Re: [PATCH 01/10] Documentation

[Nauman Rafique <nauman@google.com>]

On Wed, Mar 18, 2009 at 2:55 PM, Vivek Goyal <vgoyal@redhat.com> wrote:
> On Wed, Mar 18, 2009 at 03:23:29PM +0800, Gui Jianfeng wrote:
>> Vivek Goyal wrote:
>> >> Hi Vivek,
>> >>
>> >> I would be interested in knowing if these are the results expected?
>> >>
>> >
>> > Hi Dhaval,
>> >
>> > Good question. Keeping current expectation in mind, yes these are expected
>> > results. To begin with, current expectations are that try to emulate
>> > cfq behavior and the kind of service differentiation we get between
>> > threads of different priority, same kind of service differentiation we
>> > should get from different cgroups.
>> >
>> > Having said that, in theory a more accurate estimate should be amount
>> > of actual disk time a queue/cgroup got. I have put a tracing message
>> > to keep track of total service received by a queue. If you run "blktrace"
>> > then you can see that. Ideally, total service received by two threads
>> > over a period of time should be in same proportion as their cgroup
>> > weights.
>> >
>> > It will not be easy to achive it given the constraints we have got in
>> > terms of how to accurately we can account for disk time actually used by a
>> > queue in certain situations. So to begin with I am targetting that
>> > try to meet same kind of service differentation between cgroups as
>> > cfq provides between threads and then slowly refine it to see how
>> > close one can come to get accurate numbers in terms of "total_serivce"
>> > received by each queue.
>>
>>   Hi Vivek,
>>
>>   I simply tested with blktrace opened. I create two groups and set ioprio
>>   4 and 7 respectively(the corresponding weight should 4:1, right?),
>
> Hi Gui,
>
> Thanks for testing. You are right about weight proportions.
>
>> and
>>   start two dd concurrently. UUIC, Ideally, the proportion of service two
>>   dd got should be 4:1 in a period of time when they are running. I extract
>>   *served* value from blktrace output and sum them up. I found the proportion
>>   of the sum of *served* value is about 1.7:1
>>   Am i missing something?
>
> Actually getting the service proportion in same ratio as weight proportion
> is quite hard for sync queues. The biggest issue is that many a times sync
> queues are not continuously backlogged and they do some IO and then dispatch
> a next round of requests.
>
> Most of the time idling seems to be the solution for giving an impression
> that sync queue is continuously backlogged but it also has potential to
> reduce throughput on faster hardware.
>
> Anyway, can you please send me your complete blkparse output. There are
> many a places where code has been designed to favor throughput than
> fairness. Looking at your blkparse output, will give me better idea what's
> the issue in your setup.
>
> Also please try the attached patch. I have experimented with waiting for
> new request to come before sync queue is expired. It helps me in getting
> the fairness numbers at least with noop on non-queueing rotational media.
>
> I also have introduced a new tunable "fairness". Above code will kick in
> only if this variable is set to 1. Many a places where we favor throughput
> over fairness, I plan to use this variable as condition to let user
> decide whether to choose fairness over throughput. I am not sure at how many
> places it really makes sense, but it atleast gives us something to play and
> compare the throughput in two cases.
>
> This patch applies on my current tree after removing tomost patceh
> "anticipatory scheduling changes". My code has changed a bit since the
> posting, so you might have to message this patch a bit.
>
> Thanks
> Vivek
>
>
> DESC
> io-controller: idle for sometime on sync queue before expiring it
> EDESC
>
> o When a sync queue expires, in many cases it might be empty and then
>  it will be deleted from the active tree. This will lead to a scenario
>  where out of two competing queues, only one is on the tree and when a
>  new queue is selected, vtime jump takes place and we don't see services
>  provided in proportion to weight.
>
> o In general this is a fundamental problem with fairness of sync queues
>  where queues are not continuously backlogged. Looks like idling is
>  only solution to make sure such kind of queues can get some decent amount
>  of disk bandwidth in the face of competion from continusouly backlogged
>  queues. But excessive idling has potential to reduce performance on SSD
>  and disks with commnad queuing.
>
> o This patch experiments with waiting for next request to come before a
>  queue is expired after it has consumed its time slice. This can ensure
>  more accurate fairness numbers in some cases.

Vivek, have you introduced this option just to play with it, or you
are planning to make it a part of the patch set. Waiting for a new
request to come before expiring time slice sounds problematic.

>
> o Introduced a tunable "fairness". If set, io-controller will put more
>  focus on getting fairness right than getting throughput right.
>
>
> ---
>  block/blk-sysfs.c   |    7 ++++
>  block/elevator-fq.c |   85 +++++++++++++++++++++++++++++++++++++++++++++-------
>  block/elevator-fq.h |   12 +++++++
>  3 files changed, 94 insertions(+), 10 deletions(-)
>
> Index: linux1/block/elevator-fq.h
> ===================================================================
> --- linux1.orig/block/elevator-fq.h     2009-03-18 17:34:46.000000000 -0400
> +++ linux1/block/elevator-fq.h  2009-03-18 17:34:53.000000000 -0400
> @@ -318,6 +318,13 @@ struct elv_fq_data {
>        unsigned long long rate_sampling_start; /*sampling window start jifies*/
>        /* number of sectors finished io during current sampling window */
>        unsigned long rate_sectors_current;
> +
> +       /*
> +        * If set to 1, will disable many optimizations done for boost
> +        * throughput and focus more on providing fairness for sync
> +        * queues.
> +        */
> +       int fairness;
>  };
>
>  extern int elv_slice_idle;
> @@ -340,6 +347,7 @@ enum elv_queue_state_flags {
>        ELV_QUEUE_FLAG_idle_window,       /* elevator slice idling enabled */
>        ELV_QUEUE_FLAG_wait_request,      /* waiting for a request */
>        ELV_QUEUE_FLAG_slice_new,         /* no requests dispatched in slice */
> +       ELV_QUEUE_FLAG_wait_busy,         /* wait for this queue to get busy */
>        ELV_QUEUE_FLAG_NR,
>  };
>
> @@ -362,6 +370,7 @@ ELV_IO_QUEUE_FLAG_FNS(sync)
>  ELV_IO_QUEUE_FLAG_FNS(wait_request)
>  ELV_IO_QUEUE_FLAG_FNS(idle_window)
>  ELV_IO_QUEUE_FLAG_FNS(slice_new)
> +ELV_IO_QUEUE_FLAG_FNS(wait_busy)
>
>  static inline struct io_service_tree *
>  io_entity_service_tree(struct io_entity *entity)
> @@ -554,6 +563,9 @@ static inline struct io_queue *elv_looku
>  extern ssize_t elv_slice_idle_show(struct request_queue *q, char *name);
>  extern ssize_t elv_slice_idle_store(struct request_queue *q, const char *name,
>                                                size_t count);
> +extern ssize_t elv_fairness_show(struct request_queue *q, char *name);
> +extern ssize_t elv_fairness_store(struct request_queue *q, const char *name,
> +                                               size_t count);
>
>  /* Functions used by elevator.c */
>  extern int elv_init_fq_data(struct request_queue *q, struct elevator_queue *e);
> Index: linux1/block/elevator-fq.c
> ===================================================================
> --- linux1.orig/block/elevator-fq.c     2009-03-18 17:34:46.000000000 -0400
> +++ linux1/block/elevator-fq.c  2009-03-18 17:34:53.000000000 -0400
> @@ -1837,6 +1837,44 @@ void elv_ioq_served(struct io_queue *ioq
>                        ioq->total_service);
>  }
>
> +/* Functions to show and store fairness value through sysfs */
> +ssize_t elv_fairness_show(struct request_queue *q, char *name)
> +{
> +       struct elv_fq_data *efqd;
> +       unsigned int data;
> +       unsigned long flags;
> +
> +       spin_lock_irqsave(q->queue_lock, flags);
> +       efqd = &q->elevator->efqd;
> +       data = efqd->fairness;
> +       spin_unlock_irqrestore(q->queue_lock, flags);
> +       return sprintf(name, "%d\n", data);
> +}
> +
> +ssize_t elv_fairness_store(struct request_queue *q, const char *name,
> +                         size_t count)
> +{
> +       struct elv_fq_data *efqd;
> +       unsigned int data;
> +       unsigned long flags;
> +
> +       char *p = (char *)name;
> +
> +       data = simple_strtoul(p, &p, 10);
> +
> +       if (data < 0)
> +               data = 0;
> +       else if (data > INT_MAX)
> +               data = INT_MAX;
> +
> +       spin_lock_irqsave(q->queue_lock, flags);
> +       efqd = &q->elevator->efqd;
> +       efqd->fairness = data;
> +       spin_unlock_irqrestore(q->queue_lock, flags);
> +
> +       return count;
> +}
> +
>  /* Functions to show and store elv_idle_slice value through sysfs */
>  ssize_t elv_slice_idle_show(struct request_queue *q, char *name)
>  {
> @@ -2263,10 +2301,11 @@ void __elv_ioq_slice_expired(struct requ
>        assert_spin_locked(q->queue_lock);
>        elv_log_ioq(efqd, ioq, "slice expired upd=%d", budget_update);
>
> -       if (elv_ioq_wait_request(ioq))
> +       if (elv_ioq_wait_request(ioq) || elv_ioq_wait_busy(ioq))
>                del_timer(&efqd->idle_slice_timer);
>
>        elv_clear_ioq_wait_request(ioq);
> +       elv_clear_ioq_wait_busy(ioq);
>
>        /*
>         * if ioq->slice_end = 0, that means a queue was expired before first
> @@ -2482,8 +2521,9 @@ void elv_ioq_request_add(struct request_
>                 * immediately and flag that we must not expire this queue
>                 * just now
>                 */
> -               if (elv_ioq_wait_request(ioq)) {
> +               if (elv_ioq_wait_request(ioq) || elv_ioq_wait_busy(ioq)) {
>                        del_timer(&efqd->idle_slice_timer);
> +                       elv_clear_ioq_wait_busy(ioq);
>                        blk_start_queueing(q);
>                }
>        } else if (elv_should_preempt(q, ioq, rq)) {
> @@ -2519,6 +2559,9 @@ void elv_idle_slice_timer(unsigned long
>
>        if (ioq) {
>
> +               if (elv_ioq_wait_busy(ioq))
> +                       goto expire;
> +
>                /*
>                 * expired
>                 */
> @@ -2546,7 +2589,7 @@ out_cont:
>        spin_unlock_irqrestore(q->queue_lock, flags);
>  }
>
> -void elv_ioq_arm_slice_timer(struct request_queue *q)
> +void elv_ioq_arm_slice_timer(struct request_queue *q, int wait_for_busy)
>  {
>        struct elv_fq_data *efqd = &q->elevator->efqd;
>        struct io_queue *ioq = elv_active_ioq(q->elevator);
> @@ -2563,15 +2606,27 @@ void elv_ioq_arm_slice_timer(struct requ
>                return;
>
>        /*
> -        * still requests with the driver, don't idle
> +        * idle is disabled, either manually or by past process history
>         */
> -       if (efqd->rq_in_driver)
> +       if (!efqd->elv_slice_idle || !elv_ioq_idle_window(ioq))
>                return;
>
>        /*
> -        * idle is disabled, either manually or by past process history
> +        * This queue has consumed its time slice. We are waiting only for
> +        * it to become busy before we select next queue for dispatch.
>         */
> -       if (!efqd->elv_slice_idle || !elv_ioq_idle_window(ioq))
> +       if (efqd->fairness && wait_for_busy) {
> +               elv_mark_ioq_wait_busy(ioq);
> +               sl = efqd->elv_slice_idle;
> +               mod_timer(&efqd->idle_slice_timer, jiffies + sl);
> +               elv_log(efqd, "arm idle: %lu wait busy=1", sl);
> +               return;
> +       }
> +
> +       /*
> +        * still requests with the driver, don't idle
> +        */
> +       if (efqd->rq_in_driver)
>                return;
>
>        /*
> @@ -2628,6 +2683,12 @@ void *elv_fq_select_ioq(struct request_q
>                }
>        }
>
> +       /* We are waiting for this queue to become busy before it expires.*/
> +       if (efqd->fairness && elv_ioq_wait_busy(ioq)) {
> +               ioq = NULL;
> +               goto keep_queue;
> +       }
> +
>        /*
>         * The active queue has run out of time, expire it and select new.
>         */
> @@ -2802,10 +2863,14 @@ void elv_ioq_completed_request(struct re
>                        elv_ioq_set_prio_slice(q, ioq);
>                        elv_clear_ioq_slice_new(ioq);
>                }
> -               if (elv_ioq_slice_used(ioq) || elv_ioq_class_idle(ioq))
> +               if (elv_ioq_class_idle(ioq))
>                        elv_ioq_slice_expired(q, 1);
> -               else if (sync && !ioq->nr_queued)
> -                       elv_ioq_arm_slice_timer(q);
> +               else if (sync && !ioq->nr_queued) {
> +                       if (elv_ioq_slice_used(ioq))
> +                               elv_ioq_arm_slice_timer(q, 1);
> +                       else
> +                               elv_ioq_arm_slice_timer(q, 0);
> +               }
>        }
>
>        if (!efqd->rq_in_driver)
> Index: linux1/block/blk-sysfs.c
> ===================================================================
> --- linux1.orig/block/blk-sysfs.c       2009-03-18 17:34:28.000000000 -0400
> +++ linux1/block/blk-sysfs.c    2009-03-18 17:34:53.000000000 -0400
> @@ -282,6 +282,12 @@ static struct queue_sysfs_entry queue_sl
>        .show = elv_slice_idle_show,
>        .store = elv_slice_idle_store,
>  };
> +
> +static struct queue_sysfs_entry queue_fairness_entry = {
> +       .attr = {.name = "fairness", .mode = S_IRUGO | S_IWUSR },
> +       .show = elv_fairness_show,
> +       .store = elv_fairness_store,
> +};
>  #endif
>  static struct attribute *default_attrs[] = {
>        &queue_requests_entry.attr,
> @@ -296,6 +302,7 @@ static struct attribute *default_attrs[]
>        &queue_iostats_entry.attr,
>  #ifdef CONFIG_ELV_FAIR_QUEUING
>        &queue_slice_idle_entry.attr,
> +       &queue_fairness_entry.attr,
>  #endif
>        NULL,
>  };
>