Fixed-Resource Policy

The dynamic selection API is an experimental feature in the oneAPI DPC++ Library (oneDPL) that selects an execution resource based on a chosen selection policy. There are several policies provided as part of the API. Policies encapsulate the logic and any associated state needed to make a selection.

The fixed-resource policy always returns the same resource selection. fixed_resource_policy is designed for two primary scenarios:

  1. debugging the use of dynamic selection

  2. special casing a dynamic selection capable application for a specific resource when it is known to be best on that platform.

namespace oneapi::dpl::experimental {

  template<typename Backend = sycl_backend>
  class fixed_resource_policy {
  public:
    // useful types
    using resource_type = typename Backend::resource_type;
    using wait_type = typename Backend::wait_type;

    class selection_type {
    public:
      fixed_resource_policy<Backend> get_policy() const;
      resource_type unwrap() const;
    };

    // constructors
    fixed_resource_policy(deferred_initialization_t);
    fixed_resource_policy(std::size_t offset = 0);
    fixed_resource_policy(const std::vector<resource_type>& u,
                          std::size_t offset = 0);

    // deferred initializers
    void initialize(std::size_t offset = 0);
    void initialize(const std::vector<resource_type>& u,
                    std::size_t offset = 0);

    // queries
    auto get_resources() const;
    auto get_submission_group();

    // other implementation defined functions...
  };

}

This policy can be used with all the dynamic selection functions, such as select, submit, and submit_and_wait. It can also be used with policy_traits.

Example

In the following example, a fixed_resource_policy is used when the code is compiled so that it selects a specific device. When USE_CPU is defined at compile-time, this example always uses the CPU queue. When USE_GPU is defined at compile-time, it always uses the GPU queue. Otherwise, it uses an auto_tune_policy to dynamically select between these two queues. Such a scenario could be used for debugging or simply to maintain the dynamic selection code even if the best device to use is known for some subset of platforms.

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

namespace ex = oneapi::dpl::experimental;

int main() {
  std::vector<sycl::queue> r { sycl::queue{sycl::cpu_selector_v},
                               sycl::queue{sycl::gpu_selector_v} };

  const std::size_t N = 10000;
  std::vector<float> av(N, 0.0);
  std::vector<float> bv(N, 0.0);
  std::vector<float> cv(N, 0.0);
  for (int i = 0; i < N; ++i) {
    av[i] = bv[i] = i;
  }

#if USE_CPU
  ex::fixed_resource_policy p{r};    // (1) uses index 0 of r, the cpu
#elif USE_GPU
  ex::fixed_resource_policy p{r, 1}; // (2) uses index 1 of r, the gpu
#else
  ex::auto_tune_policy p{r};
#endif

  {
    sycl::buffer<float> a_b(av);
    sycl::buffer<float> b_b(bv);
    sycl::buffer<float> c_b(cv);


    for (int i = 0; i < 6; ++i) {
      ex::submit_and_wait(p, [&](sycl::queue q) { // (3)
        // (4)
        std::cout << (q.get_device().is_cpu() ? "using cpu\n" : "using gpu\n");
        return q.submit([&](sycl::handler &h) {   // (5)
          sycl::accessor a_a(a_b, h, sycl::read_only);
          sycl::accessor b_a(b_b, h, sycl::read_only);
          sycl::accessor c_a(c_b, h, sycl::read_write);
          h.parallel_for(N, [=](auto i) { c_a[i] = a_a[i] + b_a[i]; });
        });
      });
    };
  }

  for (int i = 0; i < N; ++i) {
    if (cv[i] != 2*i) {
       std::cout << "ERROR!\n";
    }
  }
  std::cout << "Done.\n";
}

The key points in this example are:

  1. If USE_CPU is defined, a fixed_resouce_policy is constructed that targets the CPU.

  2. If USE_GPU is defined, a fixed_resouce_policy is constructed that targets the GPU.

  3. submit_and_wait is invoked with the policy as the first argument. The selected queue will be passed to the user-provided function.

  4. For clarity when run, the type of device is displayed.

  5. The queue is used in a function to perform an asynchronous offload. The SYCL event returned from the call to submit is returned. Returning an event is required for functions passed to submit and submit_and_wait.

Selection Algorithm

The selection algorithm for fixed_resource_policy always returns the same specific resource from its set of resources. The index of the resource is set during construction or deferred initialization.

Simplified, expository implementation of the selection algorithm:

template<typename... Args>
selection_type fixed_resource_policy::select(Args&& ...) {
  if (initialized_) {
    return selection_type{*this, resources_[fixed_offset_]};
  } else {
    throw std::logic_error(“select called before initialization”);
  }
}

where resources_ is a container of resources, such as std::vector of sycl::queue, and fixed_offset_ stores a fixed integer offset. Both resources_ and fixed_offset are set during construction or deferred initialization of the policy and then remain constant.

Constructors

fixed_resource_policy provides three constructors.

fixed_resource_policy constructors

Signature

Description

fixed_resource_policy(deferred_initialization_t);

Defers initialization. An initialize function must be called prior to use.

fixed_resource_policy(std::size_t offset = 0);

Sets the index for the resource to be selected. Uses the default set of resources.

fixed_resource_policy(const std::vector<resource_type>& u, std::size_t offset = 0);

Overrides the default set of resources and optionally sets the index for the resource to be selected.

Deferred Initialization

A fixed_resource_policy that was constructed with deferred initialization must be initialized by calling one its initialize member functions before it can be used to select or submit.

fixed_resource_policy constructors

Signature

Description

initialize(std::size_t offset = 0);

Sets the index for the resource to be selected. Uses the default set of resources.

initialize(const std::vector<resource_type>& u, std::size_t offset = 0);

Overrides the default set of resources and optionally sets the index for the resource to be selected.

Queries

A fixed_resource_policy has get_resources and get_submission_group member functions.

fixed_resource_policy constructors

Signature

Description

std::vector<resource_type> get_resources();

Returns the set of resources the policy is selecting from.

auto get_submission_group();

Returns an object that can be used to wait for all active submissions.

Reporting Requirements

If a resource returned by select is used directly without calling submit or submit_and_wait, it may be necessary to call report to provide feedback to the policy. However, the fixed_resource_policy does not require any feedback about the system state or the behavior of the workload. Therefore, no explicit reporting of execution information is needed, as is summarized in the table below.

fixed_resource_policy reporting requirements

execution_info

is reporting required?

task_submission

No

task_completion

No

task_time

No

In generic code, it is possible to perform compile-time checks to avoid reporting overheads when reporting is not needed, while still writing code that will work with any policy, as demonstrated below:

auto s = select(my_policy);
if constexpr (report_info_v<decltype(s), execution_info::task_submission_t>)
{
  s.report(execution_info::task_submission);
}