# Seed random number generator when using pmap

**URL:** <https://discourse.julialang.org/t/seed-random-number-generator-when-using-pmap/78973>\
**Category:** New to Julia\
**Tags:** pmap\
**Created:** [April 4, 2022, 1:37am UTC](https://discourse.julialang.org/t/seed-random-number-generator-when-using-pmap/78973 "2022-04-04T01:37:57Z")\
**Posts on this page:** 3\
**Page:** 1

<div class="post-metadata">

**Author:** ![jisutich](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/jisutich/32/33342_2.png) [@jisutich](https://discourse.julialang.org/u/jisutich)\
**Post date:** [April 4, 2022, 1:37am UTC](https://discourse.julialang.org/t/seed-random-number-generator-when-using-pmap/78973/1 "2022-04-04T01:37:57Z")

</div>

Hi

I am trying to use `pmap` to increase the speed of my code. And I try to seed the random generator in different processors, so I have a code like

```julia
using Distributed

addprocs(4)
@everywhere using LinearAlgebra
@everywhere using StatsBase
@everywhere using Statistics
@everywhere using Random
@everywhere using StaticArrays
@everywhere using Future

#generate random number generator for each processor
@everywhere const LOCAL_R = Future.randjump(MersenneTwister(123), nprocs())[myid()]

@everywhere function rand_haar2(::Val{n}) where n
    M = randn(LOCAL_R,SMatrix{ComplexF64, n,n}) # = rand(Ginibre(2, n))
    q = qr(M).Q
    L = cispi.(2 .* @SVector(rand(n)))
    return q*diagm(L)
end

@everywhere function main2(; ps=2, theta = 1.08, Nk = 800, Nr = 10000)
    sum_z =@MVector zeros(Nk)
    sum_zlog =@MVector zeros(Nk)
    sum_s =@MVector zeros(Nk)
    sum_slog =@MVector zeros(Nk)
    
    Npool = Int(floor(10^ps))
    pool = zeros(Npool)
    pool_sample = zeros(Npool)
    logpool = zeros(Npool)
   

    Kup= @SMatrix[cos(theta) 0; 0 sin(theta)]
    Kdown = @SMatrix[sin(theta) 0; 0 cos(theta)]
    P2up = kron(@SMatrix[1. 0.;0. 1.], @SMatrix[1 0; 0 0])
    P2down = kron(@SMatrix[1 0;0 1], @SMatrix[0 0;0 1])
    
    for i in 1:Nr
        pool .= 0.5
        pool_sample .= 0
     

        for k in 1:Nk
            poolcount = 0
            while poolcount < Npool
                z1 = pool[rand(1:Npool)]
                rho1 = diagm(@SVector[z1,1-z1])

                z2 = pool[rand(1:Npool)]
                rho2 = diagm(@SVector[z2,1-z2])

                u1 = rand_haar2(Val{2}())
                u2 = rand_haar2(Val{2}())

                K1up = u1*Kup*u1'
                K1down = u1*Kdown*u1'
                
                K2up = u2*Kup*u2'
                K2down = u2*Kdown*u2'

                rho1p = K1up*rho1*K1up'
                rho2p = K2up*rho2*K2up'

                p1 = real(tr(rho1p+rho1p'))/2
                p2 = real(tr(rho2p+rho2p'))/2
                
                if rand(LOCAL_R)<p1
                    rho1p = (rho1p+rho1p')/(2*p1)
                else 
                    rho1p = K1down*rho1*K1down'/((1-p1)) 
                end

                if rand(LOCAL_R)<p2
                    rho2p = (rho2p+rho2p')/(2*p2)
                else
                    rho2p = K2down*rho2*K2down'/((1-p2))
                end
             
                rho = kron(rho1p,rho2p)

                U = rand_haar2(Val{4}())
                rho_p = P2up*U*rho*U'*P2up'
                p = real(tr(rho_p+rho_p'))/2
             
                if rand(LOCAL_R)<p
                    temp =(rho_p+rho_p')/2
                    #rho_f = @SMatrix[tr(temp[1:2,1:2]) tr(temp[1:2,3:4]); tr(temp[3:4,1:2]) tr(temp[3:4,3:4])]/(p)
                    rho_f = @SMatrix[temp[1,1]+temp[2,2] temp[1,3]+temp[2,4]; temp[3,1]+temp[4,2] temp[3,3]+temp[4,4]]/(p)
                else
                    temp = P2down*U*rho*U'*P2down'
                    rho_f = @SMatrix[temp[1,1]+temp[2,2] temp[1,3]+temp[2,4]; temp[3,1]+temp[4,2] temp[3,3]+temp[4,4]]/(1-p)
                end
                rho_f = (rho_f+rho_f')/2
                t = abs(tr(rho_f*rho_f))
                z = (1-t)/(1+sqrt(2*t-1))
                if !iszero(abs(z))
                    poolcount = poolcount+1
                    pool_sample[poolcount] = abs(z)
                end
            end

            pool .= pool_sample
            pool_sample .= 0

            logpool .=log.(pool)
      

            sum_z[k] += mean(pool)
            sum_zlog[k] += mean(logpool)/sqrt(Npool)
            sum_s[k] += (std(pool)/sqrt(Npool))^2
            sum_slog[k] += (std(logpool)/sqrt(Npool))^2
        end

    
    end
    return sum_z/Nr, sqrt.(sum_s)/Nr, sum_zlog/Nr, sqrt.(sum_slog)/Nr
end

@time ret = pmap(main2,Nr = [2500,2500,2500,2500])

```

But when I try to run this code, I got an error

```julia
ERROR: On worker 2:
MethodError: no method matching getindex(::MersenneTwister, ::Int64)
Stacktrace:
   @ Distributed D:\Julia-1.7.2\share\julia\stdlib\v1.7\Distributed\src\macros.jl:219
 [4] top-level scope
   @ D:\Julia-1.7.2\share\julia\stdlib\v1.7\Distributed\src\macros.jl:203

```

Also, I have some keywords arguments in function `main2`. `Nr =10000` represent the total number of iterations of one loop in the function. Now I want to split this loop to four processors and each one does just 2500 iterations. So I try `@time ret = pmap(main2,Nr = [2500,2500,2500,2500])`, is this correct or not?

I am sorry that I know very little about the parallel computing in Julia. I tried to read the manual, but I still don’t understand how it works.

Thanks

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<div class="post-metadata">

**Author:** ![affans](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/affans/32/11911_2.png) [@affans](https://discourse.julialang.org/u/affans)\
**Post date:** [April 4, 2022, 3:24am UTC](https://discourse.julialang.org/t/seed-random-number-generator-when-using-pmap/78973/2 "2022-04-04T03:24:59Z")

</div>

Could you not just seed each process in `main2`?

```julia
@everywhere function main2(; ps=2, theta = 1.08, Nk = 800, Nr = 10000, processID)
    Random.seed!(seedvalue) # pass in the process ID here (see additional argument)
end

```

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<div class="post-metadata">

**Author:** ![jisutich](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/jisutich/32/33342_2.png) [@jisutich](https://discourse.julialang.org/u/jisutich)\
**Post date:** [April 4, 2022, 1:26pm UTC](https://discourse.julialang.org/t/seed-random-number-generator-when-using-pmap/78973/3 "2022-04-04T13:26:11Z")

</div>

Yes I think this is also a possible way. I will try it thanks.