linux-kernel - Re: [PATCH] block: Improve think time sampling for CFQ

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Message-ID: <20090715215214.GA3994@redhat.com>
Date:	Wed, 15 Jul 2009 17:52:14 -0400
From:	Vivek Goyal <vgoyal@...hat.com>
To:	Divyesh Shah <dpshah@...gle.com>
Cc:	linux-kernel@...r.kernel.org, jens.axboe@...cle.com,
	nauman@...gle.com, mrubin@...gle.com
Subject: Re: [PATCH] block: Improve think time sampling for CFQ

On Thu, Jul 16, 2009 at 02:56:54AM +0530, Divyesh Shah wrote:
> Avoid taking a think time sample when the cfqq is not a sync queue or not
> currently active or till its first request in the ongoing timeslice
> completes.
> 

Hi Divyesh,

It would be nice to give some more details in changelog regarding why are
you donig this change.

> Signed-off by: Divyesh Shah <dpshah@...gle.com>
> ---
> This applies to Linus's kernel tree.
> 
>  block/cfq-iosched.c |   15 +++++++++++----
>  1 files changed, 11 insertions(+), 4 deletions(-)
> diff --git a/block/cfq-iosched.c b/block/cfq-iosched.c
> index fd7080e..1657d4f 100644
> --- a/block/cfq-iosched.c
> +++ b/block/cfq-iosched.c
> @@ -1904,10 +1904,17 @@ err:
>  }
>  
>  static void
> -cfq_update_io_thinktime(struct cfq_data *cfqd, struct cfq_io_context *cic)
> +cfq_update_io_thinktime(struct cfq_data *cfqd, struct cfq_queue *cfqq,
> +			struct cfq_io_context *cic)
>  {
> -	unsigned long elapsed = jiffies - cic->last_end_request;
> -	unsigned long ttime = min(elapsed, 2UL * cfqd->cfq_slice_idle);
> +	unsigned long elapsed, ttime;
> +
> +	if (!cfq_cfqq_sync(cfqq) || cfqq != cfqd->active_queue || 
> +	    cfq_cfqq_slice_new(cfqq))
> +		return;

If we take a valid sample only when cfqq is the active queue, I think it
will take a long time before idling is enabled back?

For example consider a queue for which idling got disabled for some
reason. Now that queue will be scheduled in and after completion of
request, we will not idle and immediately expire the queue. For sequential
readers, next request most likely will come after the expiry of the queue.
Now this queue is not active, so when next request comes in, that sample
will not be valid and we will never enable the idling on this queue?

I am not sure what are you trying to solve here. I guess that you are
concerned about the case where a reader can drive queue depth more than 1.
So after first request when next request comes in, it is probably not very
fair to compare it with cic->last_end_request. Instead it should be
compared with arrival time of previous request?

If yes, then we can probably maintain another variable, cic->last_req_arrival
and calculate elapsed time based on last_req_arrival only if it is after
the last_end_request. May be something like as follows.

	if (time_after(cic->last_req_arrival, cic->last_end_request))
		elapsed = jiffies - cic->last_req_arrival;
	else
		elapsed = jiffies - cic->last_end_request;

One more question, why are you not considering a sample valid if slice_new
is set for the active queue? I think with introduction of
last_req_arrival, we probably would not need it.

Thanks
Vivek

> +
> +	elapsed = jiffies - cic->last_end_request;
> +	ttime = min(elapsed, 2UL * cfqd->cfq_slice_idle);
>  
>  	cic->ttime_samples = (7*cic->ttime_samples + 256) / 8;
>  	cic->ttime_total = (7*cic->ttime_total + 256*ttime) / 8;
> @@ -2072,7 +2079,7 @@ cfq_rq_enqueued(struct cfq_data *cfqd, struct cfq_queue *cfqq,
>  	if (rq_is_meta(rq))
>  		cfqq->meta_pending++;
>  
> -	cfq_update_io_thinktime(cfqd, cic);
> +	cfq_update_io_thinktime(cfqd, cfqq, cic);
>  	cfq_update_io_seektime(cfqd, cic, rq);
>  	cfq_update_idle_window(cfqd, cfqq, cic);
>  
--
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