cpufreq on SandyBridge in 3.9

Ken Moffat zarniwhoop at ntlworld.com
Sat Mar 30 21:09:00 PDT 2013


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



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