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Date: Thu, 23 Feb 2012 16:22:33 -0800
From: Patricia Shanahan <pats@acm.org>
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Subject: Re: O.T. optimising file placement
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On 2/23/2012 3:16 PM, Martin Gregorie wrote:
> On Wed, 22 Feb 2012 21:40:19 -0500, Arne Vajhøj wrote:
>
>>
>> Most OS'es support async IO.
>>
> Yes, I know, but its not relevant to a single-threaded process since its
> logic generally requires it to wait for a read or write to complete
> before it continues[1]. Hence my comment that this prevents head movement
> being optimized unless a lot of processes are active because there's only
> one outstanding IOP per process.
>
> [1] unless you're deliberately doing async i/o using poll() or
>      select() (in C) or nio (in Java), in which case the process is often
>      best regarded as a half-way house between single and multi-threaded
>      logic.
>
>

Last time I looked at operating system internals, there was a lot of
prefetch logic that could generate multiple parallel requests from a
one-at-a-time sequence.

Patricia