if anybody cares (;-) In the 3.9-rc kernel there is a new cpufreq driver for Intel SandyBridge CPUs, enabled by CONFIG_X86_INTEL_PSTATE. The help says:
This driver provides a P state for Intel core processors. The driver implements an internal governor and will become the scaling driver and governor for Sandy bridge processors. When this driver is enabled it will become the perferred scaling driver for Sandy bridge processors. Since I have a SandyBridge processor on this machine, I enabled this setting. The first thing I noticed was that my own cpufreq initscript failed - I set 'ondemand' as the .config default, and in the initscript I set that for each core. But this driver only provides powersave or performance. A workaround to set the old acpi driver is to add intel_pstate=disable to the grub command line. After fairly extensive testing, I can confirm that the new driver is reasonable - my .config default of 'ondemand' brings up the 'powersave' setting which is not very useful - a setting slightly slower than with powersave in the acpi driver, probably good for battery life but painful when compiling :) The 'performance' setting, OTOH, is good - with the acpi driver 'performance' stays at full speed (3300 MHz in my case), but with the new P-state driver it falls back to the default 48% speed (1584 MHz in my case) quickly, and ramps up towards full speed equally quickly. Testing builds of gcc-4.7.2 shows the new 'performance' setting is a little faster (with -j4) than the old acpi 'ondemand'. A kernel (x86_64) -j4 defconfig build is, however, marginally slower and one build (hopefully just an outlier!) was quite a lot slower [ typically 6m52 for acpi, 6m57 for intel_pstate but that one build took 7m20 ]. So, for kernels >= 3.9 on SandyBridge use the 'performance' scaling governor instead of 'ondemand' if you enable this new driver [ e.g. test for the /sys/devices/system/cpu/intel_pstate/ directory in your own initscript, or perhaps change the default setting in your .config ]. This was a public information bulletin. ĸen -- das eine Mal als Tragödie, das andere Mal als Farce -- http://linuxfromscratch.org/mailman/listinfo/lfs-chat FAQ: http://www.linuxfromscratch.org/faq/ Unsubscribe: See the above information page
