* Re: Fw: missing #if for 1000 HZ
[not found] <20030725203843.569863bc.akpm@osdl.org>
@ 2003-07-28 21:55 ` george anzinger
0 siblings, 0 replies; only message in thread
From: george anzinger @ 2003-07-28 21:55 UTC (permalink / raw)
To: Andrew Morton; +Cc: john stultz, Albert Cahalan, linux-kernel, Ulrich.Windl
Andrew Morton wrote:
> Guys?
If I understand what is being done, he is right. By this measure the
current error in the div (by shifting) is 2.4%.
I wonder how this plays with(line 639):
time_phase += time_adj;
if (time_phase <= -FINENSEC) {
long ltemp = -time_phase >> (SHIFT_SCALE - 10);
time_phase += ltemp << (SHIFT_SCALE - 10);
delta_nsec -= ltemp;
}
where, we are also using >>10 ~= /1000. I _think_ this ladder
approximation only delays part of the correction but does not loose
it, but the more eyes the better.
-g
>
> Begin forwarded message:
>
> Date: 22 Jul 2003 21:01:16 -0400
> From: Albert Cahalan <albert@users.sourceforge.net>
> To: linux-kernel@vger.kernel.org, Ulrich.Windl@rz.uni-regensburg.de
> Cc: Andrew Morton <akpm@digeo.com>, alan@redhat.com
> Subject: missing #if for 1000 HZ
>
>
> This should improve timekeeping a bit @ 1000 HZ.
>
>
> diff -Naurd old/kernel/timer.c new/kernel/timer.c
> --- old/kernel/timer.c 2003-07-18 17:27:01.000000000 -0400
> +++ new/kernel/timer.c 2003-07-18 17:32:19.000000000 -0400
> @@ -606,6 +606,15 @@
> else
> time_adj += (time_adj >> 2) + (time_adj >> 5);
> #endif
> +#if HZ == 1000
> + /* Compensate for (HZ==1000) != (1 << SHIFT_HZ).
> + * Add 1.5625% and 0.78125% to get 1023.4375; => only 0.05% error (p. 14)
> + */
> + if (time_adj < 0)
> + time_adj -= (-time_adj >> 6) + (-time_adj >> 7);
> + else
> + time_adj += (time_adj >> 6) + (time_adj >> 7);
> +#endif
> }
>
> /* in the NTP reference this is called "hardclock()" */
>
>
>
>
--
George Anzinger george@mvista.com
High-res-timers: http://sourceforge.net/projects/high-res-timers/
Preemption patch: http://www.kernel.org/pub/linux/kernel/people/rml
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