Tell me about evacuation in Go maps before version 1.24: what is it for and when does it happen?

In Go versions before 1.24, an operation called evacuation related to redistribution of elements during map growth occurred when working with maps (map).

When a map was resized, elements were copied from the old bucket array to a new one. Evacuation is the process of moving elements from the old structure to the new one. During this process, read and write operations could work with both structures to ensure correctness and avoid data races.

Evacuation is necessary for:

• Ensuring safety during concurrent access to the map while it is being resized.
• Gradually transferring elements to avoid costly copying operations all at once.

It occurs when new elements are added, and the current map reaches a fill threshold, triggering the expansion process and copying elements to a new structure.

In Go 1.24 and above, the mechanism has been redesigned to improve performance and simplify internal implementation.