The Go Programming Language Specification
Appending to and copying slices
The variadic function append appends zero or more values x to s of
type S, which must be a slice type, and returns the resulting slice,
also of type S.
append(s S, x ...T) S // T is the element type of S
If the capacity of s is not large enough to fit the additional values,
append allocates a new, sufficiently large underlying array that fits
both the existing slice elements and the additional values. Otherwise,
append re-uses the underlying array.
针对您的示例,平均每个操作分配了 [40, 41) 字节用于在需要时增加切片的容量。使用摊销常数时间算法来增加容量:如果长度为 1024 或以下,则将其增加到原来的两倍,否则增加到原来的 1.25 倍。平均每个操作有 [0, 1) 次分配。
例如,
func BenchmarkMem(b *testing.B) {
b.ReportAllocs()
var x []int64
var a, ac int64
b.ResetTimer()
for i := 0; i < b.N; i++ {
c := cap(x)
x = append(x, int64(i))
if cap(x) != c {
a++
ac += int64(cap(x))
}
}
b.StopTimer()
sizeInt64 := int64(8)
B := ac * sizeInt64
b.Log("op", b.N, "B", B, "alloc", a, "lx", len(x), "cx", cap(x))
}
输出:
BenchmarkMem-4 50000000 26.6 ns/op 40 B/op 0 allocs/op
--- BENCH: BenchmarkMem-4
bench_test.go:32: op 1 B 8 alloc 1 lx 1 cx 1
bench_test.go:32: op 100 B 2040 alloc 8 lx 100 cx 128
bench_test.go:32: op 10000 B 386296 alloc 20 lx 10000 cx 12288
bench_test.go:32: op 1000000 B 45188344 alloc 40 lx 1000000 cx 1136640
bench_test.go:32: op 50000000 B 2021098744 alloc 57 lx 50000000 cx 50539520
对于 op = 50000000
,
B/op = floor(2021098744 / 50000000) = floor(40.421974888) = 40
allocs/op = floor(57 / 50000000) = floor(0.00000114) = 0
阅读:
Go Slices:用法和内部原理
数组、切片(和字符串):“append”的机制
'append'复杂度
为了在追加时拥有零B/op(和零allocs/op),请在追加之前使用足够容量的切片进行分配。
例如,使用var x = make([]int64, 0, b.N)
,
func BenchmarkZero(b *testing.B) {
b.ReportAllocs()
var x = make([]int64, 0, b.N)
var a, ac int64
b.ResetTimer()
for i := 0; i < b.N; i++ {
c := cap(x)
x = append(x, int64(i))
if cap(x) != c {
a++
ac += int64(cap(x))
}
}
b.StopTimer()
sizeInt64 := int64(8)
B := ac * sizeInt64
b.Log("op", b.N, "B", B, "alloc", a, "lx", len(x), "cx", cap(x))
}
输出:
BenchmarkZero-4 100000000 11.7 ns/op 0 B/op 0 allocs/op
--- BENCH: BenchmarkZero-4
bench_test.go:51: op 1 B 0 alloc 0 lx 1 cx 1
bench_test.go:51: op 100 B 0 alloc 0 lx 100 cx 100
bench_test.go:51: op 10000 B 0 alloc 0 lx 10000 cx 10000
bench_test.go:51: op 1000000 B 0 alloc 0 lx 1000000 cx 1000000
bench_test.go:51: op 100000000 B 0 alloc 0 lx 100000000 cx 100000000
请注意,基准CPU时间从约26.6 ns/op降至约11.7 ns/op。