Are Synchronized Methods Slower In Single Threaded Applications?

Question

I've been debating this to myself for the last few minutes, and I'm seeing reasons for both yes and no. This stemmed from looking at the answers to Java HashMap vs. Hashtable and seeing several people say Hashtable is in fact slower.

It seems to me that a synchronized method should act absolutely no different than its unsynchronized counterpart if running in a single thread, since the action of synchronizing shouldn't block anything. That said, I would imagine the compiler handles the two cases differently and that's why people are saying synchronized is slower.

Not that it's by any means conclusive, but I ran some simple tests on HashMap vs Hashtable, and saw little difference in speed.

Answer 1

Yes, single-theaded Java programs that use synchronization may be slightly slower than they would be without synchronization. For early Java releases, synchronization was expensive. For any modern release, however, uncontended synchronization is pretty cheap. I wouldn't worry about this.

Note that Java 6 has and Java 7 is to have good optimizations around locking:

• Lock coarsening
• Lock elision
• Adaptive Spin locking
• Biased locking

For more information, see the Java SE 6 Performance White Paper. Also note that uncontended synchronization appears to be more expensive on multi-core CPUs than on single-core CPUs, perhaps due to the Java Memory Model requirements of synchronization forcing local CPU caches to be shared with other CPUs, or some other memory barrier. For example, read Do Java 6 threading optimizations actually work? - Part II. (The Part I was not as insightful as the Part II.)

Answer 2

Yes. They are going to be slightly slower due to the extra overhead of maintaining locks.