Re: cpufreq & dual core CPUs.

[Dave Jones <davej@redhat.com>]

On Tue, May 30, 2006 at 09:40:37PM -0700, Pallipadi, Venkatesh wrote:
 
 > I am assuming speedstep-centrino is being used here.

yes, sorry for omitting that, though I'm not sure this problem
is unique to Intel CPUs, though powernow-k8 does seem to have
some awareness of the cpu_core_map

 > This one core
 > reducing the frequency of the other core is not supposed to happen. The
 > way it is handled in Core Duo or later CPUs is:
 > 1) We can either use software coordination when BIOS and OS supports it.
 > In which case, OS will know that these two CPUs are tied together and
 > controls both of them using one interface. However, the users of this
 > one common interface, cpuspeed or ondemand governor should be aware of
 > this fact that one interface controls two CPUs (by looking at the
 > affected_cpus in cpufreq directory). Ondemand handles this today. But,
 > cpuspeed. I am not sure.

Looking at that file on my core duo laptop, I see ..

(00:47:47:davej@exile:~)$ cat /sys/devices/system/cpu/cpu0/cpufreq/affected_cpus
0
(00:47:54:davej@exile:~)$ cat /sys/devices/system/cpu/cpu1/cpufreq/affected_cpus
1

That implies to me that they're separate cores no?

 > 2) More easier way is to do this coordination in hardware. Each Cpu will
 > write to their own MSR. Hardware will pick the maximum frequency of the
 > two MSR and controls the package at that frequency. This is the default
 > mode on most of Core Duo based platforms and we have found that this is
 > more effient in the sense that each CPU can indepently control the
 > frequency, with no set_cpu_allowed() and friends in the path.
 > 
 > Basically, with hardware coordination, you should not see this
 > particular issue. I feel, there is something broken in this particular
 > case and I will look into it closer tomorrow. My guess at this point is,
 > this is due to userlevel governor, which looks at utilization over a
 > period of time (few seconds) and then decides on the frequency. This can
 > be checked with ondemand governor, which should handle this case much
 > better.

ondemand does behave better than userspace does, but something is
still amiss given that they can both still be set to individual
values.
This..

(00:52:15:davej@exile:~)$ grep MHz /proc/cpuinfo
cpu MHz         : 1000.000
cpu MHz         : 2167.000

Should never happen on a dual core system that has synchronised cores.
Either they're both running at 1GHz, or they're both running at 2.1GHz.

(That was with both cpu0 & cpu1 set to use ondemand governor).

		Dave

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