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

Measuring the Memory Footprint of a Go Map

When working with large maps in Go, it's crucial to manage memory consumption effectively. However, there is no built-in method for directly computing the byte length of a map.

The "encoding/binary" package provides a Size function for slices and fixed values, but it's not applicable to maps. Manually extracting key-value pairs and calculating their length would be tedious and imprecise, as it would exclude the internal Go overhead.

To overcome this limitation, we can delve into the underlying structure of a map. A map in Go consists of two main components: a header and an array of buckets.

Map Header Structure

type hmap struct {
    count int // # live cells == size of map
    flags uint32
    hash0 uint32 // hash seed
    B     uint8  // log_2 of # of buckets
}

The size of the header is simply the size of the hmap struct: unsafe.Sizeof(hmap).

Bucket Structure

Each bucket holds a specific number of key-value pairs. The number of buckets is determined by the B field in the header.

type bmap struct {
    tophash [bucketCnt]uint8
    // Followed by bucketCnt keys and then bucketCnt values.
}

The constant bucketCnt defines the number of key-value pairs per bucket:

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

Calculating Map Memory Footprint

Knowing the sizes of the header and buckets, we can calculate the memory footprint of a map as follows:

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

• unsafe.Sizeof(hmap): Size of the map header
• (len(theMap) * 8): Total size of keys, assuming they are strings (8 bytes each)
• (len(theMap) * 8 * unsafe.Sizeof(x)): Total size of values, assuming they are of type x
• (len(theMap) * 8 * unsafe.Sizeof(y)): Total size of overflow buckets, if any (assuming overflow buckets are also of type y)

This formula provides an accurate estimate of the memory consumed by a map, taking into account both the header overhead and the contents of the map.

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