On 12/18/06, Erik Mouw <[email protected]> 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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