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Date:      Wed, 03 Feb 2010 12:52:00 +0100
From:      Jordi Espasa Clofent <jespasac@minibofh.org>
To:        freebsd-stable@freebsd.org
Subject:   Re: ionice in FreeBSD?
Message-ID:  <4B696360.3070209@minibofh.org>
In-Reply-To: <4B695A1A.1000505@incunabulum.net>
References:  <4B685EBA.4020501@minibofh.org> <4B695A1A.1000505@incunabulum.net>

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On 02/03/2010 12:12 PM, Bruce Simpson wrote:
> On 02/02/2010 17:19, Jordi Espasa Clofent wrote:
>>
>> In FreeBSD we've nice(1), renice(8) and even rtprio, idprio(1) but if
>> I'm understanding correctly, they're related to CPU priorty only, not
>> to I/O.
>
> That's not entirely true.
>
> A thread's CPU priority is still going to affect its ability to be
> scheduled on the CPU, and if it's waiting in the read() or write()
> syscalls, then this will make a difference to how quickly it can
> complete the next call.

Yes. I've already supposed it.

> However, it doesn't explicitly affect relative I/O prioritization. This
> is another story entirely. I suspect in a lot of cases adding a weight
> to per thread I/O, isn't going to make much difference for disk I/Os
> which are being sorted for the geometry (e.g. AHCI NCQ).
>
> So I guess my question is, 'why do you need I/O scheduling, and what
> aspect of system performance are you trying to solve with it' ?

Some shell-scripts based on dd or rsync, for example. Even a daily 
antivirus (ClamAV) scanner means an extensive I/O.

-- 
I must not fear. Fear is the mind-killer. Fear is the little-death that 
brings total obliteration. I will face my fear. I will permit it to pass 
over me and through me. And when it has gone past I will turn the inner 
eye to see its path. Where the fear has gone there will be nothing. Only 
I will remain.

Bene Gesserit Litany Against Fear.



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