lkml.org 
[lkml]   [2006]   [Oct]   [20]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectRe: various laptop nagles - any suggestions? (note: 2.6.19-rc2-mm1 but applies to multiple kernels)

* Andrew Morton <akpm@osdl.org> wrote:

> > > I don't know how many machines will be affected by this, but I'd
> > > expect it's quite a few - the Vaio has a less-than-one-year-old
> > > Intel CPU in it.
> >
> > Is this still the broken lapic issue ?
>
> yup. iirc the standard FC5 SMP kernel runs dog-slowly on that machine
> too.

hm. This is how lapic timer calibration works.

the lapic timer is really simple - it counts down from a value and
generates an irq if that counter reaches 0. Then it starts counting down
again.

the 'count down from' value is programmed via __setup_APIC_LVTT().

we first write a 'really large' number into it (1 billion):

__setup_APIC_LVTT(1000000000);

the unit of counting is '16 system bus cycles'.

i.e. if your system has a system bus of 333 MHz, then a value of 1
billion takes 48 seconds to count down. (so the calibration ought to be
pretty robust in this regard.)

then we use the wait_timer_tick() function, which waits until the PIT
counter reaches 0 (which is attached to the PIT whose frequency we know
and thus the PIT is already programmed correctly). Hence by calling
wait_timer_tick() we can generate a delay of one jiffy - and we can read
out the current lapic timer count and determine the calibration factor.

then we calculate the result as:

result = (tt1-tt2)*APIC_DIVISOR/LOOPS;

where tt1 is the counter before we start calibration, tt2 is the lapic
timer counter after we did calibration. (APIC_DIVISOR is 16)

i dont see where the error is - but there must be some calibration
problem as your system shows a systematic 1:60 difference between
expected and real lapic timer frequency.

Ingo
-
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/

\
 
 \ /
  Last update: 2006-10-20 22:41    [W:0.102 / U:0.484 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site