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SubjectRe: [PATCH 1/4] cfq-iosched: Keep track of average think time for the sync-noidle workload.
On Tue, May 18, 2010 at 02:20:17PM -0400, Jeff Moyer wrote:
> This patch uses an average think time for the entirety of the sync-noidle
> workload to determine whether or not to idle on said workload. This brings
> it more in line with the policy for the sync queues in the sync workload.
>
> Testing shows that this provided an overall increase in throughput for
> a mixed workload on my hardware RAID array.
>
> Signed-off-by: Jeff Moyer <jmoyer@redhat.com>
> ---
> block/cfq-iosched.c | 44 +++++++++++++++++++++++++++++++++++++++-----
> 1 files changed, 39 insertions(+), 5 deletions(-)
>
> diff --git a/block/cfq-iosched.c b/block/cfq-iosched.c
> index 838834b..46a7fe5 100644
> --- a/block/cfq-iosched.c
> +++ b/block/cfq-iosched.c
> @@ -83,9 +83,14 @@ struct cfq_rb_root {
> unsigned total_weight;
> u64 min_vdisktime;
> struct rb_node *active;
> + unsigned long last_end_request;
> + unsigned long ttime_total;
> + unsigned long ttime_samples;
> + unsigned long ttime_mean;
> };
> #define CFQ_RB_ROOT (struct cfq_rb_root) { .rb = RB_ROOT, .left = NULL, \
> - .count = 0, .min_vdisktime = 0, }
> + .count = 0, .min_vdisktime = 0, .last_end_request = 0, \
> + .ttime_total = 0, .ttime_samples = 0, .ttime_mean = 0 }
>
> /*
> * Per process-grouping structure
> @@ -962,8 +967,10 @@ cfq_find_alloc_cfqg(struct cfq_data *cfqd, struct cgroup *cgroup, int create)
> goto done;
>
> cfqg->weight = blkcg->weight;
> - for_each_cfqg_st(cfqg, i, j, st)
> + for_each_cfqg_st(cfqg, i, j, st) {
> *st = CFQ_RB_ROOT;
> + st->last_end_request = jiffies;
> + }
> RB_CLEAR_NODE(&cfqg->rb_node);
>
> /*
> @@ -1795,9 +1802,12 @@ static bool cfq_should_idle(struct cfq_data *cfqd, struct cfq_queue *cfqq)
>
> /*
> * Otherwise, we do only if they are the last ones
> - * in their service tree.
> + * in their service tree and the average think time is
> + * less than the slice length.
> */
> - if (service_tree->count == 1 && cfq_cfqq_sync(cfqq))
> + if (service_tree->count == 1 && cfq_cfqq_sync(cfqq) &&
> + (!sample_valid(service_tree->ttime_samples ||
> + cfqq->slice_end - jiffies < service_tree->ttime_mean)))
> return 1;

This comparision will also might break some logic in select_queue() where
we wait for a queue/group to get busy even if queue's time slice has
expired.

********************************************************************
if (cfq_slice_used(cfqq) && !cfq_cfqq_must_dispatch(cfqq)) {
/*
* If slice had not expired at the completion of last
* request
* we might not have turned on wait_busy flag. Don't
* expire
* the queue yet. Allow the group to get backlogged.
*
* The very fact that we have used the slice, that means
* we
* have been idling all along on this queue and it should
* be
* ok to wait for this request to complete.
*/
if (cfqq->cfqg->nr_cfqq == 1 && RB_EMPTY_ROOT(&cfqq->sort_list)
&& cfqq->dispatched && cfq_should_idle(cfqd, cfqq)) {
cfqq = NULL;
goto keep_queue;
}

*************************************************************************

With this change, now above condition will never be true as
cfq_should_idle() will always return false as slice has already expired.
And that will affect group loosing its fair share.

So I guess we can define new functions to check more conditions instead of
putting it in cfq_should_idle()

Vivek


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