[Devel] Re: [PATCH 13/25] io-controller: Wait for requests to complete from last queue before new queue is scheduled

[Vivek Goyal]

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 at redhat.com> 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 at redhat.com>
> > ---
> >  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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