Fork me on GitHub

Constant-pool erlang hack


Erlang R12B-0 added a per-module memory area for constants, the literal values in a module are stored there. Before, they were allocated on the heap every time they were referenced. This meant that some kinds of optimization that avoided literal lookup-tables became irrelevant in one go (without even recompiling the source). A great example of the kind of improvements that OTP focus on: removing speed-bumps to having beautiful code.

From the release notes of R12B-0

OTP-6850: Literal lists, tuples, and binaries are no longer constructed at run-time as they used to be, but are stored in a per-module constant pool. Literals that are used more than once are stored only once.

This is not a change to the language, only in the details of its implementation. Therefore, the implications of this change is described in the Efficiency Guide.

The erlang efficiency guide on constant pools pretty much say the same thing. But Björn Gustavsson adds this very interesting remark about unloading a module and the constant pool.

The hack

If one has very assymetric access patterns to some value, Maybe millions of times more reads than updates, and this is a measured problem, one can reach for hacks such as generating a module containing the values as literals and thus have a global configuration value that will not grow your heap unecessary.


As always, remember when to optimize.