Round-Robin Policy
##################

The dynamic selection API is an experimental feature in the |onedpl_long|
(|onedpl_short|) 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 round-robin policy cycles through the set of resources at each selection. ``round_robin_policy``
is useful for offloading kernels of similar cost to devices of similar
capabilities. In those cases, a round-robin assignment of kernels to devices
will achieve a good load balancing.

.. code:: cpp

  namespace oneapi::dpl::experimental {
  template <typename ResourceType = sycl::queue, typename ResourceAdapter = oneapi::dpl::identity,
          typename Backend = default_backend<ResourceType, ResourceAdapter>>
    class round_robin_policy
      : public policy_base<round_robin_policy<ResourceType, ResourceAdapter, Backend>,
                           ResourceAdapter, Backend>
    {
      public:
        using resource_type = ResourceType;
        using backend_type = Backend;

        round_robin_policy(deferred_initialization_t);
        round_robin_policy();
        round_robin_policy(const std::vector<ResourceType>& u, ResourceAdapter adapter = {});

        // deferred initializer
        void initialize();
        void initialize(const std::vector<resource_type>& u);
        // other implementation defined functions...
    };
  
  }

This policy can be used with all the dynamic selection :doc:`free functions <functions>`,
as well as with :ref:`policy traits <policy-traits>`.

Example
-------

The following example demonstrates a simple approach to send work to each 
queue in a set of queues, and then wait for all devices to complete the work
before repeating the process. A ``round_robin_policy`` is used rotate through
the available devices.

.. code:: cpp

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

  const std::size_t N = 10000;
  namespace ex = oneapi::dpl::experimental;

 void f(sycl::handler& h, float* v);


  int round_robin_example(std::vector<sycl::queue>& similar_devices,
                          std::vector<float*>& usm_data) {

    ex::round_robin_policy p{similar_devices}; // (1)

    auto num_devices = p.get_resources().size();
    auto num_arrays = usm_data.size();

    // (2)
    auto submission_group_size = (num_arrays < num_devices) ? num_arrays : num_devices;

    std::cout << "Running with " << num_devices << " queues\n"
              << "             " << num_arrays  << " usm arrays\n"
              << "Will perform " << submission_group_size << " concurrent offloads\n";

    for (int i = 0; i < 100; i += submission_group_size) { // (3)
      for (int j = 0; j < submission_group_size; ++j) {  // (4)
        ex::submit(p, [&](sycl::queue q) { // (5)
          float* data = usm_data[j];
          return q.submit([=](sycl::handler &h) { // (6)
            f(h, data);
          });
        }); 
      }   
      ex::wait(p.get_submission_group()); // (7)
    }
    return 0;
  }

The key points in this example are:

#. A ``round_robin_policy`` is constructed that rotates between the CPU and GPU queues.
#. The total number of concurrent offloads, ``submission_group_size``, will be limited to the number of USM arrays or the number of queues, whichever is smaller. 
#. The outer ``i``-loop iterates from 0 to 99, stepping by the ``submission_group_size``. This number of submissions will be offloaded concurrently.
#. The inner ``j``-loop iterates over ``submission_group_size`` submissions.
#. ``submit`` is used to select a queue and pass it to the user's function, but does not block until the event returned by that function completes. This provides the opportunity for concurrency across the submissions.
#. 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``.
#. ``wait`` is called to block for all the concurrent ``submission_group_size`` submissions to complete.

Selection Algorithm
-------------------
 
The selection algorithm for ``round_robin_policy`` rotates through
the elements of the set of available resources. A simplified, expository 
implementation of the selection algorithm follows:
 
.. code:: cpp

  //not a public function, for exposition purposes only
  template<typename ...Args>
  selection_type round_robin_policy::select(Args&&...) {
    if (initialized_) {
      auto& r = resources_[next_context_++ % num_resources_];
      return selection_type{*this, r};
    } else {
      throw std::logic_error("selected called before initialization");
    }
  }

where ``resources_`` is a container of resources, such as 
``std::vector`` of ``sycl::queue``, ``next_context_`` is 
a counter that increments at each selection, and ``num_resources_``
is the size of the ``resources_`` vector.

Constructors
------------

``round_robin_policy`` provides three constructors.

.. list-table::
  :widths: 50 50
  :header-rows: 1

  * - Signature
    - Description
  * - ``round_robin_policy(deferred_initialization_t);``
    - Defers initialization. An ``initialize`` function must be called prior to use.
  * - ``round_robin_policy();``
    - Initialized to use the default set of resources.
  * - | ``round_robin_policy(``
      |   ``const std::vector<ResourceType>& u,``
      |   ``ResourceAdapter adapter = {});``
    - Overrides the default set of resources with an optional resource adapter.

Deferred Initialization
-----------------------

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

.. list-table::
  :widths: 50 50
  :header-rows: 1

  * - Signature
    - Description
  * - ``initialize();``
    - Initialize to use the default set of resources.
  * - ``initialize(const std::vector<resource_type>& u);``
    - Overrides the default set of resources.

Queries
-------

A ``round_robin_policy`` has ``get_resources`` and ``get_submission_group``
member functions.

.. list-table::
  :widths: 50 50
  :header-rows: 1

  * - 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.
