thread synchronization
also known as intraprocess communication
mutex: A single variable that can be locked or unlocked, uses the hardware instruction “test and set” which entirely locks the RAM from all the cores
semaphore: if count is negative n, n threads are blocked on the semaphore, allows you to count how many threads are waiting.
barriers: wait until that many threads hit the barrier (implemented with semaphores)
early days of Linux the kernel only had one lock which lead to a lot of slowness
fairness: threads don’t sleep for too long progress: threads don’t sleep forever
kill sends to any thread in that process. pthread sends to the specific thread, and pthread_cond_signal sends to anyone blocking
