locking effects in Java

Question

does anybody know the effects of locking in Java specifically on the speed of execution of the program? if any, how do we overcome this effect? concrete references are appreciated..

Answer 1

If your application is mainly single threaded you may see no or very little performance degradation when doing too much locking.

In general a single lock can be grabbed very fast and efficiently when using the java.concurrent libraries or the build-in synchronized keyword. They use techniques like Lock elision, adaptive locking and lock coarsening (see Synchronization optimizations in Mustang), which basically means the compiler and/or library do some very smart things to optimize lock usage.

This especially plays out in situations where the lock wouldn't really be needed; the compiler optimizes it away.

However, when a lot of threads need to grab the same locks you'll eventually notice that the CPU load of your cores will never reach 100% for a pure computational problem. In effect, something is not going as fast as it possibly could, and your CPU is just sitting idle.

In absence of (heavy) IO this is often a sign of excessive lock contention and this thus indeed degrades overall system performance.