从部件构建 Vector{UInt8} 时减少内存分配

Question

我希望从不同的部分构建一个 Vector{UInt8}，如下所示：

using BenchmarkTools
using Random

const a = Vector{UInt8}("Number 1: ")
const b = Vector{UInt8}(", Number 2: ")
const c = Vector{UInt8}(", Number 3: ")
const d = Vector{UInt8}(", Number 4: ")

function main(num2, num4)::Vector{UInt8}

    return vcat(
        a,
        Vector{UInt8}(string(rand(1:100))),
        b,
        Vector{UInt8}(string(num2)),
        c,
        Vector{UInt8}(string(rand(1:100))),
        d,
        Vector{UInt8}(string(num4)),
    )

end

@btime main(70.45, 12) # 486.224 ns (13 allocations: 1.22 KiB)
#Example output: "Number 1: 50, Number 2: 70.45, Number 3: 10, Number 4: 12"

先转字符串再转Vector{UInt8}好像不对。我不介意加入向量时发生的 1 分配。

Answer 1

将整数转换为 UInt8 格式的数字向量可以非常高效地完成。转换浮点数有点棘手。

总而言之，我认为你的代码已经相当高效了。这是加速整数代码的建议。浮点数代码，我没能改进：

function tobytes(x::Integer)
    N = ndigits(x)
    out = Vector{UInt8}(undef, N)
    for i in N:-1:1
        (x, r) = divrem(x, 0x0a)
        out[i] = UInt8(r) + 0x30
    end
    return out
end

tobytes(x) = Vector{UInt8}(string(x))

# notice that we generate random UInt8 values instead of rand(1:100), as this is faster. They still have to be converted according to the character interpretation, though.
function main2(num2, num4)
    [a; tobytes(rand(0x01:0x64)); b; tobytes(num2); c; tobytes(rand(0x01:0x64)); d; tobytes(num4)]
end

tobytes for intergers 现在接近最优，运行时间主要由预分配 Vector{UInt8}.

的时间决定

从部件构建 Vector{UInt8} 时减少内存分配

Reducing memory allocations when building Vector{UInt8} from parts

string

byte

allocation

low-latency

julia