# QR Decomposition Extremely Slow

**URL:** https://discourse.julialang.org/t/qr-decomposition-extremely-slow/16946
**Category:** General Usage
**Tags:** linearalgebra
**Created:** [October 30, 2018, 1:24am UTC](https://discourse.julialang.org/t/qr-decomposition-extremely-slow/16946 "2018-10-30T01:24:25Z")
**Posts on this page:** 1
**Showing post:** 3

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### Author: ![jpsamaroo](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/jpsamaroo/32/46804_2.png) [@jpsamaroo](https://discourse.julialang.org/u/jpsamaroo)
#### Post date: [October 30, 2018, 2:26am UTC](https://discourse.julialang.org/t/qr-decomposition-extremely-slow/16946/3 "2018-10-30T02:26:10Z")

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> [@QR decomposition in julia 2x slower than octave](https://discourse.julialang.org/t/qr-decomposition-in-julia-2x-slower-than-octave/10043/3):
>
> I suspect they may be doing different things. I don’t know what [qr method](https://octave.sourceforge.io/octave/function/qr.html) Octave calls when no output arguments are provided, but my guess is that it’s just doing the 1st phase (computing the Householder reflectors and upper-triangular part) and not the 2nd (building Q). This would roughly match [qrfact](https://docs.julialang.org/en/stable/stdlib/linalg/#Base.LinAlg.qrfact) in Julia. On my machine, I get: julia\> @time qrfact(A); 5.906331 seconds (25.99 k allocations: 194.912 MiB, 2.13% gc time) julia\> @time qr(A); 16.202900 seconds (93.73 k allocations: 770.718 …

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