Messages in this thread | | | Date | Mon, 28 Dec 2009 13:16:57 +0100 | From | Jens Axboe <> | Subject | Re: [PATCH]cfq-iosched: don't take requests with long distence as close |
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On Mon, Dec 28 2009, Shaohua Li 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.
Sorry for the lack of response in this thread, christmas vacation is upon us. The below looks good, I've added it. Thanks!
-- Jens Axboe
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