Locking overhead

Requesting locks, waiting for locks, and releasing locks add processing overhead.

• A program that supports multiprocessing always does the same lock and unlock processing, even though it is running in a uniprocessor or is the only user in a multiprocessor system of the locks in question.
• When one thread requests a lock held by another thread, the requesting thread may spin for a while or be put to sleep and, if possible, another thread dispatched. This consumes processor time.
• The existence of widely used locks places an upper bound on the throughput of the system. For example, if a given program spends 20 percent of its execution time holding a mutual-exclusion lock, at most five instances of that program can run simultaneously, regardless of the number of processors in the system. In fact, even five instances would probably never be so nicely synchronized as to avoid waiting for one another (see Multiprocessor throughput scalability).