# Creating random \`MMatrix\` with large size is too slow

**URL:** <https://discourse.julialang.org/t/creating-random-mmatrix-with-large-size-is-too-slow/90323>\
**Category:** Performance\
**Tags:** performance, array, staticarrays\
**Created:** [November 16, 2022, 3:57am UTC](https://discourse.julialang.org/t/creating-random-mmatrix-with-large-size-is-too-slow/90323 "2022-11-16T03:57:04Z")\
**Posts on this page:** 6\
**Page:** 1

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**Author:** ![singularitti](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/singularitti/32/17678_2.png) [@singularitti](https://discourse.julialang.org/u/singularitti)\
**Post date:** [November 16, 2022, 3:57am UTC](https://discourse.julialang.org/t/creating-random-mmatrix-with-large-size-is-too-slow/90323/1 "2022-11-16T03:57:05Z")

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I want to create a `StaticArrays.MMatrix` with a somewhat medium or large size using `rand`, but it seems much slower than the plain `Matrix` and it took a long time for it to display:

```julia
using StaticArray: MMatrix

N = 10
m = rand(N, N)

julia> @btime MMatrix{N,N}(m);
  1.067 μs (22 allocations: 1.89 KiB)

julia> @btime rand(N, N);
  136.503 ns (1 allocation: 896 bytes)

N = 40

julia> @btime m;
  2.125 ns (0 allocations: 0 bytes)

julia> @btime rand(N, N);
  1.179 μs (1 allocation: 12.62 KiB)

```

What should I do?

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**Author:** ![Mason](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/mason/32/2423_2.png) [@Mason](https://discourse.julialang.org/u/Mason)\
**Post date:** [November 16, 2022, 4:01am UTC](https://discourse.julialang.org/t/creating-random-mmatrix-with-large-size-is-too-slow/90323/2 "2022-11-16T04:01:28Z")

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As it says in the README of StaticArrays.jl:

> Note that in the current implementation, working with large `StaticArray`s puts a lot of stress on the compiler, and becomes slower than `Base.Array` as the size increases. A very rough rule of thumb is that you should consider using a normal `Array` for arrays larger than 100 elements.

Regarding

> [@singularitti](#):
>
> What should I do?

that depends entirely on what you’re trying to do. Do you really need static sizing here? Are you trying to stack allocate everything?

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<div class="post-metadata">

**Author:** ![singularitti](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/singularitti/32/17678_2.png) [@singularitti](https://discourse.julialang.org/u/singularitti)\
**Post date:** [November 16, 2022, 4:04am UTC](https://discourse.julialang.org/t/creating-random-mmatrix-with-large-size-is-too-slow/90323/3 "2022-11-16T04:04:17Z")

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Oh, thank you! I do not know that it has this limitation.

> Do you really need static sizing here? Are you trying to stack allocate everything?

Not really, I rarely use `MMatrix` and thought they would be performant if I want to store them with fixed sizes.

I just want a performant `Matrix` and `Array` type. I do not know if the builtin types are enough.

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**Author:** ![Oscar\_Smith](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/oscar_smith/32/25343_2.png) [@Oscar\_Smith](https://discourse.julialang.org/u/Oscar_Smith)\
**Post date:** [November 16, 2022, 4:29am UTC](https://discourse.julialang.org/t/creating-random-mmatrix-with-large-size-is-too-slow/90323/4 "2022-11-16T04:29:15Z")

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`Matrix` is already fast.

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<div class="post-metadata">

**Author:** ![stillyslalom](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/stillyslalom/32/45687_2.png) [@stillyslalom](https://discourse.julialang.org/u/stillyslalom)\
**Post date:** [November 16, 2022, 4:38am UTC](https://discourse.julialang.org/t/creating-random-mmatrix-with-large-size-is-too-slow/90323/5 "2022-11-16T04:38:11Z")

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If all you need is fixed size, note the existence of [`SizedArray`](https://juliaarrays.github.io/StaticArrays.jl/dev/pages/api/#StaticArraysCore.SizedArray) (and `SizedVector`/`SizedMatrix`) within StaticArrays.jl, which lets you assign fixed dimensions to standard Julia arrays.

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<div class="post-metadata">

**Author:** ![mikmoore](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/mikmoore/32/31109_2.png) [@mikmoore](https://discourse.julialang.org/u/mikmoore)\
**Post date:** [November 16, 2022, 3:57pm UTC](https://discourse.julialang.org/t/creating-random-mmatrix-with-large-size-is-too-slow/90323/6 "2022-11-16T15:57:52Z")

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While 40x40 is beyond the size where you really should be using a `MMatrix`, 10x10 is totally acceptable. What you’re seeing is a benchmarking artifact of global variables `m` and `N`.

```julia-repl
using StaticArrays
using BenchmarkTools

N = 10;
m = rand(10, 10);

@btime rand(MMatrix{N,N}); # N is not a known constant so this performs badly
# 1.420 μs (22 allocations: 1.89 KiB)

@btime MMatrix{10,10}($m); # literals are fine
# 34.441 ns (1 allocation: 816 bytes)

const Nconst = 10
@btime MMatrix{Nconst,Nconst}($m); # const N is fine
# 33.972 ns (1 allocation: 816 bytes)

m = rand(40, 40);
@btime MMatrix{40,40}($m); # bigger than a MMatrix really should be
# 1.520 μs (1 allocation: 12.56 KiB)

```

Although, really, you should not generate random matrices this way. Use either of these equivalent methods:

```julia-repl
julia> @btime @MMatrix rand(10,10);
  153.006 ns (1 allocation: 816 bytes)

julia> @btime rand(MMatrix{10,10});
  153.129 ns (1 allocation: 816 bytes)

```

I’ll reiterate that if the size of any `StaticArrays` array is not known at compile time, there is little benefit to using it over a normal `Array` unless it will be passed down into other functions where its size can be known.