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

[Gui Jianfeng <guijianfeng@cn.fujitsu.com>]

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@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.

I see your point, whether can we add the extra sync queue check into cfq_should_idle
for common use?

Thanks,
Gui

> 
> Thanks,
> Shaohua
> 
> 
>