How Much Memory Does an Unspecified Go Map Allocate?

Memory Allocation in Golang Maps: Unveiling the Underlying Structures

When creating a Go map without specifying the initial memory allocation, the actual memory usage remains a mystery. This article sheds light on how to determine the memory allocated by the implementation.

Insights from the Go Map Implementation

Delving into the source code of the Go map type, we discover that a map comprises two components: a header (hmap) and an array of buckets (bmap). Remarkably, a single bucket is allocated if the initial allocation is unspecified during map creation.

Structure of a Header (hmap)

The header consists of seven fields:

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

On 64-bit machines, the size of int, uintptr, and unsafe.Pointer is 8 bytes each, giving a total of 40 bytes for the header.

Structure of a Bucket (bmap)

A bucket is an array with eight elements of type uint8, resulting in an additional 8 bytes.

Final Memory Allocation

Thus, the memory allocated for a map with one bucket consists of:

• Header size: 40 bytes
• Bucket size: 8 bytes
• Total size: 48 bytes

This calculation is applicable to 64-bit architectures, and the actual size may vary slightly on different platforms.

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