What exactly is "allocation" in Julia?

[giordano]

https://docs.julialang.org/en/v1.9-dev/manual/performance-tips/#Measure-performance-with-[@time](@ref)-and-pay-attention-to-memory-allocation

We should clarify that what @time reports is specifically heap allocations, which are typically needed for either mutable objects or for creating/growing variable-sized containers (such as Array or Dict, strings, or “type-unstable” objects whose type is only known at runtime). Allocating (or deallocating) such blocks of memory may require an expensive system call (e.g. via malloc in C), and they must be tracked for garbage collection. In contrast, immutable values like numbers (except bignums), tuples, and immutable structs can be stored much more cheaply, e.g. in stack or CPU-register memory, so one doesn’t typically worry about the performance cost of “allocating” them.

Unexpected memory allocation is almost always a sign of some problem with your code, usually a problem with type-stability or creating many small temporary arrays. Consequently, in addition to the allocation itself, it’s very likely that the code generated for your function is far from optimal. Take such indications seriously and follow the advice below.