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Workspace placement

Workspace placement#

The usage of prepended namespace specifiers oneapi::mkl::dft is omitted below for conciseness.

DFT implementations often require temporary storage for intermediate data whilst computing DFTs. This temporary storage is referred to as a workspace. Whilst descriptor objects manage their own workspace automatically by default (config_value::WORKSPACE_AUTOMATIC set for config_param::WORKSPACE_PLACEMENT), it may be preferable to provide them with an external workspace (config_value::WORKSPACE_EXTERNAL set for config_param::WORKSPACE_PLACEMENT) for the following reasons:

  • to reduce the number of mallocs / frees;

  • to reduce memory consumption.

For some backends and configurations, externally-managed workspaces may reduce performance.

A typical workflow for using externally-managed workspaces is given below.

Automatically- and externally-managed workspaces#

For any descriptor object, its configuration value associated with configuration parameter config_param::WORKSPACE_PLACEMENT determines whether it manages its workspace automatically (configuration value config_value::WORKSPACE_AUTOMATIC) or if it is to be provided with an external workspace (configuration value config_value::WORKSPACE_EXTERNAL).

Automatically-managed workspace

The default value for the config_param::WORKSPACE_PLACEMENT is config_value::WORKSPACE_AUTOMATIC.

When set to config_value::WORKSPACE_AUTOMATIC the user does not need to provide an external workspace. The workspace will be automatically managed by the backend library.

Externally-managed workspace

The configuration parameter config_param::WORKSPACE_PLACEMENT can be set to config_value::WORKSPACE_EXTERNAL to allow the workspace to be set manually.

When a descriptor is committed with config_value::WORKSPACE_EXTERNAL set for config_param::WORKSPACE_PLACEMENT, the user must provide an external workspace before calling any compute function. More details can be found in the section dedicated to the set_workspace member function and in the typical usage illustrated below.

Typical usage of externally-managed workspaces#

The usage of config_value::WORKSPACE_EXTERNAL typically involves the following order of operations:

  1. config_value::WORKSPACE_EXTERNAL is set for the uncommitted descriptor’s config_param::WORKSPACE_PLACEMENT.

  2. The descriptor is committed.

  3. The required workspace size is queried.

  4. A workspace of sufficient size is provided to the descriptor.

  5. Compute functions following the type of external workspace provided are called.

  6. The user is responsible for freeing the external workspace.

This is shown in the following example code:

namespace dft = oneapi::mkl::dft;
// Create a descriptor
dft::descriptor<dft::precision::SINGLE, dom> desc(n);
// 1. Set the workspace placement to WORKSPACE_EXTERNAL
desc.set_value(dft::config_param::WORKSPACE_PLACEMENT,
               dft::config_value::WORKSPACE_EXTERNAL);
// Set further configuration parameters
// ...
// 2. Commit the descriptor
desc.commit(myQueue);
// 3. Query the required workspace size
std::int64_t workspaceBytes{0};
desc.get_value(dft::config_param::WORKSPACE_EXTERNAL_BYTES, &workspaceBytes);
// Obtain a sufficiently large USM allocation or buffer. For this example, a USM allocation is used.
float* workspaceUsm = sycl::malloc_device<float>(workspaceBytes / sizeof(float), myQueue);
// 4. Set the workspace
desc.set_workspace(workspaceUsm);
// 5. Now USM compute functions can be called.

Parent topic: DFT-related scoped enumeration types

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