Re: [PATCH]cfq-iosched: don't stop async queue with async requestspending

[Shaohua Li]

On Thu, Jan 14, 2010 at 04:21:52PM +0800, Gui Jianfeng wrote:
> Shaohua Li wrote:
> > 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@xxxxxxxxx> 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.
> 
> I see your point, whether can we add the extra sync queue check into cfq_should_idle
> for common use?
yes.

Thanks,
Shaohua
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