Apple Open Sources Snow Leopard's Grand Central Dispatch (Apple Insider)

if you are running batch jobs that do not interact with anything then the most efficiant way to run them is also the most unfair, you pick one per core and run it to completion with no context switches at all.

now in the real world, nothing short of HPC clusters really doesn't interact with anything. even normal batch jobs need to get data from disk, at which point that thread must sit idle waiting for the disk and another thread could run instead.

don't think of this like you would a normal kernel scheduler. think of this like you would a userspace batch engine. it tries to maximize the total throughput at the expense of latency