Julia parallelism: @distributed (+) slower than serial?

Question

After seeing a couple tutorials on the internet on Julia parallelism I decided to implement a small parallel snippet for computing the harmonic series.

The serial co

Answer 1

You're distributed version should be

function harmonic_distr2(n::Int64)
    x = @distributed (+) for i in n:-1:1
        1/i # no x assignment here
    end
    x
end

The @distributed loop will accumulate values of 1/i on every worker an then finally on the master process.

Note that it is also generally better to use BenchmarkTools's @btime macro instead of @time for benchmarking:

julia> using Distributed; addprocs(4);

julia> @btime harmonic(1_000_000_000); # serial
  1.601 s (1 allocation: 16 bytes)

julia> @btime harmonic_distr2(1_000_000_000); # parallel
  754.058 ms (399 allocations: 36.63 KiB)

julia> @btime harmonic_distr(1_000_000_000); # your old parallel version
  4.289 s (411 allocations: 37.13 KiB)

The parallel version is, of course, slower if run only on one process:

julia> rmprocs(workers())
Task (done) @0x0000000006fb73d0

julia> nprocs()
1

julia> @btime harmonic_distr2(1_000_000_000); # (not really) parallel
  1.879 s (34 allocations: 2.00 KiB)