Large Matrix Takes Forever



Hi, I am new to Julia and I am writing a program that makes a random matrix drawn from the Haar distribution. It works fine for small matrices ~200x200 but when I try for 1000x1000 the program takes a very long time to run. I am not sure what the issue is here.

using Random, LinearAlgebra
using Plots

function mkHaar(dimMat)
    #initialize random number generatore and create complex 4x4 normal random matrix
    rng = MersenneTwister()
    rndMat = randn(rng, ComplexF64, (dimMat, dimMat))

    Z = qr(rndMat)
    L = Diagonal(Z.R)/abs.(Diagonal(Z.R))

    rndHaar = Z.Q*L

    #We can check to see that rndHaar is Unitary

    #and that the eigenvalue phases are uniformly distributed
    EV = eigvals(rndHaar)
    phaseEV = angle.(EV)

    #Make histogram
    gr(size = (300,300), legend = false)
    #create binning breaks
    binbrks = range(-pi, stop = pi, length = 25)
    p = histogram(phaseEV,bins=binbrks)



It looks like matrix multiplication with Z.Q (which is not actually a plain matrix but a special type for the result of the QR factorization) is very slow because it ends up calling the getindex function here: which looks pretty slow to me.

You can get around this by just copying it to a matrix first:

rndHaar = Matrix(Z.Q) * L

There’s probably a better way to fix this in Julia itself, but I don’t know that part of the code well enough.


Here’s an implementation that will work (you can remove the two Compat. prefixes if you are on 1.0):


There is a fast method for multiplying the special “Q” matrix type by a StridedMatrix:
but this doesn’t get called for multiplying by a Diagonal matrix, for which it ends up calling *(::AbstractMatrix, ::Diagonal) instead.

To avoid having an explosion of the number of *(...) methods we need, it might be nice to have some kind of ExpensiveIndexing trait that generic methods can check to see if they need to make a Matrix copy before executing indexing-heavy algorithms.



Thanks for all the replies they were very helpful. For now I think I will just add the Matrix() since it is the simplest but I’ll do some performance checks and see what’s the best way to handle this in the future.