# Why is the result of the operation between \`SharedMatrix\`es not a \`SharedMatrix\`?

**URL:** <https://discourse.julialang.org/t/why-is-the-result-of-the-operation-between-sharedmatrix-es-not-a-sharedmatrix/8013>\
**Category:** Julia at Scale\
**Created:** [December 27, 2017, 3:27am UTC](https://discourse.julialang.org/t/why-is-the-result-of-the-operation-between-sharedmatrix-es-not-a-sharedmatrix/8013 "2017-12-27T03:27:44Z")\
**Posts on this page:** 2\
**Page:** 1

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**Author:** ![usefulhyun](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/usefulhyun/32/1770_2.png) [@usefulhyun](https://discourse.julialang.org/u/usefulhyun)\
**Post date:** [December 27, 2017, 3:27am UTC](https://discourse.julialang.org/t/why-is-the-result-of-the-operation-between-sharedmatrix-es-not-a-sharedmatrix/8013/1 "2017-12-27T03:27:44Z")

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I created two sharedmatrix as the below

sm1 = SharedMatrix{Float64}( rand(1000, 1000) )  
sm2 = SharedMatrix{Float64}( rand(1000, 1000) )  
sm3 = sm1 \* sm2 ---- (1)

However, sm3 is of type Matrix{Float64} not of type SharedMatrix{Float64}

So I have tried like the below

sm3 = SharedMatrix{Float64}(sm1\*sm2) ----- (2)

But, this still has a performance problem.

(2) is much slower than (1).

I use version 0.6.2

Does anyone help me? I want to find a way that returns the result of type SharedMatrix and as fast as (1) do.

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<div class="post-metadata">

**Author:** ![mohamed82008](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/mohamed82008/32/18171_2.png) [@mohamed82008](https://discourse.julialang.org/u/mohamed82008)\
**Post date:** [December 27, 2017, 5:57am UTC](https://discourse.julialang.org/t/why-is-the-result-of-the-operation-between-sharedmatrix-es-not-a-sharedmatrix/8013/2 "2017-12-27T05:57:30Z")

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The conversion from `SharedArray` to `Array` happens because of the call to BLAS, but I think it should be converted back before returning. However performance-wise, the following 2 functions are very similar:

```julia
julia> using BenchmarkTools

julia> function f1(a::SharedMatrix{T}, b::SharedMatrix{S}) where {T, S}
           c = a*b
           return SharedMatrix{promote_type(T,S)}(c)
       end
f1 (generic function with 1 method)

julia> function f2(a::SharedMatrix{T}, b::SharedMatrix{S}) where {T, S}
           c = a*b
           return c
       end
f2 (generic function with 1 method)

julia> sm1 = SharedMatrix{Float64}(rand(1000,1000));

julia> sm2 = SharedMatrix{Float64}(rand(1000,1000));

julia> @btime sm3 = f1($sm1, $sm2);
  86.727 ms (835 allocations: 7.66 MiB)

julia> @btime sm4 = f2($sm1, $sm2);
  85.523 ms (2 allocations: 7.63 MiB)

julia> sm1 = SharedMatrix{Float64}(rand(10000,10000));

julia> sm2 = SharedMatrix{Float64}(rand(10000,10000));

julia> @time sm3 = f1(sm1, sm2);
 51.399829 seconds (813 allocations: 762.967 MiB, 0.14% gc time)

julia> @time sm4 = f2(sm1, sm2);
 50.057509 seconds (6 allocations: 762.940 MiB, 0.12% gc time)

```

Edit: when you convert, the array is actually being copied so yes I agree it might be a performance problem. I hope someone more knowledgeable will give you a better answer.