Kernel space: Progress on ACPI and power management

Data centers demand cooler-running Linux boxes, and several projects are starting to deliver.

By Jonathan Corbet, LinuxWorld.com, 07/11/07

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Len Brown can only be a glutton for punishment; he is, after all, the maintainer of the Linux ACPI subsystem. That is a difficult position to be in: ACPI involves getting into the BIOS layer, an area of system software which is not always known for careful, high-quality work. Supporting ACPI is a complex task which, among other things, requires the embedding of a specialized interpreter within the kernel, a hard sell at best. Even with that background in mind, one must wonder just how much masochism is required to lead one to deliver three separate talks at the 2007 Ottawa Linux Symposium. That is just what Len did, however; the end result was a good view into several aspects of the power management problem.

 

Getting more from tickless

The first talk (on the tickless kernel) was supposed to be given by Suresh Siddha, who was unable to attend the event. The dynamic tick patches have been covered here before. Suresh/Len's talk was not really about how these patches work, but, instead, about the work which remains to be done to take full advantage of the tickless design. It seems that the work which has been done so far is just the beginning.

The problem is that, on a system used by Suresh and company, the average processor sleep time was still less than 1ms even after the dynamic tick code was enabled. Given that one of the driving reasons for dynamic tick was to let the processor sleep for long periods of time - thus saving power - this is a disappointing result. It turns out that there is a lot which can be done to improve the situation, though.

Step number one is to address a kernel-space problem: there are a lot of active kernel timers which tend to spread out over time. As a result, the kernel wakes up much more often than it would if the timers were sufficiently well coordinated to expire at the same time whenever possible. As it happens, many kernel timers do not need great precision; a timer which fires some number of milliseconds later than scheduled is not a problem. So, if the kernel could defer some timers to fire at the same time as others, it can reduce the number of wakeups. The deferrable timers patch does exactly that; the round_jiffies() function added in 2.6.19 can also help the kernel line up events. Adding this code brought the average sleep time up to 20ms, with the system handling 90 interrupts per second.

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