阵列大小对性能的影响

Question

I just started learning Go and I found myself creating a simple program that:

• Creates an int array of size SIZE (expected to be >= 1000)
• Iterates through its elements from 0 to 999, setting them to 0
• Prints how long everything took

Something like this:

package main

import (
    "time"
    "fmt"
)

const SIZE = 1000

func main() {
    start := time.Now()
    a := [SIZE]int {}
    for i := 0; i < 1000; i++ { a[i] = 0 }
    fmt.Println("Time: ", time.Since(start))
}

I got the following results after running on my machine 5 times:

• 3.375µs
• 2.831µs
• 2.698µs
• 2.655µs
• 2.59µs

However, if I increase SIZE to 100000 (100x) the program becomes slower. These are the observed results on the same machine:

• 407.844µs
• 432.607µs
• 397.67µs
• 465.959µs
• 445.101µs

Why is the value of SIZE making such a big difference? The number of iterations will always be the same (1000)...

Answer 1

In one sentence

Because you are also counting the time to allocate the slice

Explanation

In your code, you are creating the start variable just before your big slice a.

func main() {
    start := time.Now()
    a := [SIZE]int {}
    //...
}

By doing this, you are ignoring the fact that allocating a slice with 1000 items is much faster than allocating a slice with 100000 items.

You have to move this initialization after the allocation. The size of the slice should not affect the time to loop through it up to a certain number of (equal) iterations.

In my benchmarks:

| Items  | ns/op | Count slice allocation? |
|--------|-------|-------------------------|
| 1000   | 1411  | YES                     |
| 100000 | 66565 | YES                     |
| 1000   | 837   | NO                      |
| 100000 | 855   | NO                      |