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Quickstart

Terminology

Because CUDA is the most popular GPU programming environment, we can use it as a reference for defining terminology in KA. A workgroup is called a block in NVIDIA CUDA and designates a group of threads acting in parallel, preferably in lockstep. For the GPU, the workgroup size is typically around 256, while for the CPU, it is usually a multiple of the natural vector-width. An ndrange is called a grid in NVIDIA CUDA and designates the total number of work items. If using a workgroup of size 1 (non-parallel execution), the ndrange is the number of items to iterate over in a loop.

Writing your first kernel

Kernel functions are marked with the @kernel. Inside the @kernel macro you can use the [kernel language](@ref api_kernel_language). As an example, the mul2 kernel below will multiply each element of the array A by 2. It uses the @index macro to obtain the global linear index of the current work item.

@kernel function mul2_kernel(A)
  I = @index(Global)
  A[I] = 2 * A[I]
end

Launching kernel on the host

You can construct a kernel for a specific backend by calling the kernel with mul2_kernel(OpenCLBackend(), 16). The first argument is a backend of type KA.Backend, the second argument being the workgroup size. This returns a generated kernel executable that is then executed with the input argument A and the additional argument being a static ndrange.

dev = OpenCLBackend()
A = ones(1024, 1024)
ev = mul2_kernel(dev, 64)(A, ndrange=size(A))
synchronize(dev)
all(A .== 2.0)

All kernels are launched asynchronously. The synchronize blocks the host until the kernel has completed on the backend.

Launching kernel on the backend

To launch the kernel on a backend-supported backend isa(backend, KA.GPU) (e.g., CUDABackend(), ROCBackend(), oneBackend()), we generate the kernel for this backend.

First, we initialize the array using the Array constructor of the chosen backend with

using CUDA: CuArray
A = CuArray(ones(1024, 1024))
using AMDGPU: ROCArray
A = ROCArray(ones(1024, 1024))
using oneAPI: oneArray
A = oneArray(ones(1024, 1024))

The kernel generation and execution are then

backend = get_backend(A)
mul2_kernel(backend, 64)(A, ndrange=size(A))
synchronize(backend)
all(A .== 2.0)

Synchronization

!!! danger All kernel launches are asynchronous, use synchronize(backend) to wait on a series of kernel launches.

The code around KA may heavily rely on GPUArrays, for example, to intialize variables.

function mymul(A)
    A .= 1.0
    backend = get_backend(A)
    ev = mul2_kernel(backend, 64)(A, ndrange=size(A))
    synchronize(backend)
    all(A .== 2.0)
end
function mymul(A, B)
    A .= 1.0
    B .= 3.0
    backend = get_backend(A)
    @assert get_backend(B) == backend
    mul2_kernel(backend, 64)(A, ndrange=size(A))
    mul2_kernel(backend, 64)(B, ndrange=size(B))
    synchronize(backend)
    all(A .+ B .== 8.0)
end

Using task programming to launch kernels in parallel.

TODO