Re: [PATCH]cfq-iosched: don't take requests with long distence as close

[Corrado Zoccolo <czoccolo@gmail.com>]

Hi Shaohua,
On Mon, Dec 28, 2009 at 3:03 AM, Shaohua Li <shaohua.li@intel.com> wrote:
> On Fri, Dec 25, 2009 at 06:16:27PM +0800, Corrado Zoccolo wrote:
>> Hi Shaohua,
>> On Thu, Dec 24, 2009 at 1:55 AM, Shaohua Li <shaohua.li@intel.com> wrote:
>> > df5fe3e8e13883f58dc97489076bbcc150789a21
>> > b3b6d0408c953524f979468562e7e210d8634150
>> > The coop merge is too aggressive. For example, if two tasks are reading two
>> > files where the two files have some adjecent blocks, cfq will immediately
>> > merge them. cfq_rq_close() also has trouble, sometimes the seek_mean is very
>> > big. I did a test to make cfq_rq_close() always checks the distence according
>> > to CIC_SEEK_THR, but still saw a lot of wrong merge. (BTW, why we take a long
>> > distence far away request as close. Taking them close doesn't improve any thoughtput
>> > to me. Maybe we should always use CIC_SEEK_THR as close criteria).
>> Yes, when deciding if two queues are going to be merged, we should use
>> the constant CIC_SEEK_THR.
>
> seek_mean could be very big sometimes, using it as close criteria is meanless
> as this doen't improve any performance. So if it's big, let's fallback to
> default value.

meanless -> meaningless (also in the comment within code)

>
> Signed-off-by: Shaohua Li<shaohua.li@intel.com>
>
> diff --git a/block/cfq-iosched.c b/block/cfq-iosched.c
> index e2f8046..8025605 100644
> --- a/block/cfq-iosched.c
> +++ b/block/cfq-iosched.c
> @@ -1682,6 +1682,10 @@ static inline int cfq_rq_close(struct cfq_data *cfqd, struct cfq_queue *cfqq,
>        if (!sample_valid(cfqq->seek_samples))
>                sdist = CFQQ_SEEK_THR;
>
> +       /* if seek_mean is big, using it as close criteria is meanless */
> +       if (sdist > CFQQ_SEEK_THR)
> +               sdist = CFQQ_SEEK_THR;
> +
>        return cfq_dist_from_last(cfqd, rq) <= sdist;
>  }
>
>
This changes also the cfq_should_preempt behaviour, where a large
seek_mean could be meaningful, so I think it is better to add a
boolean parameter to cfq_rq_close, to distinguish whether we are
preempting or looking for queue merges, and make the new code
conditional on merging.

Thanks,
Corrado