Linux creator Linus Torvalds announced the release of the 2.6.21 kernel, "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 (just as an example: we were apparently unlucky enough to hit what looks like a previously unknown hardware errata in one of the ethernet drivers that got updated etc)." Regarding the the dynticks code which was merged in -rc1 [story] he said, "the big change during 2.6.21 is all the timer changes to support a tickless system (and even with ticks, more varied time sources). Thanks (when it no longer broke for lots of people ;) go to Thomas Gleixner and Ingo Molnar and a cadre of testers and coders." He went on to note, "of course, the timer stuff was just the most painful and core part (and thus the one that I remember most): there's a lot of changes all over. The appended changelog is just for the fixes since -rc7, so that doesn't look very impressive, the full changes since 2.6.20 are obviously a *lot* bigger (and you're better off reading the individual -rc changelogs)." Linus finished with a running joke about the many debates centered around current CPU scheduler efforts [story], quipping, "we now return you to your regular scheduler discussions".

Avi Kivity suggested that combining KVM, the Kernel-based Virtual Machine [story], with the dyntick patch [story] could improve overall KVM performance. He noted that it would likely improve performance of both the host by "avoiding expensive vmexits due to useless timer interrupts," as well as on the guest by "reducing the load on the host when the guest is idling (currently an idle guest consumes a few percent cpu)". Ingo Molnar [interview] pointed out that KVM with his -rt kernel already works with dynticks enabled on both the host and the guest, "using the dynticks code from the -rt kernel makes the overhead of an idle guest go down by a factor of 10-15". Ingo added that he hopes the dyntick patch will be ready to be merged into the upcoming mainline 2.6.21 kernel.

Rik van Riel [interview] noted that there were other ways to reduce the load of the guest when it's idling, "you do not need dynticks for this actually. Simple no-tick-on-idle like Xen has works well enough." Ingo explained, "s390 (and more recently Xen too) uses a next_timer_interrupt() based method to stop the guest tick - which works in terms of reducing guest load, but it doesnt stop the host-side interrupt. The highest quality approach is to have dynticks on both the host and the guest, and this also gives high-resolution timers and a modernized time/timer-events subsystem for both the host and the guest."

Thomas Gleixner and Ingo Molnar [interview] posted an update of their high-res timers kernel patches for the 2.6.17 kernel, "upon which we based a tickless kernel (dyntick) implementation and a 'dynamic HZ' feature as well". The patch currently works for x86, with ports to x86_64, PPC and ARM in the works. Thomas explains, "the high-res timers feature (CONFIG_HIGH_RES_TIMERS) enables POSIX timers and nanosleep() to be as accurate as the hardware allows (around 1usec on typical hardware). This feature is transparent - if enabled it just makes these timers much more accurate than the current HZ resolution." He goes on to discribe the tickless kernel:

"The tickless kernel feature (CONFIG_NO_HZ) enables 'on-demand' timer interrupts: if there is no timer to be expired for say 1.5 seconds when the system goes idle, then the system will stay totally idle for 1.5 seconds. This should bring cooler CPUs and power savings: on our (x86) testboxes we have measured the effective IRQ rate to go from HZ to 1-2 timer interrupts per second.

"This feature is implemented by driving 'low res timer wheel' processing via special per-CPU high-res timers, which timers are reprogrammed to the next-low-res-timer-expires interval. This tickless-kernel design is SMP-safe in a natural way and has been developed on SMP systems from the beginning."