Re: [PATCH] Bad rounding in timeval_to_jiffies [was: Re: Odd Timer behavior in 2.6 vs 2.4 (1 extra tick)]

[Russell King <rmk+lkml@arm.linux.org.uk>]

On Wed, Apr 20, 2005 at 11:40:16PM -0400, Steven Rostedt wrote:
> Is 11 jiffies correct for 10ms?

Consider the 1 jiffy case.  How long does waiting one jiffy actually wait?

j=0            1              2
+--------------+--------------+--> t
 A     B      C D

If you start timing one jiffy from A, you're looking for j=1, so your
timer expires close to D and you have waited one jiffy.

If you start timing one jiffy from B, you're still looking for j=1.
Your timer expires at the same point (D) but you've waited less than
one jiffy.

If you start timing one jiffy from C, it's the same - expires at D.
This time, you've waited virtually no time at all.

The problem is that when you add a timer, you don't have any idea
which point you're going to be starting your timer at.

This is why we always round up to the next jiffy when we convert
times to jiffies - this ensures that you will get at _least_ the
timeout you requested, which is in itself a very important
guarantee.

-- 
Russell King
 Linux kernel    2.6 ARM Linux   - http://www.arm.linux.org.uk/
 maintainer of:  2.6 Serial core