Tip ‘o the hat, wag o’ the finger — Linux power savings for laptop users
It’s interesting to see how far, and yet how much more work we need to do on power management for Linux. I recently got a new laptop — a Lenovo Thinkpad X61s — and using the powertop tool, I was able to configure my system to the point where in what I can “airplane mail reading mode” (mailbox preloaded into memory, USB disabled, wireless and ethernet disabled, backlight down to 30% brightness, sloppily written power hogs like Firefox and Notes — every single application writer should be forced to run powertap and explain why their program feels it necessary to constantly wake up the CPU), I can get my usage down to about 9.8 watts. Using the 8 hours extended battery, that’s 8 hours of battery life, although granted doing very little. On the flip side, if I’m doing a major kernel compile, I can drive up utilization up to almost 30 watts, which means less than 3 hours of battery life. So that’s definitely the good news; Linux can sharply reduce its power consumption to the point where it is highly competitive with Windows. (And probably better than Vista, just because that OS is so heavy and bloated.) So thanks and and a tip of the hat to Intel and to Arjan van de Ven for making such a useful tool like Powertop available.
So now for the bad news. Getting down to this level of power saving thriftness, where the laptop is carefully sipping only the minimal amounts of power from the battery, is definitely a work in progress. First of all, you can only get this level of power savings by unloading a specific USB driver, uhci_hcd. This will disable low speed devices (including unfortunately the fingerprint reader and the EVDO WWAN device if you were silly enough to buy one that was built into the laptop as opposed to a stand-alone card that you can swap between laptops and lend out to friends as necessary). But how many users are going to open up a terminal window, su to root and type the command “rmmod uhci_hcd”? And know how to reload the driver using “modprobe uhci_hcd” when they need to use the USB devices again?
A similar problem exists for Network Manager; when the user disables the network by right-clicking on the applet, why doesn’t it automatically bring down the interface, instead of forcing the user to manually su to root and then type the command “ifconfig eth0 down; ifconfig wlan0 down”?
A more serious problem is the Intel Wireless driver for the 4965. Even with the wlan0 interface configured down, and with the RF kill switch enabled, keeping the iwl4965 driver loaded will still cost you an extra full watt of power. When you’re down to 9.6 watts, that means that keeping the iwl4965 driver loaded when you don’t need it will cost you a 10% reduction in your battery life! That’s just sloppy, and hopefully it will be fixed in a future update to the iwl4965 driver, but as long as you don’t mind manually removing and reloading it, you can work around this power-saving oversight.
A bigger issue, though, one for which no workaround exists, is that unlike the ipw3945 drivers, which at least had private, non-standard iwpriv commands to engage the 802.11’s power-saving features, the iwl4965 driver has neither the non-standard Intel iwpriv interfaces, nor the standard iwconfig interfaces for enabling any kind of powersaving features, including changing the transmit power of the card. So while powertop deserves plenty of kudos, iwl4965 deserves a wag of the finger from a power saving viewpoint. No doubt Intel just needs to allocate more money to its Open Source Technology Center so it get more of its crack developers to work improving Linux support for their processors and chipsets.
Speaking of which, I’m still waiting for an Intel x.org 965GM driver that can support compiz/beryl and simultaneously show video clips at the same time… And being able turn off the 50 interrupts/second generated by the video card when they aren’t needed because 3-d graphics aren’t currently in use, without requiring a restart of the X server, would also be a nice touch. The bottom line is that Linux power savings and Linux support for laptops in general is much better than it was a year ago, and a lot of credit has to go to the efforts of Intel’s teams producing such good work as powertop, their wireless drivers, and their open X server drivers. We still have a lot of work left to be done, though!