> As a central part of resource management, the OS thread scheduler must maintain the following, simple, invariant: make sure that ready threads are scheduled o

> As a central part of resource management, the OS thread scheduler must maintain the following, simple, invariant: make sure that ready threads are scheduled on available cores. As simple as it may seem, we found that this invariant is often broken in Linux. Cores may stay idle for seconds while ready threads are waiting in runqueues. This oddly lines up with an observation I've made about traffic lights. It is very surprising how often you'll be stopped at an intersection where everybody is waiting and nobody is allowed to go, sometimes for what feels like 10-30 seconds. It seems like the lowest-hanging fruit for improving traffic is to aim never to have degenerate intersections. If someone is waiting and nobody is going, the light should change quickly.