linux-kernel - Re: Linux disk performance.

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Message-ID: <652016d30612182222h7fde4ea5jbc0927c8ebeae76a@mail.gmail.com>
Date:	Tue, 19 Dec 2006 12:07:14 +0545
From:	"Manish Regmi" <regmi.manish@...il.com>
To:	"Erik Mouw" <mouw@...linux.org>, nickpiggin@...oo.com.au
Cc:	kernelnewbies@...linux.org, linux-kernel@...r.kernel.org
Subject: Re: Linux disk performance.

On 12/18/06, Erik Mouw <mouw@...linux.org> wrote:
<...snip...>
> >
> > But isn't O_DIRECT supposed to bypass buffering in Kernel?
>
> It is.
>
> > Doesn't it directly write to disk?
>
> Yes, but it still uses an IO scheduler.
>

Ok. but i also tried with noop to turnoff disk scheduling effects.
There was still timing differences. Usually i get 3100 microseconds
but upto 20000 microseconds at certain intervals. I am just using
gettimeofday between two writes to read the timing.

> In your first message you mentioned you were using an ancient 2.6.10
> kernel. That kernel uses the anticipatory IO scheduler. Update to the
> latest stable kernel (2.6.19.1 at time of writing) and it will default
> to the CFQ scheduler which has a smoother writeout, plus you can give
> your process a different IO scheduling class and level (see
> Documentation/block/ioprio.txt).

Thanks... i will try with CFQ.

Nick Piggin:
> but
> they look like they might be a (HZ quantised) delay coming from
> block layer plugging.

Sorry i didn´t understand what you mean.

To minimise scheduling effects i tried giving it maximum priority.

-- 
---------------------------------------------------------------
regards
Manish Regmi

---------------------------------------------------------------
UNIX without a C Compiler is like eating Spaghetti with your mouth
sewn shut. It just doesn't make sense.
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