The pause instruction is supposed to be used as a "yield hint" for programs to signal that they are waiting for a concurrent action in another CPU to unblock, e

The pause instruction is supposed to be used as a "yield hint" for programs to signal that they are waiting for a concurrent action in another CPU to unblock, e.g. the contended case in a spinlock implementation, where the current thread needs another thread to release the lock before it can make progress. In Skylake, they have changed the microarchitecture so that pause is a much stronger hint; executing a pause will "yield" the current hyperthread, allowing the other hyperthread on the same core to use more of the CPU's resources (ROB slots). This improves performance for well-tuned synchronization, but hurts patterns where pause is executed too often. Unrolling the spinlock loop would help in this case, probably.