linux-kernel - Re: [PATCH 2/2] cfq-iosched: Take care of corner cases of group losing share due to deletion

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Date:	Wed, 9 Dec 2009 14:56:39 +0100
From:	Jens Axboe <jens.axboe@...cle.com>
To:	Vivek Goyal <vgoyal@...hat.com>
Cc:	jmoyer@...hat.com, linux-kernel@...r.kernel.org,
	guijianfeng@...fujitsu.com
Subject: Re: [PATCH 2/2] cfq-iosched: Take care of corner cases of group
	losing share due to deletion

On Tue, Dec 08 2009, Vivek Goyal wrote:
> If there is a sequential reader running in a group, we wait for next request
> to come in that group after slice expiry and once new request is in, we expire
> the queue. Otherwise we delete the group from service tree and group looses
> its fair share.
> 
> So far I was marking a queue as wait_busy if it had consumed its slice and
> it was last queue in the group. But this condition did not cover following
> two cases.
> 
> 1.If a request completed and slice has not expired yet. Next request comes
>   in and is dispatched to disk. Now select_queue() hits and slice has expired.
>   This group will be deleted. Because request is still in the disk, this queue
>   will never get a chance to wait_busy.
> 
> 2.If request completed and slice has not expired yet. Before next request
>   comes in (delay due to think time), select_queue() hits and expires the
>   queue hence group. This queue never got a chance to wait busy.
> 
> Gui was hitting the boundary condition 1 and not getting fairness numbers
> proportional to weight.
> 
> This patch puts the checks for above two conditions and improves the fairness
> numbers for sequential workload on rotational media. Check in select_queue()
> takes care of case 1 and additional check in should_wait_busy() takes care
> of case 2.

I think this (and 1/2) look fine, just one minor comment:

> @@ -3250,6 +3264,36 @@ static void cfq_update_hw_tag(struct cfq_data *cfqd)
>  		cfqd->hw_tag = 0;
>  }
>  
> +static inline bool
> +cfq_should_wait_busy(struct cfq_data *cfqd, struct cfq_queue *cfqq)
> +{

That's too large to inline.

-- 
Jens Axboe

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