CODEWITHSUNDEEP
parallel-processingjulia
AdM
AdM

Reputation: 83

Multi-threading issue in Julia

I'm trying parallelise some bits of a code but I do not understand why the following functions main1() and main2() give different results using Julia's multi-threading:

a = rand(4,4);b = rand(4,4);c = rand(4,4);d = rand(4,4)

function main1(a,b,c,d)
    L = zeros(2,2,16)
    FF = zeros(2,2,16)
    FT = zeros(2,2,16)
    F = Array{Float32}(undef,2,2)
    # L = Array{Array{Float32, 1}, 4}

    for i = 1:4
        for j = 1:4
            ic = i + j*(i-1)
            F[1,1] = a[i,j]
            F[1,2] = b[i,j]
            F[2,1] = c[i,j]
            F[2,2] = d[i,j]
            L[:,:,ic] .= F * F'
            FF[:,:,ic] .= F
            FT[:,:,ic] .= F'
        end
    end
    return L,FF,FT
end

function main2(a,b,c,d)

    L  = zeros(2,2,16)
    FF = zeros(2,2,16)
    FT = zeros(2,2,16)
    F  = Array{Float32}(undef,2,2)
    # L = Array{Array{Float32, 1}, 4}

    Threads.@threads for i = 1:4
        Threads.@threads for j = 1:4
            ic = i + j*(i-1)
            F[1,1] = a[i,j]
            F[1,2] = b[i,j]
            F[2,1] = c[i,j]
            F[2,2] = d[i,j]
            L[:,:,ic] .= F * F'
            FF[:,:,ic] .= F
            FT[:,:,ic] .= F'
        end
    end
    return L,FF,FT
end

How could the parallelisation of main1() be properly fixed?

Upvotes: 2

Views: 174

Answers (1)

Przemyslaw Szufel
Przemyslaw Szufel

Reputation: 42194

You cannot nest @threads loops so normally you should do:

Threads.@threads for u in vec(CartesianIndices((4,4)))
    i,j = u.I
    # your code goes here
end

However, in your code you get the same ic value for different pair of values of (i,j). In the main1 you are overwriting the same parts of L, FF, FT many times which is an obvious bug. Multi-threading will change the order the data is overwritten so it will yields different results. In conclusion, first fix main1 and than parallelize it.

Upvotes: 1

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