Fighting for the power.

Alexander Motin mav at
Sun May 3 22:18:25 UTC 2009

I would like to summarize some of my knowledge on reducing FreeBSD power
consumption and describe some new things I have recently implemented in
8-CURRENT. The main character of this story is my 12" Acer TravelMate
6292 laptop with C2D T7700 2.4GHz CPU, 965GM chipset and SATA HDD, under
amd64 8-CURRENT.

Modern systems, especially laptops, are implementing big number of 
power-saving technologies. Some of them are working automatically, other 
have significant requirements and need special system tuning or 
trade-offs to be effectively used.

So here is the steps:

1. CPU
CPU is the most consuming part of the system. Under the full load it
alone may consume more then 40W of power, but for real laptop usage the
most important is idle consumption.
Core2Duo T7700 CPU has 2 cores, runs on 2.4GHz frequency, supports EIST
technology with P-states at 2400, 2000, 1600, 1200 and 800MHz levels,
supports C1, C2 and C3 idle C-states, plus throttling. So how can we use it:
  P-states and throttling
Enabling powerd allows to effectively control CPU frequency/voltage
depending on CPU load. powerd on recent system can handle it quite
transparently. By default, frequency controlled via mix of EIST and
throttling technologies. First one controls both core frequency and
voltage, second - only core frequency. Both technologies give positive
power-saving effect. But effect of throttling is small and can be
completely hidden by using C2 state, that's why I recommend to disable
throttling control by adding to /boot/loader.conf:
In my case frequency/voltage control saves about 5W of idle power.
  - C1 stops clock on some parts of CPU core during inactivity. It is
safe, cheap and supported by CPUs for ages. System uses C1 state by default.
  - C2 state allows CPU to turn off all core clocks on idle. It is also
cheap, but requires correct ACPI-chipset-CPU interoperation to be used.
Use of C2 state can be enabled by adding to /etc/rc.conf:
Effect from this state is not so big when powerd is used, but still
  - C3 state allows CPU completely stop all internal clocks, reduce
voltage and disconnect from system bus. This state gives additional
power saving effect, but it is not cheap and require trade-offs.
As soon as CPU is completely stopped in C3 state, local APIC timers in
each CPU core, used by FreeBSD as event sources on SMP, are not
functioning. It stops system time, breaks scheduling that makes system
close to dead. The only solution for this problem is to use some
external timers. Originally, before SMP era, FreeBSD used i8254 (for HZ)
and RTC (for stats) chipset timers. I have made changes to 8-CURRENT to
resurrect them for SMP systems. To use them, you can disable local APIC
timers by adding to /boot/loader.conf:
Also, to drop/rise voltage on C3, CPU needs time (57us for my system).
It means that C3 state can't be effectively used when system is waking
up often. To increase inactivity periods we should reduce interrupt rate
as much as possible by adding to loader.conf:
It may increase system response time a bit, but it is not significant
for laptop. Also we may avoid additional 128 interrupts per second per
core, by the cost of scheduling precision, with using i8254 timer also
for statistic collection purposes instead of RTC clock, by using another
newly added option:
As result, system has only 100 interrupts per core and CPUs are using C3
with high efficiency:
%sysctl dev.cpu |grep cx
dev.cpu.0.cx_supported: C1/1 C2/1 C3/57
dev.cpu.0.cx_lowest: C3
dev.cpu.0.cx_usage: 0.00% 0.00% 100.00% last 7150us
dev.cpu.1.cx_supported: C1/1 C2/1 C3/57
dev.cpu.1.cx_lowest: C3
dev.cpu.1.cx_usage: 0.00% 0.00% 100.00% last 2235us
Result of effective C3 state usage, comparing to C2+powerd, is about 2W.

2. PCI devices
PCI bus provides method to control device power. For example, I have
completely no use for my FireWire controller and most of time - EHCI USB
controller. Disabling them allows me to save about 3W of power. To
disable all unneeded PCI devices you should build kernel without their
drivers and add to loader.conf:
To enable devices back all you need to do is just load their drivers as

3. Radios
WiFi and Bluetooth adapters can consume significant power when used (up
to 2W when my iwn WiFi is connected) or just enabled (0.5W). Disabling
them with mechanical switch on laptop case saves energy even when they
are not connected.

4. HDA modem
I was surprised, but integrated HDA modem consumed about 1W of power
even when not used. I have used the most radical solution - removed it
mechanically from socket. Case surface in that area become much cooler.

5. HDA sound
To reduce number of sound generated interrupts I have added to the

6. HDD
First common recommendation is use tmpfs for temporary files. RAM is
cheap, fast and anyway with you.
Also you may try to setup automatic idle drive spin-down, but if it is
the only system drive you should be careful, as every spin-up reduces
drive's life time.
For several months (until I have bought SATA SSD) I have successfully
used SDHC card in built-in PCI sdhci card reader as main filesystem. On
random read requests it is much faster then HDD, but it is very slow on
random write. Same time it consumes almost nothing. USB drives could
also be used, but effect is much less as EHCI USB controller consumes
much power.
Spinning-down my 2.5" Hitachi SATA HDD saves about 1W of power. Removing
it completely saves 2W.

Comparing to PATA, SATA interface uses differential signaling for data
transfer. To work properly it has to transmit pseudo-random scrambled
sequence even when idle. As you understand, that requires power. But
SATA implements two power saving modes: PARTIAL and SLUMBER. These modes
could be activated by either host or device if both sides support them.
PARTIAL mode just stops scrambling, but keeps neutral link state, resume
time is 50-100us. SLUMBER mode powers down interface completely, but
respective resume time is 3-10ms.
I have added minimal SATA power management to AHCI driver. There are
hint.ata.X.pm_level loader tunables can be used to control it now.
Setting it to 1 allows drive itself to initiate power saving, when it
wish. Values 2 and 3 make AHCI controller to initiate PARTIAL and
SLUMBER transitions after every command completion.
Note that SATA power saving is not compatible with drive hot-swap, as
controller unable to detect drive presence when link is powered-down.
In my case PARTIAL mode saves 0.5W and SLUMBER - 0.8W of power.

So what have I got? To monitor real system power consumption I am using
information provided by ACPI battery via `acpiconf -i0` command:

Original  system:
Design capacity:        4800 mAh
Last full capacity:     4190 mAh
Technology:             secondary (rechargeable)
Design voltage:         11100 mV
Capacity (warn):        300 mAh
Capacity (low):         167 mAh
Low/warn granularity:   32 mAh
Warn/full granularity:  32 mAh
Model number:           Victoria
Serial number:          292
Type:                   LION
OEM info:               SIMPLO
State:                  discharging
Remaining capacity:     93%
Remaining time:         2:24
Present rate:           1621 mA
Voltage:                12033 mV

Tuned system:
%acpiconf -i0
Design capacity:        4800 mAh
Last full capacity:     4190 mAh
Technology:             secondary (rechargeable)
Design voltage:         11100 mV
Capacity (warn):        300 mAh
Capacity (low):         167 mAh
Low/warn granularity:   32 mAh
Warn/full granularity:  32 mAh
Model number:           Victoria
Serial number:          292
Type:                   LION
OEM info:               SIMPLO
State:                  discharging
Remaining capacity:     94%
Remaining time:         4:47
Present rate:           826 mA
Voltage:                12231 mV

So I have really doubled my on-battery time by this tuning - 4:47 hours 
instead of 2:24 with default settings. Preinstalled vendor-tuned Windows 
XP on the same system, provides maximum 3:20 hours.

Alexander Motin

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