How Much Memory Does a Go Map Initially Reserve?

How Much Memory Do Go Maps Reserve?

In Go, maps are allocated dynamically, with the initial capacity left unspecified. This means that the memory allocation is implementation-specific. To determine the initial memory allocation, consult the map's internal structure.

Map Structure

A map consists of a header (type hmap) and an array of buckets (type bmap). When created without an initial capacity, a single bucket is allocated.

Header Structure and Size

The header comprises:

• 1 * int
• 2 * uint8
• 1 * uint16
• 1 * uint32
• 2 * unsafe.Pointer
• 1 * uintptr

The size of int, uintptr, and unsafe.Pointer is equal to the word size (8 bytes on 64-bit machines). Therefore, the header size is 40 bytes.

Bucket Structure and Size

Each bucket contains an array of 8 * uint8. Adding this to the header size, we get the total initial memory allocation:

Header Size (40 bytes) + Bucket Size (8 bytes) = **48 bytes (64-bit architecture)**

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