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SubjectRe: Linux disk performance.
On 12/18/06, Erik Mouw <> wrote:
> >
> > 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 ( 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.

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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