so slow with --direct=1? In the previous test
> (no direct IO), I had cleared the caches using
> (echo 3 > /proc/sys/vm/drop_caches) so reads could not have come from page
> cache?

O_DIRECT bypasses the page cache, and hence the readahead code.  Try
driving deeper queue depths and/or using larger I/O sizes.

Cheers,
Jeff
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