Re: [Xen-users] io scheduler

[Mogens Valentin <mogensv@xxxxxxxxxxxxxxxx>]

Anand wrote:
> Looking in the dmesg for a domU, i saw the following:
>
> io scheduler noop registered
> io scheduler anticipatory registered
> io scheduler deadline registered
> io scheduler cfq registered
>
> Reading docs i found it was about the Linux IO Scheduler.
>
> cat /sys/block/sda1/queue/scheduler
>
> [noop] anticipatory deadline cfq
>
> Now using cfq for scheduler
>
> echo cfq > /sys/block/sda1/queue/scheduler
>
> cat /sys/block/sda1/queue/scheduler
>
> noop anticipatory deadline [cfq]
>
> Now there is  a log entry in /var/log/messages about cfq, it says
>
> Jan  9 05:39:28 host6 kernel: cfq: depth 4 reached, tagging now on
>
> The same message is also displayed on the console of domU.
>
> The scheduler docs say that using elevator= option with kernel a 
> scheduler can be specified at the command line, however if i try to do 
> that in domU, it doesn't work.
> My dom0 has anticipatory as the scheduler which is the default IO scheduler.
>
> Any one has any ideas or worked with the io scheduler ?
On non-xen, yes. There's some benchmarks somewhere (don't seem to find 
them right now) about advanteges of the various ones.
/usr/src/linux/Documentation/kernel-parameters.txt  says a few words.

As you've seen for yourself, the scheduler can be changed runtime, and 
likewise as a boot option.
My understanding is that schedulers works fairly directly on hardware, 
at least reorganizing disk queues, I -think- you cannot have different 
schedulers for dom0 and domU's.
Why would you? It's the same physical disk...
I believe cfq is usually considered the most versatile, unless specific 
needs are determined.
--
Kind regards,
Mogens Valentin


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