Parallel programming in Julia
I have been following the docs for parallel programming in julia and for my mind, which thinks like openMP or MPI, I find the design choice quite strange.
I have an application where I want data to be distributed among processes, and then I want to tell each process to apply some operation to whatever data it is assigned, yet I do not see a way of doing this in Julia. Here is an example
julia> r = remotecall(2, rand, 2)
RemoteRef{Channel{Any}}(2,1,30)
julia> fetch(r)
2-element Array{Float64,1}:
0.733308
0.45227
so on process 2 lives a random array with 2 elements. I can apply some function to this array via
julia> remotecall_fetch(2, getindex, r, 1)
0.7333080770447185
but why does it not work if i apply a function which should change the vector, like:
julia> remotecall_fetch(2, setindex!, r, 1,1)
ERROR: On worker 2:
MethodError: `setindex!` has no method matching setindex!(::RemoteRef{Channel{Any}}, ::Int64, ::Int64)
in anonymous at multi.jl:892
in run_work_thunk at multi.jl:645
[inlined code] from multi.jl:892
in anonymous at task.jl:63
in remotecall_fetch at multi.jl:731
in remotecall_fetch at multi.jl:734
I don't quite know how to describe it, but it seems like the workers can only return "new" things. I don't see how I can send some variables and a function to a worker and have the function modify the variables in place. In the above example, I'd like the array to live on a single process and ideally I'd be able to tell that process to perform some operations on that array. After all the operations are finished I could then fetch results etc.
I think you can achive this with the macro @spawnat
:
julia> addprocs(2)
2-element Array{Int64,1}:
2
3
julia> r = remotecall(2, rand, 2)
RemoteRef{Channel{Any}}(2,1,3)
julia> fetch(r)
2-element Array{Float64,1}:
0.149753
0.687653
julia> remotecall_fetch(2, getindex, r, 1)
0.14975250913699378
julia> @spawnat 2 setindex!(fetch(r), 320.0, 1)
RemoteRef{Channel{Any}}(2,1,6)
julia> fetch(r)
2-element Array{Float64,1}:
320.0
0.687653
julia> @spawnat 2 setindex!(fetch(r), 950.0, 2)
RemoteRef{Channel{Any}}(2,1,8)
julia> fetch(r)
2-element Array{Float64,1}:
320.0
950.0
But with remotecall_fetch
, it looks like the returned array is really a copy:
julia> remotecall_fetch(2, setindex!, fetch(r), 878.99, 1)
2-element Array{Float64,1}:
878.99
950.0
julia> remotecall_fetch(2, setindex!, fetch(r), 232.99, 2)
2-element Array{Float64,1}:
320.0
232.99
julia> fetch(r)
2-element Array{Float64,1}:
320.0
950.0
with: Julia Version 0.4.3
根据您的需求描述,您可能会发现分布式数组很有用。
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