This question is about the speed of accessing elements of arrays and slices, not about the efficiency of passing them to functions as arguments.
I would expect arrays to be faster than slices in most cases because a slice is a data structure describing a contiguous section of an array and so there may be an extra step involved when accessing elements of a slice (indirectly the elements of its underlying array).
So I wrote a little test to benchmark a batch of simple operations. There are 4 benchmark functions, the first 2 test a global slice and a global array, the other 2 test a local slice and a local array:
var gs = make([]byte, 1000) // Global slice
var ga [1000]byte // Global array
func BenchmarkSliceGlobal(b *testing.B) {
for i := 0; i < b.N; i++ {
for j, v := range gs {
gs[j]++; gs[j] = gs[j] + v + 10; gs[j] += v
}
}
}
func BenchmarkArrayGlobal(b *testing.B) {
for i := 0; i < b.N; i++ {
for j, v := range ga {
ga[j]++; ga[j] = ga[j] + v + 10; ga[j] += v
}
}
}
func BenchmarkSliceLocal(b *testing.B) {
var s = make([]byte, 1000)
for i := 0; i < b.N; i++ {
for j, v := range s {
s[j]++; s[j] = s[j] + v + 10; s[j] += v
}
}
}
func BenchmarkArrayLocal(b *testing.B) {
var a [1000]byte
for i := 0; i < b.N; i++ {
for j, v := range a {
a[j]++; a[j] = a[j] + v + 10; a[j] += v
}
}
}
I ran the test multiple times, here is the typical output (go test -bench .*):
BenchmarkSliceGlobal 300000 4210 ns/op
BenchmarkArrayGlobal 300000 4123 ns/op
BenchmarkSliceLocal 500000 3090 ns/op
BenchmarkArrayLocal 500000 3768 ns/op
Analyzing the results:
Accessing the global slice is slightly slower than accessing the global array which is as I expected:
4210 vs 4123 ns/op
But accessing the local slice is significantly faster than accessing the local array:
3090 vs 3768 ns/op
My question is: What is the reason for this?
Notes
I tried varying the following things but none changed the outcome:
- the size of the array/slice (tried 100, 1000, 10000)
- the order of the benchmark functions
- the element type of the array/slice (tried
byteandint)