Medusa Pattern

Last time, we saw the Star Trek Anti-Pattern, where the number of threads would go out of hands, to infinity and beyond.The easy solution to that is to limit the number of threads.

That is what I call the Medusa Pattern, from the name of that famous Gorgona that would freeze people by just looking at them. We are going to freeze the number of threads in a thread pool:

Executors.newFixedThreadPool(n);

Now comes the big question: what should be ‘n’? How many threads should I have in my pool? The usual accepted answer is: as many threads as the number of cores, plus one because one thread is often in waiting. In his book “Concurrency in Practice”, Brian Goetz answers this question with this formula:

t = c * (1 + w / s)

Where t is the number of threads, c is the number of cores in your machine, w is the waiting time, that is the average time your threads spend waiting, and s is the service time, that is the time your threads spend in average doing some useful work. You can see that if your threads are very busy, with almost zero waiting time, there should be as many threads as the number of cores, and we come close to the usual accepted rule of thumb.


On the other hand, if your threads spend a lot of time waiting, the value of t can be quite high. For instance, I work in a project where we have a monitoring application that monitors over 300 servers. Since monitoring threads spend their time waiting for a ping, we decided to have as many threads as servers.


We’ve seen cases with one thread, an infinity of threads, a fixed number of threads, but there is a special case where you do not want any thread. We’ll cover this in the Yoda Pattern. See you next time.