# CUDA

## Allocating Device Memory

`cudaMalloc(LOCATION, SIZE)`

* LOCATION: Memory location on Device to allocate memory, an address in the GPU's memory
* SIZE: number of bytes to allocate

De-Allocate: `cudaFree()`

## Copy Data between Host and Device

`cudaMemory(DST, SRC, NUM_BYTES, DIRECTION)`

* DST: An address of the memory to copy into
* SRC: An address of the memory to copy from
* NUM\_BYTES: N \* sizeof(type)
* DIRECTION:
  * cudaMemcpyHostToDevice
  * cudaMemcpyDeviceToHost

## Define the Kernel

```cpp
__global__ void kernel(int *d_out, int *d_in) {
    d_out[0] = d_in[0];
}
```

### Thread Index

In kernel definition, built-in variable `threadIdx`  is accessible to get thread index within the thread block for each thread.

It has 3 dimensions: `threadIdx.x`, `threadIdx.y` and `threadIdx.z`.

### Block Index

Index of a block: `blockIdx.x`, `blockIdx.y` and `blockIdx.z`. 

### Indexing within Grid

```cpp
i = threadIdx.x + blockIdx.x * blockDim.x;
```

### \_\_syncthreads

To explicitly synchronize all threads (adding barriers), use `__syncthreads.`

```cpp
int temp = a[i + 1];
__syncthreads;
a[i] = temp;
__syncthreads;
```

## Launch the Kernel

```cpp
int *h_c, *d_c;

// Allocate memeory on the device
cudaMalloc((void**)&d_c, sizeof(int));
// Copy content from host to device
cudaMemcpy(d_c, h_c, sizeof(int), cudaMemcpyHostToDevice);

// Launch the kernel
dim3 grid_size(1);
dim3 block_size(1);
kernel<<<grid_size, block_size>>>(...);

// Force host to wait on the completion of the kernel
cudaDeviceSynchronize();

// Copy data back to host
cudaMemcpy(h_c, d_c, sizeof(int), cudaMemcpyDeviceToHost);

// Clean up memory
cudaFree(d_c);
free(h_c);
```


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