Dynamic Selection API#

The dynamic selection API is an experimental feature in the oneAPI DPC++ Library (oneDPL). Dynamic selection provides functions for choosing a resource using a selection policy. By default, the resources selected via these APIs in oneDPL are SYCL queues. There are several functions and selection policies provided as part of the API.

The selection policies include: fixed_resource_policy that always selects a specified resource, round_robin_policy that rotates between resources, dynamic_load_policy that chooses the resource that has the fewest outstanding submissions, and auto_tune_policy that chooses the best resources based on runtime profiling information.

Policy objects are used as arguments to the dynamic selection functions. The select function picks and returns a resource based on a policy. The submit and submit_and_wait functions select a resource and then pass the chosen resource to a developer-provided function.

The following code example shows some of the key aspects of the API. The use of any empty single_task is for syntactic demonstration purposes only; any valid command group or series of command groups can be submitted to the selected queue.

#include <oneapi/dpl/dynamic_selection>
#include <sycl/sycl.hpp>
#include <iostream>

namespace ex = oneapi::dpl::experimental;

int main() {

  // (1) create a policy object
  ex::round_robin_policy p{ { sycl::queue{ sycl::cpu_selector_v },
                              sycl::queue{ sycl::gpu_selector_v } } };

  for (int i = 0; i < 6; ++i) {

    // (2) call one of the dynamic selection functions
    //     -- pass the policy to the API function
    //     -- provide a function to be called with a selected queue
    //        -- the user function must receive a sycl queue
    //        -- the user function must return a sycl event
    auto done = ex::submit(p,
                           // (3) use the selected queue in user function
                           [=](sycl::queue q) {
                            std::cout << "submit task to "
                                      << ((q.get_device().is_gpu()) ? "gpu\n" : "cpu\n");
                            return q.single_task([]() { /* work here */ });

    // (4) each submission can be waited on using the returned object

  // (5) and/or all submissions can be waited on as a group

In the preceding example, the key points in the code include:

  1. A policy object is created. In this example, the policy is a round_robin_policy that will rotate between a CPU and GPU SYCL queue.

  2. The submit function is called in a loop. The arguments to the call include the policy object and user-provided function.

  3. The user-provided function receives a SYCL queue (selected by the policy) and returns a SYCL event that represents the end of the chain of work that was submitted to the queue.

  4. The submit function returns an object that can be waited on. Calling wait on the done object blocks the main thread until the work submitted to the queue by your function is complete.

  5. The whole group of submissions made during the loop can be waited on. In this example, the call is redundant, since each submission was already waited for inside of the loop body.

The output from this example is:

submit task to cpu
submit task to gpu
submit task to cpu
submit task to gpu
submit task to cpu
submit task to gpu

And shows that the user function is passed alternating queues.

More detailed information about the API is provided in the following sections: