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

Computing the Memory Footprint of a Map in Go

Determining the memory footprint of a map in Go can pose a challenge. The encoding/binary package's Size function, useful for calculating the length of slices or fixed values, does not extend to maps.

To address this issue, we can delve into the internal structure of a Go map. The map header (hmap) contains information such as the number of live cells and the log base 2 of the number of buckets.

Each bucket (bmap) holds a constant number of top hash values, followed by keys and values. This organization eliminates padding that would be required for certain key-value combinations.

Calculating the map's memory footprint involves summing the size of the header, the size of the buckets, and the size of the keys and values stored in the map:

memory_footprint = unsafe.Sizeof(hmap) + (len(theMap) * 8) + (len(theMap) * 8 * unsafe.Sizeof(x)) + (len(theMap) * 8 * unsafe.Sizeof(y))

Where:

• theMap is the map value
• x is a value of the map's key type
• y is a value of the map's value type

Note that sharing the hmap structure with your package requires the use of assembly code, similar to the thunk.s approach used in the Go runtime.

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