Re: [PATCH]cfq-iosched: give busy sync queue no dispatch limit

[Gui Jianfeng <guijianfeng@cn.fujitsu.com>]

Shaohua Li wrote:
> If there are a sync and an async queue and the sync queue's think time is small,
> we can ignore the sync queue's dispatch quantum. Because the sync queue will
> always preempt the async queue, we don't need to care about async's latency.
> This can fix a performance regression of aiostress test, which is introduced by
> commit f8ae6e3eb825. The issue should exist even without the commit, but the
> commit amplifies the impact.
> 
> The initial post does the same optimization for RT queue too, but since I have
> no real workload for it, Vivek suggests to drop it.
> 
> Signed-off-by: Shaohua Li <shaohua.li@intel.com>

This fix looks good to me.

Reviewed-by: Gui Jianfeng <guijianfeng@cn.fujitsu.com>

Thanks,
Gui

> ---
>  block/cfq-iosched.c |   26 ++++++++++++++++++++++++--
>  1 file changed, 24 insertions(+), 2 deletions(-)
> 
> Index: linux/block/cfq-iosched.c
> ===================================================================
> --- linux.orig/block/cfq-iosched.c	2011-03-04 09:50:22.000000000 +0800
> +++ linux/block/cfq-iosched.c	2011-03-04 10:22:16.000000000 +0800
> @@ -238,6 +238,7 @@ struct cfq_data {
>  	struct rb_root prio_trees[CFQ_PRIO_LISTS];
>  
>  	unsigned int busy_queues;
> +	unsigned int busy_sync_queues;
>  
>  	int rq_in_driver;
>  	int rq_in_flight[2];
> @@ -1372,6 +1373,8 @@ static void cfq_add_cfqq_rr(struct cfq_d
>  	BUG_ON(cfq_cfqq_on_rr(cfqq));
>  	cfq_mark_cfqq_on_rr(cfqq);
>  	cfqd->busy_queues++;
> +	if (cfq_cfqq_sync(cfqq))
> +		cfqd->busy_sync_queues++;
>  
>  	cfq_resort_rr_list(cfqd, cfqq);
>  }
> @@ -1398,6 +1401,8 @@ static void cfq_del_cfqq_rr(struct cfq_d
>  	cfq_group_service_tree_del(cfqd, cfqq->cfqg);
>  	BUG_ON(!cfqd->busy_queues);
>  	cfqd->busy_queues--;
> +	if (cfq_cfqq_sync(cfqq))
> +		cfqd->busy_sync_queues--;
>  }
>  
>  /*
> @@ -2405,6 +2410,7 @@ static bool cfq_may_dispatch(struct cfq_
>  	 * Does this cfqq already have too much IO in flight?
>  	 */
>  	if (cfqq->dispatched >= max_dispatch) {
> +		bool promote_sync = false;
>  		/*
>  		 * idle queue must always only have a single IO in flight
>  		 */
> @@ -2412,15 +2418,31 @@ static bool cfq_may_dispatch(struct cfq_
>  			return false;
>  
>  		/*
> +		 * If there is only one sync queue, and its think time is
> +		 * small, we can ignore async queue here and give the sync
> +		 * queue no dispatch limit. The reason is a sync queue can
> +		 * preempt async queue, limiting the sync queue doesn't make
> +		 * sense. This is useful for aiostress test.
> +		 */
> +		if (cfq_cfqq_sync(cfqq) && cfqd->busy_sync_queues == 1) {
> +			struct cfq_io_context *cic = RQ_CIC(cfqq->next_rq);
> +
> +			if (sample_valid(cic->ttime_samples) &&
> +				cic->ttime_mean < cfqd->cfq_slice_idle)
> +				promote_sync = true;
> +		}
> +
> +		/*
>  		 * We have other queues, don't allow more IO from this one
>  		 */
> -		if (cfqd->busy_queues > 1 && cfq_slice_used_soon(cfqd, cfqq))
> +		if (cfqd->busy_queues > 1 && cfq_slice_used_soon(cfqd, cfqq) &&
> +				!promote_sync)
>  			return false;
>  
>  		/*
>  		 * Sole queue user, no limit
>  		 */
> -		if (cfqd->busy_queues == 1)
> +		if (cfqd->busy_queues == 1 || promote_sync)
>  			max_dispatch = -1;
>  		else
>  			/*
> 
> 
>