schedules the hardware to interrupt it when it knows a future job will require its attention.
The tickless kernel provides another indirect benefit when it comes to power efficiency: It enables better use of virtualization, technology that lets multiple operating systems run simultaneously on the same computer, by replacing numerous idle machines with fewer, more efficiently used ones.
Tickless kernels mean that the virtualization software that underlies all those operating systems isn't unduly taxed with needless interruptions. So theoretically, administrators can consolidate servers more aggressively.
"If you have a server running 50 virtualized guests, and each guest has a timer tick 1,000 times per second, that is 50 thousand ticks per second, without even doing any work yet," van de Ven said. "With tickless, you go from 1,000 to maybe 10, and suddenly it becomes manageable to do 50 guests."
Michael Larabel, editor of the Phoronix site that tests Linux hardware performance, found the tickless kernel can cut power consumption from 28 watts to 26 watts in IBM's Pentium M-based ThinkPad R52 running Fedora 7.
"A tickless kernel, in conjunction with (processor-based) power-saving technologies, can go a long way in extending the life of the battery and reducing the heat output," Larabel said.
Peeking with PowerTop
A tickless kernel isn't much good if higher-level software requires the kernel to schedule frequent wakeup calls. That's where PowerTop comes in.
"A typical Linux distribution has many components that wake the processor up frequently for no good reason," van de Ven said in an announcement of the software. "PowerTop will provide an indication of which...software components are the biggest offenders in slurping up your battery time."
Scrutiny with PowerTop has uncovered power-draw culprits. The Gvim text editor's blinking cursor wakes up the kernel. Evolution e-mail software needs to check for new jobs to do 10 times a second. The GAIM (now called Pidgin) instant-messaging software checks every 5 seconds to see if it should set itself as "idle".
As well as fixing these sorts of issue, the Linux kernel itself needs to be spruced up to better support its own "ticklessness".
"Even though the kernel itself now has all the fundamental timer-handling knowledge, most of the kernel subsystems use some timers for their own handling, and tuning that usage will probably go on for some time," Torvalds said.
And other frontiers need work, van de Ven said. Device drivers--the software modules that let the kernel communicate with hardware such as network cards or keyboards--need to be revamped to better handle power issues.
Another issue is developing power-related policy management software that governs a computer's behavior based on what its user is doing. And yet another thorny one is supporting laptop suspend-resume abilities better so laptop computers can hibernate gracefully.
"On the suspend/resume side there will be a lot of rearchitecting needed, especially for suspend-to-disk," van de Ven said. "It's an ongoing discussion topic in Linux."
But much of that work can take place on a newly tickless foundation. "The heavy lifting is mostly done," Torvalds said.
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