On Thu, Jul 02, 2009 at 01:09:14PM -0700, Nauman Rafique wrote:
> On Thu, Jul 2, 2009 at 1:01 PM, Vivek Goyal<vgoyal@xxxxxxxxxx> wrote:
> > o Currently one can dispatch requests from multiple queues to the disk. This
> > is true for hardware which supports queuing. So if a disk support queue
> > depth of 31 it is possible that 20 requests are dispatched from queue 1
> > and then next queue is scheduled in which dispatches more requests.
> >
> > o This multiple queue dispatch introduces issues for accurate accounting of
> > disk time consumed by a particular queue. For example, if one async queue
> > is scheduled in, it can dispatch 31 requests to the disk and then it will
> > be expired and a new sync queue might get scheduled in. These 31 requests
> > might take a long time to finish but this time is never accounted to the
> > async queue which dispatched these requests.
> >
> > o This patch introduces the functionality where we wait for all the requests
> > to finish from previous queue before next queue is scheduled in. That way
> > a queue is more accurately accounted for disk time it has consumed. Note
> > this still does not take care of errors introduced by disk write caching.
> >
> > o Because above behavior can result in reduced throughput, this behavior will
> > be enabled only if user sets "fairness" tunable to 2 or higher.
>
> Vivek,
> Did you collect any numbers for the impact on throughput from this
> patch? It seems like with this change, we can even support NCQ.
>
Hi Nauman,
Not yet. I will try to do some impact analysis of this change and post the
results.
Thanks
Vivek
> >
> > o This patch helps in achieving more isolation between reads and buffered
> > writes in different cgroups. buffered writes typically utilize full queue
> > depth and then expire the queue. On the contarary, sequential reads
> > typicaly driver queue depth of 1. So despite the fact that writes are
> > using more disk time it is never accounted to write queue because we don't
> > wait for requests to finish after dispatching these. This patch helps
> > do more accurate accounting of disk time, especially for buffered writes
> > hence providing better fairness hence better isolation between two cgroups
> > running read and write workloads.
> >
> > Signed-off-by: Vivek Goyal <vgoyal@xxxxxxxxxx>
> > ---
> > block/elevator-fq.c | 31 ++++++++++++++++++++++++++++++-
> > 1 files changed, 30 insertions(+), 1 deletions(-)
> >
> > diff --git a/block/elevator-fq.c b/block/elevator-fq.c
> > index 68be1dc..7609579 100644
> > --- a/block/elevator-fq.c
> > +++ b/block/elevator-fq.c
> > @@ -2038,7 +2038,7 @@ STORE_FUNCTION(elv_slice_sync_store, &efqd->elv_slice[1], 1, UINT_MAX, 1);
> > EXPORT_SYMBOL(elv_slice_sync_store);
> > STORE_FUNCTION(elv_slice_async_store, &efqd->elv_slice[0], 1, UINT_MAX, 1);
> > EXPORT_SYMBOL(elv_slice_async_store);
> > -STORE_FUNCTION(elv_fairness_store, &efqd->fairness, 0, 1, 0);
> > +STORE_FUNCTION(elv_fairness_store, &efqd->fairness, 0, 2, 0);
> > EXPORT_SYMBOL(elv_fairness_store);
> > #undef STORE_FUNCTION
> >
> > @@ -2952,6 +2952,24 @@ void *elv_fq_select_ioq(struct request_queue *q, int force)
> > }
> >
> > expire:
> > + if (efqd->fairness >= 2 && !force && ioq && ioq->dispatched) {
> > + /*
> > + * If there are request dispatched from this queue, don't
> > + * dispatch requests from new queue till all the requests from
> > + * this queue have completed.
> > + *
> > + * This helps in attributing right amount of disk time consumed
> > + * by a particular queue when hardware allows queuing.
> > + *
> > + * Set ioq = NULL so that no more requests are dispatched from
> > + * this queue.
> > + */
> > + elv_log_ioq(efqd, ioq, "select: wait for requests to finish"
> > + " disp=%lu", ioq->dispatched);
> > + ioq = NULL;
> > + goto keep_queue;
> > + }
> > +
> > elv_ioq_slice_expired(q);
> > new_queue:
> > ioq = elv_set_active_ioq(q, new_ioq);
> > @@ -3109,6 +3127,17 @@ void elv_ioq_completed_request(struct request_queue *q, struct request *rq)
> > */
> > elv_ioq_arm_slice_timer(q, 1);
> > } else {
> > + /* If fairness >=2 and there are requests
> > + * dispatched from this queue, don't dispatch
> > + * new requests from a different queue till
> > + * all requests from this queue have finished.
> > + * This helps in attributing right disk time
> > + * to a queue when hardware supports queuing.
> > + */
> > +
> > + if (efqd->fairness >= 2 && ioq->dispatched)
> > + goto done;
> > +
> > /* Expire the queue */
> > elv_ioq_slice_expired(q);
> > }
> > --
> > 1.6.0.6
> >
> >
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