Memory overhead of maps in Go

Question

map[byte]byte{0:10} should be using least 2 bytes, one for value and one per key. But as each hashmap implmentation, there is also a hidden cost per item. What is the memory

Answer 1

Here's a cross-platform reimplementation of Nick's program. It includes changes where I think it was flawed. It also adds more measured data points.

Note: To allow for a wider "entries" range, the measured map bellow is map[int16]byte.

package main

import (
        "fmt"
        "runtime"
        "unsafe"
)

func Alloc() uint64 {
        var stats runtime.MemStats
        runtime.GC()
        runtime.ReadMemStats(&stats)
        return stats.Alloc - uint64(unsafe.Sizeof(hs[0]))*uint64(cap(hs))
}

var hs = []*map[int16]byte{}

func main() {
        hs := []*map[int16]byte{}
        n := 1000
        before := Alloc()
        for i := 0; i < n; i++ {
                h := map[int16]byte{}
                hs = append(hs, &h)
        }
        after := Alloc()
        emptyPerMap := float64(after-before) / float64(n)
        fmt.Printf("Bytes used for %d empty maps: %d, bytes/map %.1f\n", n, after-before, emptyPerMap)
        hs = nil

        k := 1
        for p := 1; p < 16; p++ {
                before = Alloc()
                for i := 0; i < n; i++ {
                        h := map[int16]byte{}
                        for j := 0; j < k; j++ {
                                h[int16(j)] = byte(j)
                        }
                        hs = append(hs, &h)
                }
                after = Alloc()
                fullPerMap := float64(after-before) / float64(n)
                fmt.Printf("Bytes used for %d maps with %d entries: %d, bytes/map %.1f\n", n, k, after-before, fullPerMap)
                fmt.Printf("Bytes per entry %.1f\n", (fullPerMap-emptyPerMap)/float64(k))
                k *= 2
        }

}

---

Output

jnml@fsc-r630:~/src/tmp$ go build && ./tmp && go version && uname -a
Bytes used for 1000 empty maps: 146816, bytes/map 146.8
Bytes used for 1000 maps with 1 entries: 147040, bytes/map 147.0
Bytes per entry 0.2
Bytes used for 1000 maps with 2 entries: 147040, bytes/map 147.0
Bytes per entry 0.1
Bytes used for 1000 maps with 4 entries: 247136, bytes/map 247.1
Bytes per entry 25.1
Bytes used for 1000 maps with 8 entries: 439056, bytes/map 439.1
Bytes per entry 36.5
Bytes used for 1000 maps with 16 entries: 818688, bytes/map 818.7
Bytes per entry 42.0
Bytes used for 1000 maps with 32 entries: 1194688, bytes/map 1194.7
Bytes per entry 32.7
Bytes used for 1000 maps with 64 entries: 2102976, bytes/map 2103.0
Bytes per entry 30.6
Bytes used for 1000 maps with 128 entries: 4155072, bytes/map 4155.1
Bytes per entry 31.3
Bytes used for 1000 maps with 256 entries: 6698688, bytes/map 6698.7
Bytes per entry 25.6
Bytes used for 1000 maps with 512 entries: 14142976, bytes/map 14143.0
Bytes per entry 27.3
Bytes used for 1000 maps with 1024 entries: 51349184, bytes/map 51349.2
Bytes per entry 50.0
Bytes used for 1000 maps with 2048 entries: 102467264, bytes/map 102467.3
Bytes per entry 50.0
Bytes used for 1000 maps with 4096 entries: 157214816, bytes/map 157214.8
Bytes per entry 38.3
Bytes used for 1000 maps with 8192 entries: 407031200, bytes/map 407031.2
Bytes per entry 49.7
Bytes used for 1000 maps with 16384 entries: 782616864, bytes/map 782616.9
Bytes per entry 47.8
go version devel +83b0b94af636 Sat Mar 09 16:25:30 2013 +1100 linux/amd64
Linux fsc-r630 3.2.0-38-generic #61-Ubuntu SMP Tue Feb 19 12:18:21 UTC 2013 x86_64 x86_64 x86_64 GNU/Linux
jnml@fsc-r630:~/src/tmp$ 

It's nice to see the numbers are better (by a factor of about 4x). The numbers for the release version (1.0.3) are only slightly higher:

jnml@fsc-r630:~/src/tmp$ go build && ./tmp
Bytes used for 1000 empty maps: 144192, bytes/map 144.2
Bytes used for 1000 maps with 1 entries: 144192, bytes/map 144.2
Bytes per entry 0.0
Bytes used for 1000 maps with 2 entries: 144192, bytes/map 144.2
Bytes per entry 0.0
Bytes used for 1000 maps with 4 entries: 315648, bytes/map 315.6
Bytes per entry 42.9
Bytes used for 1000 maps with 8 entries: 436288, bytes/map 436.3
Bytes per entry 36.5
Bytes used for 1000 maps with 16 entries: 885824, bytes/map 885.8
Bytes per entry 46.4
Bytes used for 1000 maps with 32 entries: 1331264, bytes/map 1331.3
Bytes per entry 37.1
Bytes used for 1000 maps with 64 entries: 2292800, bytes/map 2292.8
Bytes per entry 33.6
Bytes used for 1000 maps with 128 entries: 4935920, bytes/map 4935.9
Bytes per entry 37.4
Bytes used for 1000 maps with 256 entries: 12164160, bytes/map 12164.2
Bytes per entry 47.0
Bytes used for 1000 maps with 512 entries: 29887808, bytes/map 29887.8
Bytes per entry 58.1
Bytes used for 1000 maps with 1024 entries: 56840768, bytes/map 56840.8
Bytes per entry 55.4
Bytes used for 1000 maps with 2048 entries: 108736064, bytes/map 108736.1
Bytes per entry 53.0
Bytes used for 1000 maps with 4096 entries: 184368752, bytes/map 184368.8
Bytes per entry 45.0
Bytes used for 1000 maps with 8192 entries: 431340576, bytes/map 431340.6
Bytes per entry 52.6
Bytes used for 1000 maps with 16384 entries: 815378816, bytes/map 815378.8
Bytes per entry 49.8
jnml@fsc-r630:~/src/tmp$