New features of oneCCL with C API

New features of oneCCL with C API#

Note

This document describes the new C API that closely follows the NVIDIA Collective Communications Library (NCCL)* API standard. Documentation for the legacy C++ API can be found here.

Alongside the new C API, oneCCL introduces a plugin system for dynamic backend selection. This design allows oneCCL to load the most suitable backend for the hardware platform at runtime.

Available Plugins#

  • onecclLegacy

    • Supports both CPU and GPU collectives.

    • Selected by default if SYCL runtime is available.

    • Backend selection depends on the type of buffer and the stream argument:

      • For GPU buffers: stream must point to a SYCL queue.

      • For host buffers:

        • If stream is a SYCL queue, the collective is scheduled after previously submitted SYCL operations.

        • If stream is NULL, the collective executes immediately in blocking mode.

  • onecclLegacyCPU

    • Designed for CPU-only collectives.

    • Ignores the stream argument (should be NULL).

Hint

To inspect plugin selection, enable logging using CCL_LOG_LEVEL=info.

How Plugin Selection Works#

  • By default, oneCCL automatically loads the most appropriate plugin based on platform capabilities.

  • Users can override this behavior using the CCL_PLUGIN environment variable:

export CCL_PLUGIN=ONECCL_LEGACY_CPU
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