linux-kernel - Re: [RFC]cfq-iosched: no dispatch limit for single queue

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Date:	Thu, 3 Dec 2009 12:57:40 +0100
From:	Jens Axboe <jens.axboe@...cle.com>
To:	Shaohua Li <shaohua.li@...el.com>
Cc:	linux-kernel@...r.kernel.org, akpm@...ux-foundation.org
Subject: Re: [RFC]cfq-iosched: no dispatch limit for single queue

On Thu, Dec 03 2009, Shaohua Li wrote:
> Since commit 2f5cb7381b737e24c8046fd4aeab571fb71315f5, each queue can send
> up to 4 * 4 requests if only one queue exists. I wonder why we have such limit.
> Device supports tag can send more requests. For example, AHCI can send 31
> requests. Test (direct aio randread) shows the limits reduce about 4% disk
> thoughput.
> On the other hand, since we send one request one time, if other queue
> pop when current is sending more than cfq_quantum requests, current queue will
> stop send requests soon after one request, so sounds there is no big latency.
> 
> Signed-off-by: Shaohua Li <shaohua.li@...el.com>
> 
> diff --git a/block/cfq-iosched.c b/block/cfq-iosched.c
> index aa1e953..e05650f 100644
> --- a/block/cfq-iosched.c
> +++ b/block/cfq-iosched.c
> @@ -1298,9 +1298,9 @@ static bool cfq_may_dispatch(struct cfq_data *cfqd, struct cfq_queue *cfqq)
>  			return false;
>  
>  		/*
> -		 * Sole queue user, allow bigger slice
> +		 * Sole queue user, no limit
>  		 */
> -		max_dispatch *= 4;
> +		max_dispatch = -1;
>  	}
>  
>  	/*

As you mention, we do dispatches in bites of 1. In reality, there's
going to be little difference when we get this far in the depth process,
so I think the patch looks good. I have applied it, thanks.

-- 
Jens Axboe

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