What is meant by “fast-path” uncontended synchronization?

Question

From the Performance and Scalability chapter of the JCIP book:

The synchronized mechanism is optimized for the uncontended case(volatile is alw

Answer 1

There are two distinct concepts here.

1. Fast-path and Slow-path code
2. Uncontended and Contended synchronization

Slow-path vs Fast-path code

This is another way to identify the producer of the machine specific binary code.

With HotSpot VM, slow-path code is binary code produced by a C++ implementation, where fast-path code means code produced by JIT compiler.

In general sense, fast-path code is a lot more optimised. To fully understand JIT compilers wikipedia is a good place to start.

Uncontended and Contended synchronization

Java's synchronization construct (Monitors) have the concept of ownership. When a thread tries to lock (gain the ownership of) the monitor, it can either be locked (owned by another thread) or unlocked.

Uncontended synchronization happens in two different scenarios:

1. Unlocked Monitor (ownership gained strait away)
2. Monitor already owned by the same thread

Contended synchronization, on the other hand, means the thread will be blocked until the owner thread release the monitor lock.

Answering the question

By fast-path uncontended synchronization the author means, the fastest bytecode translation (fast-path) in the cheapest scenario (uncontended synchronization).