This is going to be a short post. There are two things I need to do frequently with manycores machines that I have:
* Switch them on/off to perform experiments in a more controlled environment.
* Scale their frequency to match their performance.
For first, there is a simple trick. All CPU cores in linux show up in sysfs
#ls /sys/devices/system/cpu/
cpu0/ cpu1/ cpu2/ cpu3/ cpufreq/ cpuidle/ kernel_max microcode/ modalias offline online possible present probe release uevent
* Switch them on/off to perform experiments in a more controlled environment.
* Scale their frequency to match their performance.
For first, there is a simple trick. All CPU cores in linux show up in sysfs
#ls /sys/devices/system/cpu/
cpu0/ cpu1/ cpu2/ cpu3/ cpufreq/ cpuidle/ kernel_max microcode/ modalias offline online possible present probe release uevent
As you can see there are 4 directories for 4 core system that I have. In each of the core* directories I have
#ls /sys/devices/system/cpu/cpu0/
cache cpufreq crash_notes microcode node0 online subsystem thermal_throttle topology uevent
So one just have to echo 1 (for ON) or 0 (for OFF) into online file. This is a sysfs file, writing to which triggers some action inside kernel. In our case this would be switching off the CPU.
#echo 0 > /sys/devices/system/cpu/cpu0/online
For this to work, you kernel must support dynamic hotplugging of CPUs. Mind that CPU0 has a special status and you can not switch if off. Although, Linux is smart enough when running on a single core, it switches to uniprocessor (UP) code.
For second, you need to have associated processor power state driver. Most of the modern Intel processor can work with P state acpi_cpufreq.ko driver. For other driver options check in:
'make menuconfig' -> Power Management and ACPI Options -> CPU Frequency Scaling
-> x86 CPU Frequency Scaling Drivers
Now get the driver in. To regulate frequency from userspace you need a userspace tool called 'cpufrequtils'. Install it. With the driver and tool in, we get something like:
#cpufreq-info
cpufrequtils 007: cpufreq-info (C) Dominik Brodowski 2004-2009
Report errors and bugs to cpufreq@vger.kernel.org, please.
analyzing CPU 0:
driver: acpi-cpufreq
CPUs which run at the same hardware frequency: 0 1 2 3
CPUs which need to have their frequency coordinated by software: 0
maximum transition latency: 10.0 us.
hardware limits: 1.60 GHz - 2.53 GHz
available frequency steps: 2.53 GHz, 2.39 GHz, 2.26 GHz, 2.13 GHz, 2.00 GHz, 1.86 GHz, 1.73 GHz, 1.60 GHz
available cpufreq governors: conservative, ondemand, userspace, performance
current policy: frequency should be within 1.60 GHz and 2.53 GHz.
The governor "userspace" may decide which speed to use
within this range.
current CPU frequency is 1.86 GHz (asserted by call to hardware).
As you can it there are multiple frequencies I can choose from. You can not arbitrarily set one frequency, it has to be one from the set. To set the frequency:
#cpufreq-set -c 0 -f 1.8GHz
where -C is core number. And -f is frequency.
And thats about it. For more details see Linux kernel documentation at
and most of other ACPI and power/performance related options are in
'make menuconfig' -> Power Management and ACPI Options
Cheers
