Intel® oneAPI Math Kernel Library (oneMKL) - Data Parallel C++ Developer Reference
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Visible to Intel only — GUID: GUID-094C2815-F54C-4D29-9ED9-CF3D24A7184F
User-Allocated Workspaces
During the FFT execution, the descriptor object may require additional memory to store intermediate results or pre-computed data. By default, the descriptor allocates this workspace internally. oneMKL DFT provides a combination of existing and new APIs to enable the user to manage this workspace.
User’s intention regarding workspace
The descriptor allocates any required workspace internally during the commit by default. The following needs to be set before commit to let oneMKL DFT know that the workspace will be provided by the user.
descriptor.set_value(oneapi::mkl::dft::config_param::WORKSPACE, oneapi::mkl::dft::config_value::WORKSPACE_EXTERNAL);
The above call will notify the descriptor to not allocate any internal workspace during the commit.
Get workspace estimate
The below optional call to get_value before the commit provides a conservative estimate of the workspace size that may be required.
size_t workspaceEstimate = 0; descriptor.get_value(oneapi::mkl::dft::config_param::WORKSPACE_ESTIMATE, &workspaceEstimate);
The estimate in bytes will be stored in the workspaceEstimate variable.
Get the exact workspace size
The below call to get_value returns the exact amount of workspace size that will be required by the descriptor for computing the FFT. This needs to be called after the commit.
size_t workspaceSize = 0; descriptor.get_value(oneapi::mkl::dft::config_param::WORKSPACE_BYTES, &workspaceSize);
The workspace size in bytes will be stored in the workspaceSize variable.
Provide the workspace to the descriptor
The below descriptor class member functions will enable the user to set/replace the workspace either in the form of SYCL buffer or device-accessible USM. If the workspace was internally allocated (config_param::WORKSPACE was not set to config_value::WORKSPACE_EXTERNAL before commit), setting the workspace will deallocate any internal workspace and user-provided workspace will be used.
Syntax
namespace oneapi::mkl::dft{
void descriptor<prec, dom>::set_workspace<data_type>(sycl::buffer<data_type, 1> &workspaceBuf);
void descriptor<prec, dom>::set_workspace<data_type>(data_type *workspaceUsm);
}
- The descriptor assumes that the workspace will be solely used for FFT, so the user must be careful not to use the same workspace area for other purposes in parallel or before finishing compute_forward/compute_backward (especially for USM as explicit synchronization is required).
In case of USM, it is assumed that the workspace is large enough. In case of Buffer, std::runtime_exception() will be thrown if the workspace size is smaller than required.
If the user calls compute_forward or compute_backward before setting the workspace a std::runtime_exception() will be thrown.
Currently, the workspace type (Buffer or USM) must be the same as the container type(Buffer or USM) used during the compute stage. Otherwise, std::runtime_exception() will be thrown.
Include Files
oneapi/mkl/dfti.hpp