Re: [PATCH] i386: Selectable Frequency of the Timer Interrupt

[Vojtech Pavlik]

On Tue, Jul 12, 2005 at 08:57:06AM -0700, George Anzinger wrote:

> >>>The PIT crystal runs at 14.3181818 MHz (CGA dotclock, found on ISA, ...)
> >>>and is divided by 12 to get PIT tick rate
> >>>
> >>>	14.3181818 MHz / 12 = 1193182 Hz
> 
> Yes, but the current code uses 1193180.  Wonder why that is...

Because it was stated so in some ancient DOS docs? I really don't know.
x86-64 uses 1193182. This is because when I was doing x86-64 timing
code, I had observably better results with this number.

> >>>   HZ   ticks/jiffie  1 second      error (ppm)
> >>>---------------------------------------------------
> >>>  100      11932      1.000015238      15.2
> >>>  200       5966      1.000015238      15.2
> >>>  250       4773      1.000057143      57.1
> >>>  300       3977      0.999931429     -68.6
> >>>  333       3583      0.999964114     -35.9
> >>>  500       2386      0.999847619    -152.4
> >>> 1000       1193      0.999847619    -152.4
> 
> If we are following the standard and trying to set up a timer, the 1
> second time MUST be >= 1 second.  Thus the values for 300 and above in
> this table don't fly. 

In that case, the table will look like this (with the CGA clock fixed to
14.31818000 MHz):

   HZ   ticks/jiffie  1 second      error (ppm)
---------------------------------------------------
   100      11932      1.000015365      15.4
   200       5966      1.000015365      15.4
   250       4773      1.000057270      57.3
   300       3978      1.000182984     183.0
   500       2387      1.000266794     266.8
  1000       1194      1.000685841     685.8
---------------------------------------------------
    82      14551      1.000000279       0.3
   216       5524      1.000001956       2.0
   432       2762      1.000001956       2.0
   864       1381      1.000001956       2.0
  1381        864      1.000001956       2.0


> If we are trying to keep system time, we'll we do just fine at 
> that by using the actual value of the jiffie (NOT 1/HZ) when we update time 
> (one of the reasons for going to nanoseconds in xtime).

Yes, adding (12.0/14318180)*LATCH to xtime in each timer interrupt,
instead of (1.0/HZ) indeed eliminates the error entirely.

> The observable thing the user sees is best seen by setting up an
> itimer to repeat every second.  Then you will see the drift AND it
> will be against the system clock which itself is quite accurate (the
> 50-100ppm you mention), even without ntp.  And the error really is in
> the range of 848ppm for HZ=1000 BECAUSE we need to follow the
> standard.  You can easily see this with the current 2.6 kernel.  We
> even have a bug report on it:
> 
> http://bugzilla.kernel.org/show_bug.cgi?id=3289
 
Going to HZ=864 would fix this problem. It would likely cause other
problems in places that expect 1/HZ to be a sane number, though.

-- 
Vojtech Pavlik
SuSE Labs, SuSE CR
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