Matthew Garrett's "Observations on Power Management" is a great intro.
- APM (Advanced Power Management): All PM policy/mechanism resides within the BIOS
- Motivated by, largely relevant only to laptops
- apmd debian package
- No longer supported in Vista. Off by default in recent Debian kernels.
- ACPI: Current, often buggy (but also often easily repairable via BIOS flash or by hand)
- C-States, which are decomposable into P-States and T-States
- P4 Thermal Throttling: Slows down or shuts off the processor based on CPU temperature
- Adjustment is either via idle cycle insertion or lowering the clock multiplier
- On Linux, cpufreq-info provides lots of good information:
[recombinator](0) $ cpufreq-info cpufrequtils 004: cpufreq-info (C) Dominik Brodowski 2004-2006 Report errors and bugs to email@example.com, please. analyzing CPU 0: driver: acpi-cpufreq CPUs which need to switch frequency at the same time: 0 hardware limits: 1.60 GHz - 2.39 GHz available frequency steps: 2.39 GHz, 1.60 GHz available cpufreq governors: ondemand, performance current policy: frequency should be within 1.60 GHz and 2.39 GHz. The governor "ondemand" may decide which speed to use within this range. current CPU frequency is 1.60 GHz. cpufreq stats: 2.39 GHz:27.25%, 1.60 GHz:72.75% (78631) analyzing CPU 1: driver: acpi-cpufreq CPUs which need to switch frequency at the same time: 1 hardware limits: 1.60 GHz - 2.39 GHz available frequency steps: 2.39 GHz, 1.60 GHz available cpufreq governors: ondemand, performance current policy: frequency should be within 1.60 GHz and 2.39 GHz. The governor "ondemand" may decide which speed to use within this range. current CPU frequency is 1.60 GHz. cpufreq stats: 2.39 GHz:4.32%, 1.60 GHz:95.68% (19387) [recombinator](0) $
- You can get C-state latency information from /proc/acpi/processor/*/power:
[wopr](0) $ cat /proc/acpi/processor/CP10/power active state: C0 max_cstate: C8 maximum allowed latency: 2000000000 usec states: C1: type[C1] promotion[--] demotion[--] latency usage duration C2: type[C2] promotion[--] demotion[--] latency usage duration C3: type[C2] promotion[--] demotion[--] latency usage duration [wopr](0) $
- On FreeBSD, sysctls from the dev.cpu and debug.cpufreq MIB hierarchies are your window into frequency control. See cpufreq(4).
- noatime -- critical for all kinds of things! don't believe me; trust ingo molnár:
i cannot over-emphasise how much of a deal it is in practice. Atime updates are by far the biggest IO performance deficiency that Linux has today. Getting rid of atime updates would give us more everyday Linux performance than all the pagecache speedups of the past 10 years, _combined_. it's also perhaps the most stupid Unix design idea of all times. Unix is really nice and well done, but think about this a bit: ' For every file that is read from the disk, lets do a ... write to the disk! And, for every file that is already cached and which we read from the cache ... do a write to the disk! ' tell that concept to any rookie programmer who knows nothing about kernels and the answer will be: 'huh, what? That's gross!'. And Linux does this unconditionally for everything, and no, it's not only done on some high-security servers that need all sorts of auditing enabled that logs every file read - no, it's done by 99% of the Linux desktops and servers. For the sake of some lazy mailers that could now be using inotify, and for the sake of ... nothing much, really - forensics software perhaps.
- SATA link state management? what is this? seen in powertop output
- turning up the writeback time / disk head parking / other debatable techniques
- The /sys/devices/system/cpu/sched_smt_power_savings tunable causes tasks (under light load) to be preferentially distributed across processing elements (ie including SMT units) and cores of physical packages (as opposed to packages themselves).
- The /sys/devices/system/cpu/sched_mc_power_savings tunable does the same, but doesn't apply to SMT.
- Task migration has overhead and associated architecture warmup (ie, caches, branch prediction and hardware prefetching). How is this affected? FIXME
- Disable Wake-on-LAN if it's not being used: ethtool -s DEVICE wol d
- PS-Poll (PowerSave Poll) turns the radio off longer in exchange for higher latencies (only when it's disabled, though)
- Requires support from access point
- Auto-association / aggressive scanning, especially when wireless is not being used
- MAC80211_DEFAULT_PS, introduced in the 2.6.32 development cycle, sets wireless powersaving by default
- Use latency requirement registration (Documentation/power/pm_qos_interface.txt) for applications which need it