# Running for loop in parallel and appending result to vector

**URL:** <https://discourse.julialang.org/t/running-for-loop-in-parallel-and-appending-result-to-vector/20487>\
**Category:** General Usage\
**Tags:** parallel\
**Created:** [February 5, 2019, 9:14pm UTC](https://discourse.julialang.org/t/running-for-loop-in-parallel-and-appending-result-to-vector/20487 "2019-02-05T21:14:19Z")\
**Posts on this page:** 1\
**Showing post:** 16

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**Author:** ![bennedich](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/bennedich/32/4894_2.png) [@bennedich](https://discourse.julialang.org/u/bennedich)\
**Post date:** [February 8, 2019, 4:18pm UTC](https://discourse.julialang.org/t/running-for-loop-in-parallel-and-appending-result-to-vector/20487/16 "2019-02-08T16:18:41Z")

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`rand()` is not thread-safe. You can solve that by creating a separate RNG per thread. See sample code below.

Also note that you should use `@btime` in BenchmarkTools instead of `@time` if you want to measure time somewhat accurately.

I also got rid of the `Distributed` and `@everywhere` stuff since it’s not needed for multi-threading. And I’ll repeat my earlier question – are you looking for multi-threading (single computer, multiple cores), or parallel computing (cluster, several distributed machines)? What does your cluster look like? The code below does multi-threading only.

Finally note that since your function `main` is so short in this example, the way you’re splitting work per thread will incur a lot of overhead. So you can’t expect to see a speedup linear to the number of threads used. There are ways around that, if indeed the work done per iteration is so small. A simple way is to handle batches at a time instead of individual elements.

```julia
const RNG = 1:Threads.nthreads() .|>_-> MersenneTwister()

function main(rng)
    roll = rand(rng)
    pars = rand(rng, 5)
    status = roll < 0.5 ? "cure" : "progression"
    (roll, status, pars)
end

function serial(f,nT)
    results = Array{Tuple{Float64,String,Array{Float64}}}(undef,nT)
    rng = MersenneTwister()
    for n = 1 : nT
        results[n] = f(rng)
    end
    return results
end

function threaded(f,nT)
    results = Array{Tuple{Float64,String,Array{Float64}}}(undef,nT)
    Threads.@threads for n = 1 : nT
        rng = RNG[Threads.threadid()]
        results[n] = f(rng)
    end
    return results
end

nT = 10^5

@time serialresults = serial(main, nT)
@time threadedresults = threaded(main, nT)

nothing

```

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