linux-kernel - Re: [PATCH 02/20] io-controller: Common flat fair queuing code in elevaotor layer

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Message-ID: <4A49B364.5000508@cn.fujitsu.com>
Date:	Tue, 30 Jun 2009 14:40:36 +0800
From:	Gui Jianfeng <guijianfeng@...fujitsu.com>
To:	Vivek Goyal <vgoyal@...hat.com>
CC:	linux-kernel@...r.kernel.org,
	containers@...ts.linux-foundation.org, dm-devel@...hat.com,
	jens.axboe@...cle.com, nauman@...gle.com, dpshah@...gle.com,
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	paolo.valente@...more.it, ryov@...inux.co.jp,
	fernando@....ntt.co.jp, s-uchida@...jp.nec.com, taka@...inux.co.jp,
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	snitzer@...hat.com, akpm@...ux-foundation.org, peterz@...radead.org
Subject: Re: [PATCH 02/20] io-controller: Common flat fair queuing code in
 elevaotor layer

Vivek Goyal wrote:
...
> +
> +/*
> + * Do the accounting. Determine how much service (in terms of time slices)
> + * current queue used and adjust the start, finish time of queue and vtime
> + * of the tree accordingly.
> + *
> + * Determining the service used in terms of time is tricky in certain
> + * situations. Especially when underlying device supports command queuing
> + * and requests from multiple queues can be there at same time, then it
> + * is not clear which queue consumed how much of disk time.
> + *
> + * To mitigate this problem, cfq starts the time slice of the queue only
> + * after first request from the queue has completed. This does not work
> + * very well if we expire the queue before we wait for first and more
> + * request to finish from the queue. For seeky queues, we will expire the
> + * queue after dispatching few requests without waiting and start dispatching
> + * from next queue.
> + *
> + * Not sure how to determine the time consumed by queue in such scenarios.
> + * Currently as a crude approximation, we are charging 25% of time slice
> + * for such cases. A better mechanism is needed for accurate accounting.
> + */

  Hi Vivek,

  The comment is out of date, would you update it accordingly?

> +void __elv_ioq_slice_expired(struct request_queue *q, struct io_queue *ioq)
> +{
> +	struct elv_fq_data *efqd = &q->elevator->efqd;
> +	struct io_entity *entity = &ioq->entity;
> +	long slice_unused = 0, slice_used = 0, slice_overshoot = 0;
> +
> +	assert_spin_locked(q->queue_lock);
> +	elv_log_ioq(efqd, ioq, "slice expired");
> +
> +	if (elv_ioq_wait_request(ioq))
> +		del_timer(&efqd->idle_slice_timer);
> +
> +	elv_clear_ioq_wait_request(ioq);
> +
> +	/*

-- 
Regards
Gui Jianfeng

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