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

[Shaohua Li <shaohua.li@intel.com>]

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.
you are the expert of iogroup. If I'm wrong, I'll be happy to change
cfq_should_idle(), which is the ideal place to be changed at my first glance.

Thanks,
Shaohua