ITCS 4/5010 Parallel Programming, B. Wilkinson, Jan 21, 2013. CUDADeviceRoutines

1. Device Routines and device variables These notes introduce: • Declaring routines that are be executed on device and on the host • Declaring local variable on device ITCS 4/5010 Parallel Programming, B. Wilkinson, Jan 21, 2013. CUDADeviceRoutines.ppt

2. CUDA extensions to declare kernel routines Host = CPU Device = GPU __global__ indicates routine can only be called from host and only executed on device __device__ indicates routine can only be called from device and only executed on device __host__ indicates routine can only be called from host and only executed on host (generally only used in combination with __device__ , see later) Two underscores each Note: Cannot call a routine from the kernel to be executed on host

3. So far we have seen __global__: … __global__ void add(int *a,int *b, int *c) { inttid = blockIdx.x * blockDim.x + threadIdx.x; if(tid < N) c[tid] = a[tid]+b[tid]; } int main(intargc, char *argv[]) { int T = 10, B = 1; // threads per block and blocks per grid int a[N],b[N],c[N]; int *dev_a, *dev_b, *dev_c; … cudaMalloc((void**)&dev_a,N * sizeof(int)); cudaMalloc((void**)&dev_b,N * sizeof(int)); cudaMalloc((void**)&dev_c,N * sizeof(int)); cudaMemcpy(dev_a, a , N*sizeof(int),cudaMemcpyHostToDevice); cudaMemcpy(dev_b, b , N*sizeof(int),cudaMemcpyHostToDevice); cudaMemcpy(dev_c, c , N*sizeof(int),cudaMemcpyHostToDevice); add<<<B,T>>>(dev_a,dev_b,dev_c); cudaMemcpy(c,dev_c,N*sizeof(int),cudaMemcpyDeviceToHost); … cudaFree(dev_a); cudaFree(dev_b); cudaFree(dev_c); cudaEventDestroy(start); cudaEventDestroy(stop); return 0; } __global__ must have void return type. Why? Executed on device Called from host Note __global__ asynchronous. Returns before complete

4. Routines to be executed on device Generally cannot call C library routines from device! However CUDA has math routines for device that are equivalent to standard C math routines with the same names, so in practice can call math routines such as sin(x) – need to check CUDA docs* before use Also CUDA has GPU-only routines implemented, faster less accurate (have __ names)* * See NVIDIA CUDA C Programming Guide for more details

5. __device__ routines __global__ void gpu_sort (int *a, int *b, int N) { … swap (&list[m],&list[j]); … } __device__ void swap (int *x, int *y) { int temp; temp = *x; *x = *y; *y = temp; } int main (intargc, char *argv[]) { … gpu_sort<<< B, T >>>(dev_a, dev_b, N); … return 0; } Recursion is possible with __device __ routines so far as I can tell

6. Routines executable on both host and device __device__ and __host__ qualifiers can be used together Then routine callable and executable on both host and device. Routine will be compiled for both. Feature might be used to create code that optionally uses a GPU or for test purposes. Generally will need statements that differentiate between host and device Note: __global__ and __host__ qualifiers cannot be used together.

7. __CUDA_ARCH__ macro Indicates compute capability of GPU being used. Can be used to create different paths thro device code for different capabilities. __CUDA_ARCH__ = 100 for 1.0 compute capability __CUDA_ARCH__ = 110 for 1.1 compute capability …

8. Example __host__ __device__ func() { #ifdef __CUDA_ARCH__ … // Device code #else … // Host code #endif } Could also select specific compute capabilities

9. Declaring local variables for host and for device

10. #include <stdio.h> #include <stdlib.h> intcpuA[10]; ... void clearArray() { for (inti = 0; i < 10; i++) cpuA[i] = 0; } void setArray(int n) { for (inti = 0; i < 10; i++) cpuA[i] = n; } int main(intargc, char *argv[]) { … clearArray(); … setArray(N); … return 0; } Local variables on host In C, scope of a variable is block it is declared in, which does not extend to routines called from block. If scope is to include main and all within it, including called routines, place declaration outside main:

11. Declaring local kernel variables #include <stdio.h> #include <stdlib.h> __device__ intgpu_A[10]; ... __global__ void clearArray() { for (inti = 0; i < 10; i++) gpuA[i] = 0; } int main(intargc, char *argv[]) { … clearArray<<<…>>>(); … setArray<<<…>>>(N); … return 0; } Declaring variable outside main but use __device__ (now used as a variable type qualifier rather than function type qualifier) Without further qualification, variable is in global (GPU) memory. Accessible by all threads

12. Accessing kernel variables from host Accessible by host using routines: • cudaMemcpyToSymbol() to copy to device • cudaMemcpyFromSymbol() to copy from device where name of kernel variable given as a String argument: int main(intargc, char *argv[]) { intcpuA[10]: … cudaMemcpyFromSymbol(&cpuA, "gpuA", sizeof(cpuA), 0, cudaMemcpyDeviceToHost); … return 0; } Contents of gpuA[] copied to cpuA[] Name of variable declared in kernel

13. #include <stdio.h> #include <cuda.h> #include <stdlib.h> intcpu_hist[10]; // accessible on cpu only __device__ intgpu_hist[10]; // accessible on gpu only void cpu_histogram(int *a, int N) { … // result in cpu_hist[] } __global__ void gpu_histogram(int *a, int N) { … // result in gpu_hist[] } int main(intargc, char *argv[]) { … gpu_histogram<<<B,T>>>(dev_a,N); // computed on gpu cpu_histogram(a,N); // computed on cpu … return 0; } Example of both local host and device variables

14. Questions