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

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

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.

Thanks,
Shaohua