To add on to every other sibling comment, switching threads or processes requires a trip back up to the kernel space (a context switch), instead of just remaini

To add on to every other sibling comment, switching threads or processes requires a trip back up to the kernel space (a context switch), instead of just remaining in user space. in this switch, all of your caches get busted. Not a problem for most folks, but when you want the greatest possible performance, you want to avoid these kinds of transitions. Basically, the same reason some folks use user-space networking stacks.