Know you contention ratios.

After blogging about how Micro-Benchmarks are often misleading and focused on proving a point, I went ahead and wrote a little micro-benchmark to curb my curiosity. What I wanted to know is this:

In the face of massive contention, how do the following compare to each other:

  • Synchronized Block
  • Unfair ReentrantLock
  • Fair ReentrantLock
  • An Atomic Object

To do this test I wrote 3 different implementations that all did that same thing:

  1. Increment a long (or AtomicLong)
  2. Thread.yield()
  3. return

The results are not at all surprising, an atomic structure comes out much faster than the others, while Fair Locks are by far the slowest. Now I already said that Micro Benchmarks are misleading, so why write this? The fact is that most systems have very low contention ratios. The interesting thing about a contention is that: the more threads trying to access a resource, the worse the bottle-neck becomes. A locked structure must always enter and exit the lock, even when there is no contention. An atomic structure will never have to enter a lock, but may have to spin a few times. If you are concerned about atomic starvation (spinning forever), you can put a limit on how many times your code is willing to spin, and then enter a lock (this is in fact how a Java monitor works).

In short: make sure you really know your contention ratio before writing loads of complex code. You may find that in a continuously high load system, an unfair lock (or synchronized block) is the best way to go.

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