linux-kernel - Re: cfq-iosched: tiobench regression

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Message-ID: <20091224091919.GA17516@sli10-desk.sh.intel.com>
Date:	Thu, 24 Dec 2009 17:19:19 +0800
From:	Shaohua Li <shaohua.li@...el.com>
To:	Gui Jianfeng <guijianfeng@...fujitsu.com>
Cc:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"jens.axboe@...cle.com" <jens.axboe@...cle.com>,
	"jmoyer@...hat.com" <jmoyer@...hat.com>,
	"czoccolo@...il.com" <czoccolo@...il.com>,
	"Zhang, Yanmin" <yanmin.zhang@...el.com>
Subject: Re: cfq-iosched: tiobench regression

On Thu, Dec 24, 2009 at 03:48:38PM +0800, Gui Jianfeng wrote:
> Shaohua Li wrote:
> > We see about 30% regression in tiobench 32 threads 80M file sequential read.
> > The regression is caused by below commits.
> > 
> > 5db5d64277bf390056b1a87d0bb288c8b8553f96
> > The commit makes the slice too small. In the test, the slice is limitted
> > to 2 * idle_slice(300ms/32 < 2*idle_slice). This dramatically impacts io
> > thoughput. The low_latency knob used to be only impact random io, now it
> > impacts sequential io too. Any idea to fix it?
> 
> Hi Shaohua,
> 
> IMHO this shouldn't be a problem. Currently, low_latency is used to improve
> the latency for the whole system. If someone would like to achieve high throughput,
> just turn off low_latency knob.
The concern is low_latency is default on. A end user is unlikely to know the knob.

Thanks,
Shaohua
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