Messages in this thread | | | From | Divyesh Shah <> | Date | Wed, 4 Nov 2009 18:44:28 -0800 | Subject | Re: [PATCH 02/20] blkio: Change CFQ to use CFS like queue time stamps |
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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?
> >> >> > + /* >> > + * 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 > -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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