slice[cap=0]: allocs: 0, 0 bytes
adding 10000 elements cost: allocs: 20, 357648 bytes
slice[cap=5000]: allocs: 1, 40960 bytes
adding 10000 elements cost: allocs: 4, 229392 bytes
slice[cap=10000]: allocs: 1, 81920 bytes
adding 10000 elements cost: allocs: 0, 0 bytes
Adding 10,000 elements to a slice with an initial capacity of 0 will become 357648 bytes. Of which ~88% (~19 other slice allocations) would be unused slices that would need to be GC'd.
| package main | |
| import ( | |
| "fmt" | |
| "runtime" | |
| ) | |
| const N = 10_000 | |
| func diff(fn func()) (int64, uint64) { | |
| runtime.GC() | |
| var before, after runtime.MemStats | |
| runtime.ReadMemStats(&before) | |
| fn() | |
| runtime.ReadMemStats(&after) | |
| return int64(after.Mallocs - before.Mallocs), after.TotalAlloc - before.TotalAlloc | |
| } | |
| func appendN(s []int, n int) []int { | |
| for i := range n { | |
| s = append(s, i) | |
| } | |
| return s | |
| } | |
| func main() { | |
| caps := []int{0, N / 2, N} | |
| for _, c := range caps { | |
| var slice []int | |
| a, m := diff(func() { | |
| slice = make([]int, 0, c) | |
| }) | |
| fmt.Printf("slice[cap=%d]: allocs: %d, %d bytes\n", cap(slice), a, m) | |
| a, m = diff(func() { | |
| _ = appendN(slice, N) | |
| }) | |
| fmt.Printf(" adding %d elements cost: allocs: %d, %d bytes\n", N, a, m) | |
| } | |
| } |