linux-ext4 - Re: [PATCH 1/4] cfq-iosched: Keep track of average think time for the sync-noidle workload.

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Date:	Tue, 18 May 2010 17:02:26 -0400
From:	Vivek Goyal <vgoyal@...hat.com>
To:	Jeff Moyer <jmoyer@...hat.com>
Cc:	linux-kernel@...r.kernel.org, linux-ext4@...r.kernel.org,
	jens.axboe@...cle.com
Subject: Re: [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@...hat.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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