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

[Corrado Zoccolo <czoccolo@gmail.com>]

On Mon, Dec 28, 2009 at 10:28 AM, Shaohua Li <shaohua.li@intel.com> wrote:
> On Mon, Dec 28, 2009 at 05:11:25PM +0800, Corrado Zoccolo wrote:
>> Hi Shaohua,
>> On Mon, Dec 28, 2009 at 9:46 AM, Shaohua Li <shaohua.li@intel.com> wrote:
>> > On Mon, Dec 28, 2009 at 04:36:39PM +0800, Corrado Zoccolo wrote:
>> >> 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)
>> > oops
>> >
>> >> > 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.
>> > can you explain why it's meaningful for cfq_should_preempt()? Unless sdist is
>> > very big, for example > 10*seek_mean, the preempt seems not meaningless.
>>
>> Disk access time is a complex function, but for rotational disks it is
>> 'sort of' increasing with the amplitude of the seek. So, if you have a
>> new request that is within the mean seek distance (even if it is
>> larger than our constant), it is good to chose this request instead of
>> waiting for an other one from the active queue (this behaviour is the
>> same exhibited by AS, so we need a good reason to change).
> I have no good reason, but just thought it's a little strange.
> If other ioscheds take the same way, let's stick.
>
>
> seek_mean could be very big sometimes, using it as close criteria is meaningless
> as this doen't improve any performance. So if it's big, let's fallback to
> default value.
>
> Signed-off-by: Shaohua Li<shaohua.li@intel.com>
Reviewed-by: Corrado Zoccolo <czoccolo@gmail.com>
>
> diff --git a/block/cfq-iosched.c b/block/cfq-iosched.c
> index e2f8046..e80bd47 100644
> --- a/block/cfq-iosched.c
> +++ b/block/cfq-iosched.c
> @@ -1675,13 +1675,17 @@ static inline sector_t cfq_dist_from_last(struct cfq_data *cfqd,
>  #define CFQQ_SEEKY(cfqq)       ((cfqq)->seek_mean > CFQQ_SEEK_THR)
>
>  static inline int cfq_rq_close(struct cfq_data *cfqd, struct cfq_queue *cfqq,
> -                              struct request *rq)
> +                              struct request *rq, bool for_preempt)
>  {
>        sector_t sdist = cfqq->seek_mean;
>
>        if (!sample_valid(cfqq->seek_samples))
>                sdist = CFQQ_SEEK_THR;
>
> +       /* if seek_mean is big, using it as close criteria is meaningless */
> +       if (sdist > CFQQ_SEEK_THR && !for_preempt)
> +               sdist = CFQQ_SEEK_THR;
> +
>        return cfq_dist_from_last(cfqd, rq) <= sdist;
>  }
>
> @@ -1709,7 +1713,7 @@ static struct cfq_queue *cfqq_close(struct cfq_data *cfqd,
>         * will contain the closest sector.
>         */
>        __cfqq = rb_entry(parent, struct cfq_queue, p_node);
> -       if (cfq_rq_close(cfqd, cur_cfqq, __cfqq->next_rq))
> +       if (cfq_rq_close(cfqd, cur_cfqq, __cfqq->next_rq, false))
>                return __cfqq;
>
>        if (blk_rq_pos(__cfqq->next_rq) < sector)
> @@ -1720,7 +1724,7 @@ static struct cfq_queue *cfqq_close(struct cfq_data *cfqd,
>                return NULL;
>
>        __cfqq = rb_entry(node, struct cfq_queue, p_node);
> -       if (cfq_rq_close(cfqd, cur_cfqq, __cfqq->next_rq))
> +       if (cfq_rq_close(cfqd, cur_cfqq, __cfqq->next_rq, false))
>                return __cfqq;
>
>        return NULL;
> @@ -3143,7 +3147,7 @@ cfq_should_preempt(struct cfq_data *cfqd, struct cfq_queue *new_cfqq,
>         * if this request is as-good as one we would expect from the
>         * current cfqq, let it preempt
>         */
> -       if (cfq_rq_close(cfqd, cfqq, rq))
> +       if (cfq_rq_close(cfqd, cfqq, rq, true))
>                return true;
>
>        return false;
>