Evaluating Locks

Before building any locks, we should first understand what our goals are, and thus we ask how to evaluate the efficacy of a particular lock implementation. To evaluate whether a lock works (and works well), we should establish some basic criteria.

Mutual exclusion

The first is whether the lock does its basic task, which is to provide mutual exclusion. Basically, does the lock work, preventing multiple threads from entering a critical section?

Fairness

The second is fairness. Does each thread contending for the lock get a fair shot at acquiring it once it is free? Another way to look at this is by examining the more extreme case: does any thread contending for the lock starve while doing so, thus never obtaining it?

Performance

The final criterion is performance, specifically the time overheads added by using the lock. There are a few different cases that are worth considering here. One is the case of no contention; when a single thread is running and grabs and releases the lock, what is the overhead of doing so? Another is the case where multiple threads are contending for the lock on a single CPU; in this case, are there performance concerns? Finally, how does the lock perform when there are multiple CPUs involved, and threads on each contending for the lock?

By comparing these different scenarios, we can better understand the performance impact of using various locking techniques, as described in the upcoming lessons.