Pentium M undervolting and underclocking
Contents
Background
Intel Pentium M processors from the Dothan and Banias families can be instructed to operate at voltage and clock frequencies lower than the nominal ones recommended by Intel and used by ThinkPads by default. Experience shows that the processor may continue working correctly at lower-than-nominal voltages and frequencies, thereby reducing power consumption, heat and fan noise.
In the Pentium M, speed and voltage are controlled by software (through the MSR registers). It is up to the operating system to choose the right voltage for each frequency. Normally this is done according to tables published by Intel or according to ACPI tables. However, this can be overriden - in the case of Linux, by a kernel patch.
For an example of what may be achieved, on one ThinkPad T43 all voltages could be reduced by 20-30% and moreover, the minimal frequency reduced from 800MHz to 533MHz; this reduced stable CPU temperature by 7-10deg under both idle and burn-in conditions. Combined with an ACPI fan control script and/or the patch for controlling fan speed, this greatly reduced the problem with fan noise.
There are several methods to control CPU voltage in Linux, and currently all of them require a patcher kernel.
Alternative 1: patch with hard-coded voltages
Instructions
- Determine the stable voltages for each frequency, using some overclocking/undervolting utility that has specific support for Pentium M ("Centrino") processors. It seems that no such utility exists for Linux. For windows, one good choice is Notebook Hardware Control (NHC), which conveniently also includes a table of nominal frequencies and voltags (in its help file).
Ascertain that at the undervolted settings the CPU actually performs correct computation (Prime95 in "Tortute Test" mode seems to be a good partial test emphasizing FPU and memory access).
Note that beside reducing voltages (undervolting), you can also try to add lower frequencies (underclocking). - Determine the model name string reported by the CPU, e.g., via the "Model name" line in /proc/cpuinfo, and likewise the cpu_family, model and stepping.
- Grab a copy of the example patch below and update it to reflect the parameters, frequencies and voltages you found.
- Apply the patch to your kernel (tested with 2.6.13.1 and 2.6.14-rc2).
- To prevent the ACPI table from overriding your table, disable the CONFIG_X86_SPEEDSTEP_CENTRINO_ACPI kernel option.
- Compile and install the new kernel.
The changes should be active now. If you want to see debug information attesting to thus, do the following:
- Reboot in single mode.
# echo 2 > /sys/module/cpufreq/parameters/debug# modprobe speedstep-centrino# dmesgand check for the message "speedstep-centrino: found "Intel(R) Pentium(R) M processor ..."
If you don't see that message, you got the cpu_id or model_name parmaeters wrong. If your system crashes, re-test the voltages and adjust accordingly.
Please report your results (including voltages)!
Linux kernel patch (example)
The following example shows how to undervolt a Pentium M 750 (1.86GHz) on a ThinkPad T43. As discussed above, the parameters are specific to this one CPU. You will need to experimentally find the correct settings for your own CPU and adjust the patch accordingly.
Note that the Pentium M 750 has a 533MHz FSB (quad-pumped 133MHz), hence the use of OP133. If you have a Pentium M with 400MHz FSB (i.e., quad-pumped 100MHz, found in Banias and older Dothan) then in the voltage table change OP133 to OP.
--- linux-2.6.13.1-vanilla/arch/i386/kernel/cpu/cpufreq/speedstep-centrino.c 2005-09-10 05:42:58.000000000 +0300
+++ linux-2.6.13.1/arch/i386/kernel/cpu/cpufreq/speedstep-centrino.c 2005-10-18 04:46:01.000000000 +0200
@@ -54,6 +54,7 @@ enum {
CPU_DOTHAN_A1,
CPU_DOTHAN_A2,
CPU_DOTHAN_B0,
+ CPU_DOTHAN_C0,
CPU_MP4HT_D0,
CPU_MP4HT_E0,
};
@@ -63,6 +64,8 @@ static const struct cpu_id cpu_ids[] = {
[CPU_DOTHAN_A1] = { 6, 13, 1 },
[CPU_DOTHAN_A2] = { 6, 13, 2 },
[CPU_DOTHAN_B0] = { 6, 13, 6 },
+ [CPU_DOTHAN_C0] = { 6, 13, 8 },
[CPU_MP4HT_D0] = {15, 3, 4 },
[CPU_MP4HT_E0] = {15, 4, 1 },
};
@@ -94,6 +97,12 @@ static struct cpufreq_driver centrino_dr
.frequency = (mhz) * 1000, \
.index = (((mhz)/100) << 8) | ((mv - 700) / 16) \
}
+/* Likewise, for processors with 133MHz FSB. */
+#define OP133(mhz, mv) \
+ { \
+ .frequency = (mhz) * 1000, \
+ .index = (((mhz)/133) << 8) | ((mv - 700) / 16) \
+ }
/*
* These voltage tables were derived from the Intel Pentium M
@@ -202,7 +211,26 @@ static struct cpufreq_frequency_table ba
OP(1700, 1484),
{ .frequency = CPUFREQ_TABLE_END }
};
+
+/* Intel Pentium M 750, drastically undervoltaged */
+static struct cpufreq_frequency_table pentium_m_750[] =
+{
+ /* MHz mV */
+ OP133( 800, 700),
+ OP133(1066, 780),
+ OP133(1200, 828),
+ OP133(1333, 876),
+ OP133(1466, 924),
+ OP133(1600, 972),
+ OP133(1733, 1020),
+ OP133(1866, 1068),
+ { .frequency = CPUFREQ_TABLE_END }
+};
+
#undef OP
+#undef OP133
#define _BANIAS(cpuid, max, name) \
{ .cpu_id = cpuid, \
@@ -225,6 +253,12 @@ static struct cpu_model models[] =
BANIAS(1500),
BANIAS(1600),
BANIAS(1700),
+ {
+ .cpu_id = &cpu_ids[CPU_DOTHAN_C0],
+ .model_name = "Intel(R) Pentium(R) M processor 1.86GHz",
+ .max_freq = 1866000,
+ .op_points = pentium_m_750
+ },
/* NULL model_name is a wildcard */
{ &cpu_ids[CPU_DOTHAN_A1], NULL, 0, NULL },
Don't forget to disable CONFIG_X86_SPEEDSTEP_CENTRINO_ACPI.
Alternative 2: patch with user-space control
The bdz.undervolt.2005.10.22.a.patch patch, written by Gentoo-Wiki user "Bdz", allows voltages to be changed without reboot.
It presents a userspace interface, as in the following example:
# cat /sys/devices/system/cpu/cpu0/cpufreq/voltage_table1356,1356,1356,1356,1356,1356,1356,1244,1116,988# echo "1084,1084,1084,1084,1084,1084,1084,988,908,860" >/sys/devices/system/cpu/cpu0/cpufreq/voltage_table
The relevant page on Gentoo-Wiki providues further information and some helpful hints and scripts for voltage adjustment.
Note that this approach only allows adjustment of the voltages for the normal clock speeds (as reported by the ACPI table in the BIOS). On some models (e.g., ThinkPad T43) this does not inlude all clock speeds supported by the processor.
Alternative 3: another patch with user-space control
The centrino-voltages.diff patch, written by Rickard Holmberg, also provides user-space control. See here and here for usage.
Gentoo Initscript
This is a gentoo initscript for the patch from Gentoo-Wiki user "Bdz".
Configuration file
# /etc/conf.d/undervoltage # voltage table sysfs interface VTABLE_SYSFS="/sys/devices/system/cpu/cpu0/cpufreq/voltage_table" # switch back to DEFAULT_VTABLE if undervoltage is stopped? [yes/no] SWITCH_BACK="no" # pentium-m banias 1,7GHz default voltages DEFAULT_VTABLE="1484,1308,1228,1116,1004,956" # pentium-m banias 1,7GHz lowered voltages [ -208mV ] MOD_VTABLE="1276,1100,1020,908,796,748"
Initscript
#!/sbin/runscript
# Copyright 1999-2005 Gentoo Foundation
# Distributed under the terms of the GNU General Public License v2
# $Header: $
sysfs_check() {
if ! [ -e ${VTABLE_SYSFS} ]; then
logger "No sysfs voltage_table present. Modifying vcore voltage failed."
eerror "It seems that the undervolting patch has not been applied to the kernel."
eerror "see http://gentoo-wiki.com/HOWTO_Undervolt_a_Pentium_M_CPU for further information"
return 1
fi
}
start() {
sysfs_check || return 1
ebegin "Switching to modified voltage table [${MOD_VTABLE}]"
echo ${MOD_VTABLE} > ${VTABLE_SYSFS}
eend $?
}
# I think it is not necessary to switch to the default voltage table on shutdown
stop() {
if [ $SWITCH_BACK = "yes" ]; then
ebegin "Switching back to default voltage table [${DEFAULT_VTABLE}]"
echo ${DEFAULT_VTABLE} > ${VTABLE_SYSFS}
eend $?
fi
}
Tested Frequencies
If you have done excessive testing with user defined frequencies (stable system for over a month). and no lookups occur when computing primes with mprime http://mersenne.org/ you can post your frequencies here.
Pentium-M Banias 1.7 - "1276,1100,1020,908,796,748"
External Sources
- A forum discussion of Pentium M undervolting
