linux-kernel - Re: [RFC,PATCH] cfq-iosched: improve async queue ramp up formula

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Date:	Fri, 27 Nov 2009 09:23:16 +0100
From:	Jens Axboe <jens.axboe@...cle.com>
To:	Corrado Zoccolo <czoccolo@...il.com>
Cc:	Linux-Kernel <linux-kernel@...r.kernel.org>,
	Jeff Moyer <jmoyer@...hat.com>,
	Vivek Goyal <vgoyal@...hat.com>, mel@....ul.ie, efault@....de
Subject: Re: [RFC,PATCH] cfq-iosched: improve async queue ramp up formula

On Thu, Nov 26 2009, Corrado Zoccolo wrote:
> The introduction of ramp-up formula for async queue depths has
> slowed down dirty page reclaim, by reducing async write performance.
> This patch improves the formula by considering the remaining slice.
> 
> The new formula will allow more dispatches at the beginning of the
> slice, reducing them at the end.
> This will ensure that we achieve good throughput, without the risk of
> overrunning the allotted timeslice.
> 
> The threshold is automatically increased when sync I/O is not
> intermingled with async, in accordance with the previous incarnation of
> the formula.

The slow ramp up is pretty much essential to being able to have low
latency for the sync reads, so I'm afraid this will break that. I would
prefer doing it through memory reclaim detection, like the other patch
you and Motohiro suggested.

-- 
Jens Axboe

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