linux-kernel - Re: [PATCH]cfq-iosched: don't stop async queue with async requests pending

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Date:	Wed, 13 Jan 2010 06:13:41 -0500
From:	Vivek Goyal <vgoyal@...hat.com>
To:	Shaohua Li <shaohua.li@...el.com>
Cc:	Corrado Zoccolo <czoccolo@...il.com>,
	"jens.axboe@...cle.com" <jens.axboe@...cle.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"jmoyer@...hat.com" <jmoyer@...hat.com>,
	"guijianfeng@...fujitsu.com" <guijianfeng@...fujitsu.com>,
	"yanmin_zhang@...ux.intel.com" <yanmin_zhang@...ux.intel.com>
Subject: Re: [PATCH]cfq-iosched: don't stop async queue with async requests
	pending

On Wed, Jan 13, 2010 at 04:23:22PM +0800, Shaohua Li wrote:
> On Wed, Jan 13, 2010 at 04:18:47PM +0800, Corrado Zoccolo wrote:
> > On Wed, Jan 13, 2010 at 8:44 AM, Shaohua Li <shaohua.li@...el.com> wrote:
> > > My SSD speed of direct write is about 80m/s, while I test page writeback,
> > > the speed can only go to 68m/s. Below patch fixes this.
> > > It appears we missused cfq_should_idle in cfq_may_dispatch. cfq_should_idle
> > > means a queue should idle because it's seekless sync queue or it's the last queue,
> > > which is to maintain service tree time slice. So it doesn't mean the
> > > last queue is always a sync queue. If the last queue is asyn queue,
> > > we definitely shouldn't stop dispatch requests because of pending async
> > > requests.
> > 
> > An other option is that cfq_should_idle returns false for async
> > queues, since cfq will never idle on them.
> I'm considering this option too, but it appears we need make async queue
> idle to maintain domain time slice.

IMHO, we don't have to wait on async write service tree. Generally aysnc
write queus contain many requests and they are not like reads where next
request is expected. So idling on aysnc write service tree is waste of
time and will lead to reduced throughput.

Vivek
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