Re: [Linux-ia64] cpu_halt() question

[Alex Williamson <alex_williamson@hp.com>]

Christian,

   I think what you're after is cpu_idle(), not cpu_halt().  cpu_halt
is what takes a processor offline, so it's not a performance issue
to make an extra PAL call before doing it.

   I've played around with calling ia64_pal_halt_light() in cpu_idle
to see if I could impact the power consumption.  Using very crude
measurements, the temperature at the back of an i2000 dropped ~10
degrees F adding something simple like:

if (pm_idle)
    (*pm_idle)();
else
    ia64_pal_halt_light();

Unfortunately, on McKinley, the PAL_HALT_LIGHT state claims the same
power consumption as the full run state.  I've verified that I get
almost no change in processor temperature by adding this call.  BTW,
this is PAL_A 0.7.31, PAL_B 0.7.36.

	Alex

Christian Hildner wrote:
> 
> Hi,
> 
> cpu_halt (in process.c) calls ia64_pal_halt_info() every time when it is
> called. I assume that cpu_halt is called often and pal_halt_info will
> never change for a specific PAL/processor. Why not call
> ia64_pal_halt_info() once at setup time and save the min_power_state in
> a global variable? This would save power (important when Itanium
> notebooks are coming) and (if you are lucky) increases performance if
> time needed by the two PAL calls is bigger then the time needed by the
> external resource to finish.
> 
> Additionally a loop including nops instead of the ia64_pal_halt call
> could bring a acceleration of the response of an external interruption
> (depending on interrupt handling in PAL).
> 
> Opinions?
> 
> Christian
> 
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--
Alex Williamson                                  Linux Development Lab
alex_williamson@hp.com                                 Hewlett Packard
970-898-9173                                          Fort Collins, CO