How to solve error while generating random number inside kernel?

General Usage
,
1

Hi,

I am a newbie to CUDA and I am trying to generate random number inside a gpu kernel using the following commands,


using CUDA
using Random

function kernel(cu_status)
	idx = threadIdx().x
    unif =  CUDA.rand(1)
    if unif < 0.5
        cu_status[idx] = true
    end
    return nothing
end

itr = 100
status = fill(false,itr)
cu_status = CuArray(status)
@cuda threads=itr kernel(cu_status)

But I am getting the following error:

ERROR: GPU compilation of kernel kernel(CuDeviceArray{Bool,1,1}) failed
KernelError: kernel returns a value of type `Union{}`

Make sure your kernel function ends in `return`, `return nothing` or `nothing`.
If the returned value is of type `Union{}`, your Julia code probably throws an exception.
Inspect the code with `@device_code_warntype` for more details.
2

rand is not supported in kernels, it’s a heavyweight API that allocates memory, accesses CPU structures, etc. Did you follow the advice the error gave you?

2 Likes
3

@maleadt: Thanks for your reply. I tried to see warning like below
@device_code_warntype @cuda threads=itr kernel(cu_status)
and I got as following:

PTX CompilerJob of kernel kernel(CuDeviceArray{Bool,1,1}) for sm_50

Variables
  #self#::Core.Compiler.Const(kernel, false)
  cu_status::CuDeviceArray{Bool,1,1}
  idx::Int64
  unif::CuArray{Float32,1}

Body::Union{}
1 ─ %1 = Main.threadIdx()::NamedTuple{(:x, :y, :z),Tuple{Int64,Int64,Int64}}
│        (idx = Base.getproperty(%1, :x))
│   %3 = CUDA.rand::Core.Compiler.Const(CUDA.rand, false)
│        (unif = (%3)(1))
│        (unif < 0.5)
│        Core.Compiler.Const(:(%5), false)
│        Core.Compiler.Const(true, false)
│        Core.Compiler.Const(:(Base.setindex!(cu_status, %7, idx)), false)
└──      Core.Compiler.Const(:(return Main.nothing), false)
ERROR: GPU compilation of kernel kernel(CuDeviceArray{Bool,1,1}) failed
KernelError: kernel returns a value of type `Union{}`

Make sure your kernel function ends in `return`, `return nothing` or `nothing`.
If the returned value is of type `Union{}`, your Julia code probably throws an exception.
Inspect the code with `@device_code_warntype` for more details.

But I don’t know how to interpret this.

Is any other option to generate random numbers in kernels?

Thanks and Regards,
Manu

4

If you know how many you need, you could pass an array of random numbers into the function.

1 Like
5

@Oscar_Smith: Thanks for your reply