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Execution Policies

Execution Policies#

C++ Standard Aligned Execution Policies#

oneDPL has the set of execution policies and related utilities that are semantically aligned with the C++ Standard, 6th edition (C++20):

// Defined in <oneapi/dpl/execution>

namespace oneapi {
  namespace dpl {
    namespace execution {

      class sequenced_policy { /*unspecified*/ };
      class parallel_policy { /*unspecified*/ };
      class parallel_unsequenced_policy { /*unspecified*/ };
      class unsequenced_policy { /*unspecified*/ };

      inline constexpr sequenced_policy seq { /*unspecified*/ };
      inline constexpr parallel_policy par { /*unspecified*/ };
      inline constexpr parallel_unsequenced_policy par_unseq { /*unspecified*/ };
      inline constexpr unsequenced_policy unseq { /*unspecified*/ };

      template <class T>
      struct is_execution_policy;

      template <class T>
      inline constexpr bool is_execution_policy_v = oneapi::dpl::execution::is_execution_policy<T>::value;
    }
  }
}

See “Execution policies” in the C++ Standard for more information.

Device Execution Policy#

A device execution policy class oneapi::dpl::execution::device_policy specifies the SYCL device and queue to run oneDPL algorithms.

// Defined in <oneapi/dpl/execution>

namespace oneapi {
  namespace dpl {
    namespace execution {

      template <typename KernelName = /*unspecified*/>
      class device_policy;

      const device_policy<> dpcpp_default;

      template <typename KernelName = /*unspecified*/>
      device_policy<KernelName>
      make_device_policy( sycl::queue );

      template <typename KernelName = /*unspecified*/>
      device_policy<KernelName>
      make_device_policy( sycl::device );

      template <typename NewKernelName, typename OldKernelName>
      device_policy<NewKernelName>
      make_device_policy( const device_policy<OldKernelName>& = dpcpp_default );
    }
  }
}

dpcpp_default is a predefined execution policy object to run algorithms on the default SYCL device.

device_policy Class#

template <typename KernelName = /*unspecified*/>
class device_policy
{
public:
    using kernel_name = KernelName;

    device_policy();
    template <typename OtherName>
    device_policy( const device_policy<OtherName>& );
    explicit device_policy( sycl::queue );
    explicit device_policy( sycl::device );

    sycl::queue queue() const;
    operator sycl::queue() const;
};

An object of the device_policy type is associated with a sycl::queue that is used to run algorithms on a SYCL device. When an algorithm runs with device_policy it is capable of processing SYCL buffers (passed via oneapi::dpl::begin/end), data in the host memory and data in Unified Shared Memory (USM), including USM device memory. Data placed in the host memory and USM can only be passed to oneDPL algorithms as pointers and random access iterators. The way to transfer data from the host memory to a device and back is unspecified; per-element data movement to/from a temporary storage is a possible valid implementation.

The KernelName template parameter, also aliased as kernel_name within the class template, is to explicitly provide a name for SYCL kernels executed by an algorithm the policy is passed to.

device_policy()

Construct a policy object associated with a queue created with the default device selector.

template <typename OtherName>
device_policy( const device_policy<OtherName>& policy )

Construct a policy object associated with the same queue as policy, by changing the kernel name of the given policy to kernel_name defined for the new policy.

explicit device_policy( sycl::queue queue )

Construct a policy object associated with the given queue.

explicit device_policy( sycl::device device )

Construct a policy object associated with a queue created for the given device.

sycl::queue queue() const

Return the queue the policy is associated with.

operator sycl::queue() const

Allow implicit conversion of the policy to a sycl::queue object.

make_device_policy Function#

The make_device_policy function templates simplify device_policy creation.

template <typename KernelName = /*unspecified*/>
device_policy<KernelName>
make_device_policy( sycl::queue queue )

Return a policy object associated with queue, with a kernel name possibly provided as the template argument, otherwise unspecified.

template <typename KernelName = /*unspecified*/>
device_policy<KernelName>
make_device_policy( sycl::device device )

Return a policy object to run algorithms on device, with a kernel name possibly provided as the template argument, otherwise unspecified.

template <typename NewKernelName, typename OldKernelName>
device_policy<NewKernelName>
make_device_policy( const device_policy<OldKernelName>& policy = dpcpp_default )

Return a policy object constructed from policy, with a new kernel name provided as the template argument. If no policy object is provided, the new policy is constructed from dpcpp_default.

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