nanosleep high cpu usage?

Question

I noticed that a little test program which calls nanosleep is showing a huge difference in CPU usage when run on Linux machines with a kernel newer than 2.6.22.

Answer 1

This is due to the introduction of NO_HZ into the mainline scheduler.

Previously, your 1,000 ns sleep was usually sleeping for a whole tick - 1,000,000 ns. Now, when the machine is otherwise idle, it's actually only sleeping for what you asked for. So it's running the while() loop and syscall around 1,000 times more frequently - hence a lot more CPU usage. If you increase tv_nsec you should see a reduction in the CPU usage.