Specify container capacity where possible in order to allocate memory for the container up front. This minimizes subsequent allocations (by copying and resizing of the container) as elements are added.
Where possible, provide capacity hints when initializing
maps with make().
make(map[T1]T2, hint)Providing a capacity hint to make() tries to right-size the
map at initialization time, which reduces the need for growing
the map and allocations as elements are added to the map.
Note that, unlike slices, map capacity hints do not guarantee complete, preemptive allocation, but are used to approximate the number of hashmap buckets required. Consequently, allocations may still occur when adding elements to the map, even up to the specified capacity.
| Bad | Good |
|---|---|
m := make(map[string]os.FileInfo)
files, _ := os.ReadDir("./files")
for _, f := range files {
m[f.Name()] = f
} |
files, _ := os.ReadDir("./files")
m := make(map[string]os.DirEntry, len(files))
for _, f := range files {
m[f.Name()] = f
} |
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Where possible, provide capacity hints when initializing slices with make(),
particularly when appending.
make([]T, length, capacity)Unlike maps, slice capacity is not a hint: the compiler will allocate enough
memory for the capacity of the slice as provided to make(), which means that
subsequent append() operations will incur zero allocations (until the length
of the slice matches the capacity, after which any appends will require a resize
to hold additional elements).
| Bad | Good |
|---|---|
for n := 0; n < b.N; n++ {
data := make([]int, 0)
for k := 0; k < size; k++{
data = append(data, k)
}
} |
for n := 0; n < b.N; n++ {
data := make([]int, 0, size)
for k := 0; k < size; k++{
data = append(data, k)
}
} |
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