Re: [PATCH]cfq-iosched: don't stop async queue with async requests pending

[Vivek Goyal <vgoyal@redhat.com>]

On Wed, Jan 13, 2010 at 03:44:42PM +0800, Shaohua Li wrote:
> My SSD speed of direct write is about 80m/s, while I test page writeback,
> the speed can only go to 68m/s. Below patch fixes this.
> It appears we missused cfq_should_idle in cfq_may_dispatch. cfq_should_idle
> means a queue should idle because it's seekless sync queue or it's the last queue,
> which is to maintain service tree time slice. So it doesn't mean the
> last queue is always a sync queue. If the last queue is asyn queue,
> we definitely shouldn't stop dispatch requests because of pending async
> requests.
> 
> Signed-off-by: Shaohua Li <shaohua.li@intel.com>
> 
> diff --git a/block/cfq-iosched.c b/block/cfq-iosched.c
> index 918c7fd..8198079 100644
> --- a/block/cfq-iosched.c
> +++ b/block/cfq-iosched.c
> @@ -2222,7 +2222,8 @@ static bool cfq_may_dispatch(struct cfq_data *cfqd, struct cfq_queue *cfqq)
>  	/*
>  	 * Drain async requests before we start sync IO
>  	 */
> -	if (cfq_should_idle(cfqd, cfqq) && cfqd->rq_in_driver[BLK_RW_ASYNC])
> +	if (cfq_cfqq_sync(cfqq) && cfq_should_idle(cfqd, cfqq)
> +		&& cfqd->rq_in_driver[BLK_RW_ASYNC])
>  		return false;

So are we driving queue depth as 1 when pure buffered writes are going on?
Because in that case service_tree->count=1 and cfq_should_idle() will
return 1 and looks like we will not dispatch next write till previous
write is over?

A general question. Why do we need to drain async requests before we start
sync IO? How does that help?

A related question, even if we have to do that, why do we check for
cfq_should_idle()? Why can't we just do following.

	if (cfq_cfqq_sync(cfqq) && cfqd->rq_in_driver[BLK_RW_ASYNC])

Thanks
Vivek