The number of available processors

In order to parallelize a function that has a well-defined product decomposition, such as the map function or one of its variants it is good that we should determine how many processors are available in a given system. This can be achieved on the JVM using the availableProcessors method of the Runtime class.

Java:

public class ProcessorCounter {
    public static void main(String[] args) {
        int processorsCount = Runtime.getRuntime().availableProcessors();
        System.out.println("The number of available processors is:" + Integer.toString(processorsCount));
    }
} 

Clojure:

(let [runtime (Runtime/getRuntime)
      processors-count (.availableProcessors runtime)]
  (println "The number of available processors is: " (str processors-count)))

Limits to parallelism:
There are two limits to parallelism:

• The number of available processors, which is determined by the available processors method.
• The overhead of threading, which states that sometimes the cost of launching parts of a computation into separate threads can outweigh the advantages of parallelism.

It doesn't make much sense to parallelize a computation if you don't have the available processors to take advantage of that. On the other hand, the overhead of threading is one reason why it isn't always good to use pmap in Clojure.