How Can I Efficiently Calculate the Memory Footprint of a Go Map?

Calculating Memory Footprint of a Go Map

Objective: Determine the byte length of a Go map efficiently without relying on external functions or cumbersome calculations.

Solution:

Map Header Size
The first step involves calculating the size of the map header (hmap). Based on the Go documentation, its structure includes:

• count (int)
• flags (uint32)
• hash0 (uint32)
• B (uint8)

The size of this header can be obtained using unsafe.Sizeof(hmap):

Bucket Size
Each bucket in the map consists of the following elements:

• tophash ([bucketCnt]uint8)
• keys (bucketCnt elements)
• values (bucketCnt elements)
• overflow pointer

The bucketCnt is defined as 8:

bucketCnt     = 1 << bucketCntBits // equals decimal 8
bucketCntBits = 3

Total Size Calculation
Finally, the total memory footprint of the map is calculated as:

• theMap: The map in question
• x: A value of the map's key type
• y: A value of the map's value type

Implementation:
Accessing the hmap structure requires sharing it with the package via assembly, similar to thunk.s in the runtime.

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