Messages in this thread | | | Date | Wed, 4 Nov 2009 11:37:52 -0500 | From | Vivek Goyal <> | Subject | Re: [PATCH 02/20] blkio: Change CFQ to use CFS like queue time stamps |
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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).
Hence I took these difnitions from CFS.
> > > + /* > > + * Maintain a cache of leftmost tree entries (it is frequently > > + * used) > > + */ > > You make it sound like there is a cache of more than one entry. Please > fix the comment.
Will fix it.
> > > +static void cfqq_served(struct cfq_queue *cfqq, unsigned long served) > > +{ > > + /* > > + * We don't want to charge more than allocated slice otherwise this > > + * queue can miss one dispatch round doubling max latencies. On the > > + * other hand we don't want to charge less than allocated slice as > > + * we stick to CFQ theme of queue loosing its share if it does not > ^^^^^^^ > losing >
Will fix it.
> > > +/* > > + * Handles three operations. > > + * Addition of a new queue to service tree, when a new request comes in. > > + * Resorting of an expiring queue (used after slice expired) > > + * Requeuing a queue at the front (used during preemption). > > + */ > > +static void cfq_service_tree_add(struct cfq_data *cfqd, struct cfq_queue *cfqq, > > + bool add_front, unsigned long service) > > service? Can we come up with a better name that actually hints at what > this is? service_time, maybe?
Ok, service_time sounds good. Will change it.
> > > Mostly this looks pretty good and is fairly easy to read.
Thanks Vivek
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