Stream: helpdesk (published)

Topic: Parallelisation changes result (unexpectadly)


view this post on Zulip Timothy (Jul 06 2021 at 10:17):

Hello,

In the following code, I have tried parallelising the for (j, pair) loop, however although the results of one iteration should be independent of other iterations, whenever I add Threads.@threads the result I get changes :frown:

    tightnesses = Vector{Float64}(undef, length(𝐑))
    besttightness = Inf
    @simpleprog length(ixy) * length(𝐑) for (i, x, y)  ixy
        D₁ = copy(D)
        pairs = D[[x,y],:]
        pairs′ = [R * pairs for R  𝐑]
        for (j, pair)  (enumerate(pairs′) |> collect)
            proginc!()
            D₁[[x,y],:] = pair
            𝚡 = basisparameterise(D₁, 𝐱cent𝛟, 𝐱𝛟)
            tightnesses[j] = basistightness(𝚡, 𝐱; k, krepr)
        end
        if minimum(tightnesses) < besttightness
            besttightness = minimum(tightnesses)
            D[[x,y],:] = pairs′[argmin(tightnesses)]
        end
        𝐃[:,:,i+1] = copy(D)
    end

If anyone has any ideas what's happening and/or how I can get this to work, that would be much appreciated!

view this post on Zulip Timothy (Jul 06 2021 at 10:18):

For reference length(𝐑) is 360.

view this post on Zulip Kwaku Oskin (Jul 06 2021 at 10:40):

Many questions arise.

What is ixy? What is @simpleprog? What is proginc!? What is D? Is there any difference between D and 𝐃? What is basisparametrise? What is 𝐱cent𝛟? And so on....

Any of this things can potentially make loop iterations-dependent, and it is impossible to say which one just by looking at this code.

One thing, though - you make D1 as a copy of D, and at the same time you change D inside iteration (in if minimum... branch). So, it looks like iterations are not independent.

view this post on Zulip Timothy (Jul 06 2021 at 11:07):

Unfortunately I the full context is rather ... large. However, I'll attempt to give an idea of what's going on here:

Regarding your comment:

One thing, though - you make D1 as a copy of D, and at the same time you change D inside iteration (in if minimum... branch). So, it looks like iterations are not independent.

D is changed in the outer loop for (i, x, y), I'm talking about the inner loop for (j, pair).

To further clarify regarding that inner loop:

view this post on Zulip Timothy (Jul 06 2021 at 11:10):

Perhaps it could help if I make a "reduced" version?

besttightness = Inf
for x  xs, y  ys
    D₁ = copy(D)
    pairs = D[[x,y],:]
    pairs′ = makerotations(pairs)
    for (j, pair)  (enumerate(pairs′) |> collect)
        D₁[[x,y],:] = pair
        tightnesses[j] = somepurefunction(D₁)
    end
    if minimum(tightnesses) < besttightness
        besttightness = minimum(tightnesses)
        D[[x,y],:] = pairs′[argmin(tightnesses)]
    end
end

view this post on Zulip Timothy (Jul 06 2021 at 11:12):

I also tried adding D₁ = copy(D) before D₁[[x,y],:] = pair at some point, but that didn't improve things.

view this post on Zulip Argel Ramírez Reyes (Jul 08 2021 at 18:28):

a minimal working example would be better but definitely it seems that that line is the one causing you problems.

By the way, it seems like the collect in there is not necessary:

julia> a = [(1,2) (1,3) (1,4)]
1×3 Matrix{Tuple{Int64, Int64}}:
 (1, 2)  (1, 3)  (1, 4)

julia> for (i,j) in enumerate(a)
       println(i," ",j)
       end
1 (1, 2)
2 (1, 3)
3 (1, 4)

julia> for (i,j) in (enumerate(a) |> collect)
       println(i," ",j)
       end
1 (1, 2)
2 (1, 3)
3 (1, 4)

view this post on Zulip Timothy (Jul 11 2021 at 04:46):

Which line are you referring to? Oh, and collect is needed when using @threads.

view this post on Zulip Gunnar Farnebäck (Jul 11 2021 at 09:22):

    for (j, pair)  (enumerate(pairs′) |> collect)
        D₁[[x,y],:] = pair
        tightnesses[j] = somepurefunction(D₁)
    end

I don't see how this possibly could be parallelized. All threads will try to update the contents of D₁ and will overwrite each other in a data race. You will need to set things up so each thread has its own copy of D₁ to play with.


Last updated: Nov 06 2024 at 04:40 UTC