Linus Torvalds has finally published the Linux 2.6.21 kernel, adding efficiency and virtualisation improvements, after a development process that dragged on for months.

"It's been over two and a half months, and while it's certainly not the longest release cycle ever, it still dragged out a bit longer than I'd have hoped for and it should have," wrote Torvalds in a release note to the kernel mailing list late on Wednesday.

He said the delay was largely due to changes in the kernel designed to support a more efficient "tickless" system.

"The big change during 2.6.21 is all the timer changes to support a tickless system," he wrote. "If the goal for 2.6.20 was to be a stable release (and it was), the goal for 2.6.21 is to have just survived the big timer-related changes and some of the other surprises."

The new kernel gives Linux a "tickless" idling mechanism called dynticks, built on top of a technology called clockevents. The tickless system means the software can allow the CPU to idle more efficiently when it's waiting for tasks to do, saving energy and, for instance, potentially extending laptop battery life.

Other big changes involve virtualisation improvements, including changes to VMI, a paravirtualisation interface compatible with VMware and Xen. The kernel also makes improvement to the up-and-coming technology KVM (Kernel-based Virtual Machine), including initial paravirtualisation support.

Paravirtualisation is a technique used by the likes of Xen to make virtual machines more efficient by making it unnecessary to completely virtualise the operating system.

Other changes include better network and audio device support, and support for the PA SEMI PWRficient CPU and Toshiba's Cell-based CellEB architecture.