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cs_dispatch_queue.h

cs_dispatch_queue.h provides 3 objects that implement a SYCL-like device task management system based on 3 objects:

  • cs_dispatch_queue: The main object to interact with, allows running tasks on the host and the device with dependency management.

  • cs_task/cs_host_task: Allow interacting with tasks launched by cs_dispatch_queue by waiting for end of a task or extracting an event to synchronize with.

  • cs_event: Represents an event triggered at the end of a task for other tasks to synchronize with. It is used as an input of cs_dispatch_queue methods to express dependencies between tasks.

cs_dispatch_queue

cs_dispatch_queue is meant to be used in a way similar to sycl::queue. It can be used to declare parallel tasks on a device that depend on (ie. wait for) each other. It holds a context that can be accessed as a member named initializer_context which is used to initialize each task created by the cs_dispatch_queue.

  • In device mode, each task is spawned from the initializer_context and its own CUDA stream for asynchronous, parallel execution.

  • In host mode, tasks are run synchronously.

Tasks will be waited upon at destruction. This behavior ensures resources held for a task are not destroyed before its termination.

cs_device_queue wraps all of cs_dispatch_queue's algorithms to run them as separated tasks represented by cs_task. An additional single_task method can be used to declare tasks run on the host and represented by cs_host_task.

cs_task and cs_host_task

  • cs_task holds resources relative to the execution of a task started with cs_dispatch_queue (ie. a CUDA stream in device mode).

  • cs_host_task is a specialization of cs_task that holds additional data necessary for the execution of a host task.

cs_event

cs_event represent a time point in a task to synchronize with.

  • In device mode, they are implemented on top of cudaEvent_t.

  • In host mode, they are only used for time measurement as the execution is synchronous.

Both cs_task and cs_host_task provide an operator to generate a cs_event for synchronization or time measurement.

Usage example

The system can be used as SYCL (without memory handle based dependency management) on top of CUDA:

inline void
_cs_sycl_like_example()
{
  // Inspired by:
  // https://enccs.github.io/sycl-workshop/task-graphs-synchronization/#how-to-specify-dependencies

  cs_dispatch_queue Q;
  Q.initializer_context.set_use_gpu(true);

  constexpr std::size_t N = 16 * 1024 * 1024;
  int                  *a, *b;

  CS_MALLOC_HD(a, N, int, CS_ALLOC_HOST_DEVICE_SHARED);
  CS_MALLOC_HD(b, N, int, CS_ALLOC_DEVICE);

  {
    // Start task A asynchronously
    auto task_a =
      Q.parallel_for(N, [=] CS_F_HOST_DEVICE(std::size_t id) { a[id] = 1; });

    // Start task B asynchronously
    auto task_b =
      Q.parallel_for(N, [=] CS_F_HOST_DEVICE(std::size_t id) { b[id] = 2; });

    // Start task C asynchronously, which depends on tasks A and B
    auto task_c =
      Q.parallel_for(N,
                     { task_a, task_b },
                     [=] CS_F_HOST_DEVICE(std::size_t id) { a[id] += b[id]; });

    // Start task D asynchronously, which depends on task C.
    // NB: a single_task runs on the host, not the device.
    int  value  = 3;
    auto task_d = Q.single_task(
      { task_c },
      [=](int *data) -> void {
        for (int i = 1; i < N; i++) {
          data[0] += data[i];
        }

        data[0] /= value;
      },
      a);

    // NB: Tasks are waited upon at destruction so this is not necessary:
    // task_d.wait();
  }

  CS_FREE_HD(a);
  CS_FREE_HD(b);
}