Re: [PATCH 02/20] blkio: Change CFQ to use CFS like queue time stamps

[Vivek Goyal <vgoyal@redhat.com>]

On Wed, Nov 04, 2009 at 06:44:28PM -0800, Divyesh Shah wrote:
> On Wed, Nov 4, 2009 at 8:37 AM, Vivek Goyal <vgoyal@redhat.com> wrote:
> > On Wed, Nov 04, 2009 at 09:30:34AM -0500, Jeff Moyer wrote:
> >> Vivek Goyal <vgoyal@redhat.com> writes:
> >>
> >
> > Thanks for the review Jeff.
> >
> >> > o Currently CFQ provides priority scaled time slices to processes. If a process
> >> >   does not use the time slice, either because process did not have sufficient
> >> >   IO to do or because think time of process is large and CFQ decided to disable
> >> >   idling, then processes looses it time slice share.
> >>                            ^^^^^^
> >> loses
> >>
> >
> > Will fix it.
> >
> >> > o One possible way to handle this is implement CFS like time stamping of the
> >> >   cfq queues and keep track of vtime. Next queue for execution will be selected
> >> >   based on the one who got lowest vtime. This patch implemented time stamping
> >> >   mechanism of cfq queues based on disk time used.
> >> >
> >> > o min_vdisktime represents the minimum vdisktime of the queue, either being
> >>                                                           ^^^^^
> >> >   serviced or leftmost element on the serviec tree.
> >>
> >> queue or service tree?  The latter seems to make more sense to me.
> >
> > Yes, it should be service tree. Will fix it.
> >
> >>
> >> > +static inline u64
> >> > +cfq_delta_fair(unsigned long delta, struct cfq_queue *cfqq)
> >> > +{
> >> > +   const int base_slice = cfqq->cfqd->cfq_slice[cfq_cfqq_sync(cfqq)];
> >> > +
> >> > +   return delta + (base_slice/CFQ_SLICE_SCALE * (cfqq->ioprio - 4));
> >> > +}
> >>
> >> cfq_scale_delta might be a better name.
> >>
> >
> > cfq_scale_delta sounds good. Will use it in next version.
> >
> >>
> >> > +static inline u64 max_vdisktime(u64 min_vdisktime, u64 vdisktime)
> >> > +{
> >> > +   s64 delta = (s64)(vdisktime - min_vdisktime);
> >> > +   if (delta > 0)
> >> > +           min_vdisktime = vdisktime;
> >> > +
> >> > +   return min_vdisktime;
> >> > +}
> >> > +
> >> > +static inline u64 min_vdisktime(u64 min_vdisktime, u64 vdisktime)
> >> > +{
> >> > +   s64 delta = (s64)(vdisktime - min_vdisktime);
> >> > +   if (delta < 0)
> >> > +           min_vdisktime = vdisktime;
> >> > +
> >> > +   return min_vdisktime;
> >> > +}
> >>
> >> Is there a reason you've reimplemented min and max?
> >
> > I think you are referring to min_t and max_t. Will these macros take care
> > of wrapping too?
> >
> > For example, if I used min_t(u64, A, B), then unsigned comparision will
> > not work right wrapping has just taken place for any of the A or B. So if
> > A=-1 and B=2, then min_t() would return B as minimum. This is not right
> > in our case.
> >
> > If we do signed comparison (min_t(s64, A, B)), that also seems to be
> > broken in another case where a value of variable moves from 63bits to 64bits,
> > (A=0x7fffffffffffffff, B=0x8000000000000000). Above will return B as minimum but
> > in our scanario, vdisktime will progress from 0x7fffffffffffffff to
> > 0x8000000000000000 and A should be returned as minimum (unsigned
> > comparison).
> 
> Can you define and use u64 versions of time_before() and time_after()
> (from include/linux/jiffies.h) for your comparisons? These take care
> of wrapping as well. Maybe call them timestamp_before()/after().
> 
> >
> > Hence I took these difnitions from CFS.
> 
> Also if these are exactly the same and you decide to continue using
> these, can we move them to a common header file (time.h or maybe add a
> vtime.h) and reuse?
> 

Ok. I will look into it. Sharing the function between CFS scheduler and
CFQ scheduler.

Thanks
Vivek