How Erlang does scheduling

In this, I describe why Erlang is different from most other language runtimes. I also describe why it often forgoes throughput for lower latency.

TL;DR – Erlang is different from most other language runtimes in that it targets different values. This describes why it often seem to perform worse if you have few processes, but well if you have many.

From time to time the question of Erlang scheduling gets asked by different people. While this is an abridged version of the real thing, it can act as a way to describe how Erlang operates its processes. Do note that I am taking Erlang R15 as the base point here. If you are a reader from the future, things might have changed quite a lot—though it is usually fair to assume things only got better, in Erlang and other systems.

Toward the operating system, Erlang usually has a thread per core you have in the machine. Each of these threads runs what is known as a scheduler. This is to make sure all cores of the machine can potentially do work for the Erlang system. The cores may be bound to schedulers, through the +sbt flag, which means the schedulers will not “jump around” between cores. It only works on modern operating systems, so OSX can’t do it, naturally. It means that the Erlang system knows about processor layout and associated affinities which is important due to caches, migration times and so on. Often the +sbt flag can speed up your system. And at times by quite a lot…