Re: [PATCH]cfq-iosched: removing unnecessary think time checking

[Vivek Goyal <vgoyal@redhat.com>]

On Tue, Mar 15, 2011 at 10:57:31AM +0800, Shaohua Li wrote:
> Removing think time checking. A high thinktime queue might means the queue
> dispatches several requests and then do away. Limitting such queue seems
> meaningless. And also this can simplify code. This is suggested by Vivek.
> 
> Signed-off-by: Shaohua Li <shaohua.li@intel.com>
> 

Looks good to me.

Acked-by: Vivek Goyal <vgoyal@redhat.com>

I really can't find a relation between why should we allow unlimited
dispatch only for low thinktime queues and not for high thinktime queue.

concern here is that due to preemtion it can happen that async IO is
starved. I think high think time queue automatically mitigates that
up to some extent that there will be time interval when async IO will
get time to dispatch.

If we run into issues of sync starving async, then it should probably
controlled by not giving unlimited queue depth to sync but by limiting
it to some higher number.

Thanks
Vivek

> ---
>  block/cfq-iosched.c |   13 ++++---------
>  1 file changed, 4 insertions(+), 9 deletions(-)
> 
> Index: linux/block/cfq-iosched.c
> ===================================================================
> --- linux.orig/block/cfq-iosched.c	2011-03-10 13:52:13.000000000 +0800
> +++ linux/block/cfq-iosched.c	2011-03-10 13:53:08.000000000 +0800
> @@ -2418,19 +2418,14 @@ 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
> +		 * If there is only one sync queue
> +		 * 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;
> -		}
> +		if (cfq_cfqq_sync(cfqq) && cfqd->busy_sync_queues == 1)
> +			promote_sync = true;
>  
>  		/*
>  		 * We have other queues, don't allow more IO from this one
>