为什么迭代地图比在Golang中迭代切片要慢得多？

Question

I was implementing a sparse matrix using a map in Golang and I noticed that my code started taking much longer to complete after this change, after dismissing other possible causes, seems that the culprit is the iteration on the map itself. Go Playground link (doesn't work for some reason).

package main

import (
    "fmt"
    "time"
    "math"
)

func main() {
    z := 50000000
    a := make(map[int]int, z)
    b := make([]int, z)

    for i := 0; i < z; i++ {
        a[i] = i
        b[i] = i
    }

    t0 := time.Now()
    for key, value := range a {
        if key != value { // never happens
            fmt.Println("a", key, value)
        }
    }
    d0 := time.Now().Sub(t0)

    t1 := time.Now()
    for key, value := range b {
        if key != value { // never happens
            fmt.Println("b", key, value)
        }
    }
    d1 := time.Now().Sub(t1)

    fmt.Println(
        "a:", d0,
        "b:", d1,
        "diff:", math.Max(float64(d0), float64(d1)) / math.Min(float64(d0), float64(d1)),
    )
}

Iterating over 50M items returns the following timings:

alix@local:~/Go/src$ go version
go version go1.3.3 linux/amd64
alix@local:~/Go/src$ go run b.go 
a: 1.195424429s b: 68.588488ms diff: 17.777154632611037

I wonder, why is iterating over a map almost 20x as slow when compared to a slice?

Answer 1

This comes down to the representation in memory. How familiar are you with the representation of different data structures and the concept of algorithmic complexity? Iterating over an array or slice is simple. Values are contiguous in memory. However iterating over a map requires traversing the key space and doing lookups into the hash-table structure.

The dynamic ability of maps to insert keys of any value without using up tons of space allocating a sparse array, and the fact that look-ups can be done efficiently over the key space despite being not as fast as an array, are why hash tables are sometimes preferred over an array, although arrays (and slices) have a faster "constant" (O(1)) lookup time given an index.

It all comes down to whether you need the features of this or that data structure and whether you're willing to deal with the side-effects or gotchas involved.

Answer 2

Seems reasonable to put my comment as an answer. The underlying structures who's iteration performance you're comparing are a hash table and an array ( https://en.wikipedia.org/wiki/Hash_table vs https://en.wikipedia.org/wiki/Array_data_structure ). The range abstraction is actually (speculation, can't find the code) iterating all the keys, accessing each value, and assigning the two to k,v := . If you're not familiar with accessing in the array it is constant time because you just add sizeof(type)*i to the starting pointer to get the item. I don't know what the internals of map are in golang but I know enough to know that it's memory representation and therefor access is nothing close that efficient.

The specs statement on the topic isn't much; http://golang.org/ref/spec#For_statements

If I find the time to look up the implementation of range for map and slice/array I will and put some more technical details.