BEAM Wisdoms — Virtual Machine Knowledge Base Help

Efficiency ELI5

Data copying

Here are general rules regarding data copying and performance.

  • Data is stored in process heap most of the time.

  • As long as data is moved inside the owning process, it is not copied, only a pointer is passed.

  • If the data ever leaves the process, a message for example, it will be copied to its new owner.

  • If the data is moved to or from ETS, it behaves similarly, it will be copied (ETS has own heap separate from any process).

  • If the data is moved between a process and a port, it will be copied (ports behave similar to processes, and own resources in a similar way).

  • Large binaries >64 bytes are never copied, instead they are reference counted and reference is shared between processes. This is why you want to GC periodically all processes which have touched a large binary, otherwise it may take a long time before the binary is freed.

Literals (Constants)

All Erlang values in your module which are larger than a single machine word are called literals. Literals are saved in the module file and then loaded when your module loads. These values are stored in special memory area called constant pool and later when you reload your code the garbage collector will move old constants to your process heap (see note below, this has changed in OTP 20.0).

Accessing a literal value of any size is cheap. You can use this trick to define large constant structures, lookup tuples with cheap element access, store large binaries in code etc.

Last modified: 10 June 2024