Lock computer science: Encyclopedia II - Lock computer science - Granularity

Lock computer science - Granularity

An important property of a lock is its granularity. The granularity is a measure of the amount of data the lock is protecting. In general, choosing a coarse granularity (a small number of locks, each protecting a large segment of data) results in less overhead when a single process is accessing the protected data, but worse performance when multiple processes are running concurrently. This is because of increased lock contention: the more coarse the lock, the higher the likelihood that the lock will stop an unrelated process from proceding. Conversely, using a fine granularity (a larger number of locks, each protecting a fairly small amount of data) increases the overhead of the locks themselves but reduces lock contention. More locks also increase the risk of deadlock.

In a database management system, for example, a lock could protect, in order of increasing granularity, a record, a data page, or an entire table. Coarse granularity, such as using table locks, tends to give the best performance for a single user, whereas fine granularity, such as record locks, tends to give the best performance for multiple users.

One strategy is to avoid locks entirely by using lock-free programming techniques.

Adapted from the Wikipedia article "Granularity", under the G.N U Free Docmentation License. Please also see http://en.wikipedia.org/wiki