Allocations in @threads loop

General Usage
1

With the following code:

function thread_test(x::Vector{Float64}, y::Vector{Float64}, z::Vector{Float64}, n::Int64)
    for j = 1:10000
        a::Vector{Int64} = rand(1:n, 128)
        b::Vector{Int64} = rand(1:n, 128)
        c::Vector{Int64} = rand(1:n, 128)
        Threads.@threads for i::Int64 = 1:128
            y[a[i]] += x[b[i]]*z[c[i]]
        end
    end
end

function no_thread_test(x::Vector{Float64}, y::Vector{Float64}, z::Vector{Float64}, n::Int64)
    for j = 1:10000
        a::Vector{Int64} = rand(1:n, 128)
        b::Vector{Int64} = rand(1:n, 128)
        c::Vector{Int64} = rand(1:n, 128)
        for i::Int64 = 1:128
            y[a[i]] += x[b[i]]*z[c[i]]
        end
    end
end

function test()
    n = 1000
    x = rand(n)
    y = rand(n)
    z = rand(n)
    thread_test(x,y,z,n)
    no_thread_test(x,y,z,n)
    @time thread_test(x,y,z,n)
    @time no_thread_test(x,y,z,n)
end

test()

I get the following output

0.177082 seconds (1.02 M allocations: 65.100 MiB, 23.14% gc time)
0.059489 seconds (30.00 k allocations: 33.417 MiB, 1.12% gc time)

Where are all these allocations coming from for the threaded case?

Additionally, I noticed that if I didn’t explicitly type i, allocations increased further:

0.344228 seconds (3.45 M allocations: 94.762 MiB)

Why is this?

2

https://github.com/JuliaLang/julia/issues/15276

3

Thanks for this. I’m trying to wrap my head around it now. Is this only applicable to explicitly typing i in my example, or is this the cause to all the allocations?

4

Everything