As written, this is creating a 1000 DArrays. I think you intended to have
only 2 of them and swap values in each iteration?
On Sunday, 4 January 2015 11:07:47 UTC+5:30, Amuthan A. Ramabathiran wrote:
>
> Hello: I recently started exploring the parallel capabilities of Julia and
> I need some help in understanding and improving the performance a very
> elementary parallel code using DArrays (I use Julia
> version 0.4.0-dev+2431). The code pasted below (based essentially on
> plife.jl) solves u''(x) = 0, x \in [0,1] with u(0) and u(1) specified,
> using the 2nd order central difference approximation. The parallel version
> of the code runs significantly slower than the serial version. It would be
> nice if someone could point out ways to improve this and/or suggest an
> alternative efficient version.
>
> function laplace_1D_serial(u::Array{Float64})
> N = length(u) - 2
> u_new = zeros(N)
>
> for i = 1:N
> u_new[i] = 0.5(u[i] + u[i + 2])
> end
>
> u_new
> end
>
> function serial_iterate(u::Array{Float64})
> u_new = laplace_1D_serial(u)
>
> for i = 1:length(u_new)
> u[i + 1] = u_new[i]
> end
> end
>
> function parallel_iterate(u::DArray)
> DArray(size(u), procs(u)) do I
> J = I[1]
>
> if myid() == 2
> local_array = zeros(length(J) + 1)
> for i = J[1] : J[end] + 1
> local_array[i - J[1] + 1] = u[i]
> end
> append!([float(u[1])], laplace_1D_serial(local_array))
>
> elseif myid() == length(procs(u)) + 1
> local_array = zeros(length(J) + 1)
> for i = J[1] - 1 : J[end]
> local_array[i - J[1] + 2] = u[i]
> end
> append!(laplace_1D_serial(local_array), [float(u[end])])
>
> else
> local_array = zeros(length(J) + 2)
> for i = J[1] - 1 : J[end] + 1
> local_array[i - J[1] + 2] = u[i]
> end
> laplace_1D_serial(local_array)
>
> end
> end
> end
>
> A sample run on my laptop with 4 processors:
> julia> u = zeros(1000); u[end] = 1.0; u_distributed = distribute(u);
>
> julia> @time for i = 1:1000
> serial_iterate(u)
> end
> elapsed time: 0.011452192 seconds (8300112 bytes allocated)
>
> julia> @time for i = 1:1000
> u_distributed = parallel_iterate(u_distributed)
> end
> elapsed time: 4.461922218 seconds (190565036 bytes allocated, 10.17% gc
> time)
>
> Thanks for your help!
>
> Cheers,
> Amuthan
>
>
>
