Overlapping Data Transfers with Kernel Computation

Applications like database analytics have much larger data set than the available memory on the acceleration device. They require the complete data to be transferred and processed in blocks. Techniques that overlap the data transfers with the computation are critical to achieve high performance for these applications.

Below is the vector add kernel from the OpenCL™ Overlap Data Transfers with Kernel Computation Example in the host category from Xilinx On-boarding Example GitHub.

kernel __attribute__((reqd_work_group_size(1, 1, 1)))
void vadd(global int* c,
          global const int* a,
          global const int* b,
          const int offset,
          const int elements) 
    int end = offset + elements;
    vadd_loop: for (int x=offset; x<end; ++x) {
        c[x] = a[x] + b[x];
There are four tasks to perform in the host application for this example: write buffer a (Wa), write buffer b (Wb), execute vadd kernel, and read buffer c (Rc). The asynchronous nature of OpenCL data transfer and kernel execution APIs allows overlap of data transfers and kernel execution as illustrated in the figure below. In this example, double buffering is used for all buffers so that the compute unit can process one set of buffers while the host can operate on the other set of buffers. The OpenCL event object provides an easy way to set up complex operation dependencies and synchronize host threads and device operations. The arrows in the figure below show how event triggering can be set up to achieve optimal performance.

Figure: Event Triggering Set Up

The host code snippet below enqueues the four tasks in a loop. It also sets up event synchronization between different tasks to ensure that data dependencies are met for each task. The double buffering is set up by passing different memory objects values to clEnqueueMigrateMemObjects API. The event synchronization is achieved by having each API to wait for other event as well as trigger its own event when the API completes.

  for (size_t iteration_idx = 0; iteration_idx < num_iterations; iteration_idx++) {
    int flag = iteration_idx % 2;

    if (iteration_idx >= 2) {
        clWaitForEvents(1, &map_events[flag]);

    buffer_a[flag] = clCreateBuffer(world.context,  
           bytes_per_iteration, &A[iteration_idx * elements_per_iteration], NULL);
    buffer_b[flag] = clCreateBuffer(world.context,  
           bytes_per_iteration, &B[iteration_idx * elements_per_iteration], NULL);
    buffer_c[flag] = clCreateBuffer(world.context,  
           bytes_per_iteration, &device_result[iteration_idx * elements_per_iteration], NULL);
    array<cl_event, 2> write_events;
    printf("Enqueueing Migrate Mem Object (Host to Device) calls\n");
    // These calls are asynchronous with respect to the main thread because we
    // are passing the CL_FALSE as the third parameter. Because we are passing
    // the events from the previous kernel call into the wait list, it will wait
    // for the previous operations to complete before continuing
        world.command_queue, 1, &buffer_a[iteration_idx % 2],
        0 /* flags, 0 means from host */,
        0, NULL, 
    set_callback(write_events[0], "ooo_queue");

        world.command_queue, 1, &buffer_b[iteration_idx % 2],
        0 /* flags, 0 means from host */,
        0, NULL, 
    set_callback(write_events[1], "ooo_queue");

    xcl_set_kernel_arg(kernel, 0, sizeof(cl_mem), &buffer_c[iteration_idx % 2]);
    xcl_set_kernel_arg(kernel, 1, sizeof(cl_mem), &buffer_a[iteration_idx % 2]);
    xcl_set_kernel_arg(kernel, 2, sizeof(cl_mem), &buffer_b[iteration_idx % 2]);
    xcl_set_kernel_arg(kernel, 3, sizeof(int), &elements_per_iteration);

    printf("Enqueueing NDRange kernel.\n");
    // This event needs to wait for the write buffer operations to complete
    // before executing. We are sending the write_events into its wait list to
    // ensure that the order of operations is correct.
    OCL_CHECK(clEnqueueNDRangeKernel(world.command_queue, kernel, 1, nullptr,
                                     &global, &local, 2 , write_events.data(),
    set_callback(kernel_events[flag], "ooo_queue");

    printf("Enqueueing Migrate Mem Object (Device to Host) calls\n");
    // This operation only needs to wait for the kernel call. This call will
    // potentially overlap the next kernel call as well as the next read
    // operations
    OCL_CHECK( clEnqueueMigrateMemObjects(world.command_queue, 1, &buffer_c[iteration_idx % 2], 
                CL_MIGRATE_MEM_OBJECT_HOST, 1, &kernel_events[flag], &read_events[flag]));

    set_callback(read_events[flag], "ooo_queue");
    clEnqueueMapBuffer(world.command_queue, buffer_c[flag], CL_FALSE, CL_MAP_READ, 0, 
            bytes_per_iteration, 1, &read_events[flag], &map_events[flag], 0);
    set_callback(map_events[flag], "ooo_queue");

The Application Timeline view below clearly shows that the data transfer time is completely hidden, while the compute unit vadd_1 is running constantly.

Figure: Data Transfer Time Hidden in Application Timeline View