Comment 634 for bug 620074

Progress has been made toward the goal of eliminating the timer tick while running in user space. The patches merged for 3.9 fix up the CPU time accounting code, printk() subsystem, and irq_work code to function without timer interrupts; further work can be expected in future development cycles.

A relatively simple scheduler patch fixes the "bouncing cow problem," wherein, on a system with more processors than running processes, those processes can wander across the processors, yielding poor cache behavior. For a "worst-case" tbench benchmark run, the result is a 15x improvement in performance.

The format of tracing events has been changed to remove some unused padding. This change created problems when it was first attempted in 2011, but it seems that the relevant user-space programs have since been fixed (by moving them to the libtraceevent library). It is worth trying again; smaller events require less bandwidth as they are communicated to user space. Anybody who observes any remaining problems would do well to report them during the 3.9 development cycle.

https://lwn.net/Articles/539179/