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Re: [Xen-users] io scheduler

To: Anand <xen.mails@xxxxxxxxx>
Subject: Re: [Xen-users] io scheduler
From: Mogens Valentin <mogensv@xxxxxxxxxxxxxxxx>
Date: Mon, 09 Jan 2006 21:17:51 +0100
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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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