linux-kernel - Re: Problems with the block-layer timeouts

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Message-Id: <20081112110840W.fujita.tomonori@lab.ntt.co.jp>
Date:	Wed, 12 Nov 2008 11:08:57 +0900
From:	FUJITA Tomonori <fujita.tomonori@....ntt.co.jp>
To:	jens.axboe@...cle.com
Cc:	stern@...land.harvard.edu, fujita.tomonori@....ntt.co.jp,
	James.Bottomley@...senPartnership.com, linux-scsi@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: Problems with the block-layer timeouts

On Tue, 11 Nov 2008 20:19:36 +0100
Jens Axboe <jens.axboe@...cle.com> wrote:

> On Tue, Nov 11 2008, Alan Stern wrote:
> > On Tue, 11 Nov 2008, FUJITA Tomonori wrote:
> > 
> > > I don't worry about anything. I just think that these round_jiffies_up
> > > are pointless because they were added for the block-layer users that
> > > care about exact timeouts, however the block-layer doesn't export
> > > blk_add_timer() so the block-layer users can't control the exact time
> > > when the timer starts. So doing round_jiffies_up calculation per every
> > > request doesn't make sense for me.
> > 
> > In fact the round_jiffies_up() routines were added for other users as
> > well as the block layer.  However none of the others could be changed
> > until the routines were merged.  Now that the routines are in the 
> > mainline, you should see them start to be called in multiple places.
> > 
> > Also, the users of the block layer _don't_ care about exact timeouts.  
> > That's an important aspect of round_jiffies() and round_jiffies_up() --
> > you don't use them if you want an exact timeout.
> > 
> > The reason for using round_jiffies() is to insure that the timeout
> > will occur at a 1-second boundary.  If several timeouts are set for
> > about the same time and they all use round_jiffies() or
> > round_jiffies_up(), then they will all occur at the same tick instead
> > of spread out among several different ticks during the course of that
> > 1-second interval.  As a result, the system will need to wake up only
> > once to service all those timeouts, instead of waking up several
> > different times.  It is a power-saving scheme.

Hmm, but for 99.9% of the cases, the timeout of the block layer
doesn't expire, the timeout rarely happens. The power-saving scheme
can be applied to only 0.1%, but at the cost of the round_jiffies
overhead per every request.

If I understand correctly, round_jiffies() is designed for timers that
will expire, such as periodic checking. The power-saving scheme nicely
works for such usages.


> I can't add anything else, can't say it any better either. The main
> point of using round_jiffies_up() is to align with other timers. I don't
> understand why you (Tomo) think that timeouts are exact? They really are
> not, and within the same second is quite adequate here.

My exact argument is for switching from round_jiffies() to
round_jiffies_up. But I wrote above, in the first place, the
round_jiffies didn't make sense to me.
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