Messages in this thread | | | Date | Thu, 14 Jan 2010 14:17:31 +0800 | From | Shaohua Li <> | Subject | Re: [PATCH]cfq-iosched: don't stop async queue with async requests pending |
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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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