Date: Thu, 14 Jan 2010 14:17:31 +0800
From: Shaohua Li <shaohua.li@intel.com>
To: Gui Jianfeng <guijianfeng@cn.fujitsu.com>
Cc: Vivek Goyal <vgoyal@redhat.com>, Corrado Zoccolo <czoccolo@gmail.com>,
       "jens.axboe@oracle.com" <jens.axboe@oracle.com>,
       "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
       "jmoyer@redhat.com" <jmoyer@redhat.com>,
       "yanmin_zhang@linux.intel.com" <yanmin_zhang@linux.intel.com>
Subject: Re: [PATCH]cfq-iosched: don't stop async queue with async requests
 pending
Message-ID: <20100114061731.GA23590@sli10-desk.sh.intel.com>
References: <20100113074442.GA10492@sli10-desk.sh.intel.com>
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 <20100113082322.GA24345@sli10-desk.sh.intel.com>
 <20100113111341.GB3087@redhat.com>
 <20100114034150.GA3922@sli10-desk.sh.intel.com>
 <4B4EAB39.1070301@cn.fujitsu.com>
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On Thu, Jan 14, 2010 at 01:27:21PM +0800, Gui Jianfeng wrote:
> Shaohua Li wrote:
> > On Wed, Jan 13, 2010 at 07:13:41PM +0800, Vivek Goyal wrote:
> >> 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@intel.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.
> > I fully agree async queue doesn't need wait. I thought the purpose we add the last
> > queue check in cfq_should_idle is we want a service tree or a group has dedicated
> > slice, because before the service tree/group slice is expired, new queue can jump
> > in and if we don't idle, the new queue can only run at next slice. Not sure if I
> > understand the code correctly.
> 
> Hi Shaohua,
> 
> If a cfq queue is the last one in the io group, if we expire this cfqq immediately,
> io group will be removed from service tree. When io group gets backlogged again, it
> will be put at the end of service tree, so it loses its previous share. so we add
> the last check here from the fairness point of view.
ya, this is what I'm understanding. So we can't return false for async queue
in cfq_should_idle if the queue is the last one of service tree.

Thanks,
Shaohua
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