FidelityFX Parallel Sort 1.2

FidelityFX Parallel Sort is a technique which uses a GPU-based radix sort algorithm to sort a provided buffer of keys, and an optional payload.

Shading language requirements


The technique

FidelityFX Parallel Sort will sort the provided key buffer and optional payload buffer using an RDNA-optimized GPU radix sort algorithm, which is one of the fastest sorting algorithms available for large data sets.

The algorithm works by operating over blocks of sequential data for optimal reads. A thread group will sort 1 or more blocks of data depending on the dataset size, and execute over a configurable amount of threads per thread group (currently set to 128).

In order to fully sort a buffer of 32-bit keys, the algorithm is invoked over 8 iterations, where each iteration will go through a 4-bit increment of the following steps:

  • Count: Counts the number of values in the local sort set according to the number of bits processed.

  • ReduceCount: Further reduces the value counts to thread group size alignments for faster offset calculation across thread groups.

  • ScanPrefix: Reduced counts are summed to provide thread group offsets.

  • ScanPrefix + Add: Thread counts are summed and summed with thread group offsets to provide the final offset location.

  • Scatter: Copies the source value to its new location (sorted up to n-bits processed thus far)

Example C++

Via static lib:


FfxParallelSortDispatchDescription  dispatchDesc = {};
dispatchDesc.commandList     = ffxGetCommandList(pCmdList);
dispatchDesc.keyBuffer       = ffxGetResource(m_pKeysToSort->GetResource(), L"ParallelSort_KeyBuffer", FFX_RESOURCE_STATE_PIXEL_COMPUTE_READ );
dispatchDesc.payloadBuffer   = ffxGetResource(m_pPayloadToSort->GetResource(), L"ParallelSort_PayloadBuffer", FFX_RESOURCE_STATE_PIXEL_COMPUTE_READ );
dispatchDesc.numKeysToSort   = <Number of keys to sort>;

FfxErrorCode  errorCode = ffxParallelSortContextDispatch (&m_ParallelSortContext, &dispatchDesc);
FFX_ASSERT(errorCode == FFX_OK );

See also