Messages in this thread | | | Date | Fri, 20 Oct 2006 22:37:31 +0200 | From | Ingo Molnar <> | Subject | Re: various laptop nagles - any suggestions? (note: 2.6.19-rc2-mm1 but applies to multiple kernels) |
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* 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.
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