# What is extra thread for idle time

Here http://tomstechnicalblog.blogspot.com/2016/02/rxjava-understanding-observeon-and.html I've found a sentence which I do not understand:

If you have a quad-core, you can optimally support 5 computational threads (4 cores + 1 extra thread for idle time).

What is that 1 extra thread for idle time for? Is it similar to those "normal" 4 cores?

• The idea is to always have a thread waiting to use a CPU should one become free (because a thread blocks). This is just a crude rule of thumb, it will obviously depend on the ratio of running time/total time of the threads. – vonbrand Feb 15 '16 at 18:13