# Mapping functions using CuArrays

**URL:** <https://discourse.julialang.org/t/mapping-functions-using-cuarrays/46300>\
**Category:** GPU\
**Tags:** cuarrays\
**Created:** [September 9, 2020, 2:11am UTC](https://discourse.julialang.org/t/mapping-functions-using-cuarrays/46300 "2020-09-09T02:11:25Z")\
**Posts on this page:** 3\
**Page:** 1

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**Author:** ![manderson](https://avatars.discourse-cdn.com/v4/letter/m/e47774/32.png) [@manderson](https://discourse.julialang.org/u/manderson)\
**Post date:** [September 9, 2020, 2:11am UTC](https://discourse.julialang.org/t/mapping-functions-using-cuarrays/46300/1 "2020-09-09T02:11:25Z")

</div>

I am attempting to map functions to the GPU using CUDA, CuArrays, and map(). My ultimate goal is to take what is currently a serial grid search and run each reduction in parallel on the GPU. For this, I need to pass a variety of static arrays and a single varying parameter to the function which then does some operations and returns a vector as a result. Stripping this down, I can easily do what I want on the CPU and the result is what I expect.

```julia
julia> function testfunction(x,y=y1)
           x .+ y
       end
testfunction (generic function with 2 methods)

julia> y1=(collect(range(1.0,stop=20,length=20)));

julia> a=ones(4);

julia> res=map(testfunction,a)
4-element Array{Array{Float64,1},1}:
 [2.0, 3.0, 4.0, 5.0, 6.0, 7.0, 8.0, 9.0, 10.0, 11.0, 12.0, 13.0, 14.0, 15.0, 16.0, 17.0, 18.0, 19.0, 20.0, 21.0]
 [2.0, 3.0, 4.0, 5.0, 6.0, 7.0, 8.0, 9.0, 10.0, 11.0, 12.0, 13.0, 14.0, 15.0, 16.0, 17.0, 18.0, 19.0, 20.0, 21.0]
 [2.0, 3.0, 4.0, 5.0, 6.0, 7.0, 8.0, 9.0, 10.0, 11.0, 12.0, 13.0, 14.0, 15.0, 16.0, 17.0, 18.0, 19.0, 20.0, 21.0]
 [2.0, 3.0, 4.0, 5.0, 6.0, 7.0, 8.0, 9.0, 10.0, 11.0, 12.0, 13.0, 14.0, 15.0, 16.0, 17.0, 18.0, 19.0, 20.0, 21.0]

```

My understanding is that I should be able to use the same exact code where the input variables are CuArrays and get the same thing back out. Unfortunately, I receive an error when attempting this. The following code gives “ERROR: CuArray only supports bits types”.

```julia
julia> using CUDA

julia> function testfunction2(x,y=y2)
           x .+ y
       end
testfunction2 (generic function with 2 methods)

julia> y2=CuArray(collect(range(1.0,stop=10,length=10)));

julia> a2=CuArray(ones(4));

julia> res=map(testfunction2,a2)
┌ Warning: Performing scalar operations on GPU arrays: This is very slow, consider disallowing these operations with `allowscalar(false)`
└ @ GPUArrays C:\Users\matda\.julia\packages\GPUArrays\eVYIC\src\host\indexing.jl:43
ERROR: CuArray only supports bits types
Stacktrace:
 [1] error(::String) at .\error.jl:33
 [2] CuArray{CuArray{Float64,1},1}(::UndefInitializer, ::Tuple{Int64}) at C:\Users\matda\.julia\packages\CUDA\dZvbp\src\array.jl:115
 [3] CuArray{CuArray{Float64,1},N} where N(::UndefInitializer, ::Tuple{Int64}) at C:\Users\matda\.julia\packages\CUDA\dZvbp\src\array.jl:124
 [4] similar(::Type{CuArray{CuArray{Float64,1},N} where N}, ::Tuple{Int64}) at .\abstractarray.jl:675
 [5] similar(::Type{CuArray{CuArray{Float64,1},N} where N}, ::Tuple{Base.OneTo{Int64}}) at .\abstractarray.jl:674
 [6] similar(::Base.Broadcast.Broadcasted{CUDA.CuArrayStyle{1},Tuple{Base.OneTo{Int64}},typeof(testfunction2),Tuple{Base.Broadcast.Extruded{CuArray{Float64,1},Tuple{Bool},Tuple{Int64}}}}, ::Type{CuArray{Float64,1}}) at C:\Users\matda\.julia\packages\CUDA\dZvbp\src\broadcast.jl:11
 [7] copy at .\broadcast.jl:877 [inlined]
 [8] materialize(::Base.Broadcast.Broadcasted{CUDA.CuArrayStyle{1},Nothing,typeof(testfunction2),Tuple{CuArray{Float64,1}}}) at .\broadcast.jl:837
 [9] map(::Function, ::CuArray{Float64,1}) at C:\Users\matda\.julia\packages\GPUArrays\eVYIC\src\host\broadcast.jl:89
 [10] top-level scope at REPL[5]:1

```

I am unsure if this is due to a misunderstanding on my part, or an actual problem. Any explanation or assistance is appreciated.

---

<div class="post-metadata">

**Author:** ![mcabbott](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/mcabbott/32/6603_2.png) [@mcabbott](https://discourse.julialang.org/u/mcabbott)\
**Post date:** [September 9, 2020, 8:56am UTC](https://discourse.julialang.org/t/mapping-functions-using-cuarrays/46300/2 "2020-09-09T08:56:58Z")

</div>

Perhaps a helpful thread, on arrays-of-arrays and GPUs: [Map Performance with CuArrays - #10 by maleadt](https://discourse.julialang.org/t/map-performance-with-cuarrays/33497/10)

---

<div class="post-metadata">

**Author:** ![manderson](https://avatars.discourse-cdn.com/v4/letter/m/e47774/32.png) [@manderson](https://discourse.julialang.org/u/manderson)\
**Post date:** [September 9, 2020, 4:12pm UTC](https://discourse.julialang.org/t/mapping-functions-using-cuarrays/46300/3 "2020-09-09T16:12:00Z")

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Thank you for the reply. This does shed some light on the issue.