NCCL v2.28.9-1 Release

NCCL v2.28 Update 2 -

• Fix operation ordering between main thread and proxy thread to prevent hangs at large scale.
• Fix Issue #1893, a bug fix in GIN.

NCCL v2.28.7-1 Release

GPU-Initiated Networking (GIN)

• Provides device-side API for integrating GPU-Initiated Networking
  capability into application kernels.
• New transport layer called DOCA GPUNetIO.
• New ncclGin construct to create, destroy and manipulate GIN contexts.
• New ncclGinBarrierSession to provide synchronization functionality.
• New put, signal, counter operations for data movement and signaling.
• GIN API signatures and functionalities are subject to change.
• GIN Support Requirements
  • CUDA 12.2 or later when compiling the GPU code
  • NVIDIA GPUs: Volta or newer. NVIDIA GPU drivers >= 510.40.3
  • NVIDIA NICs: CX4 or newer. rdma-core >= 44.0
  • Requires nvidia-peermem or DMABUF support. When using DMABUF, linux
    kernel >= 6.1 is required.

New ncclCommRevoke API for fault tolerance

• Introduces ncclCommRevoke to quiesce ongoing NCCL work on a
  communicator without freeing resources.
• This answers the need for a lightweight way to cancel in-flight
  collectives and bring a communicator to a safe state before
  split/shrink/finalize/destroy.
• Includes optional cross-rank coordination (global barrier) and
  supports blocking/non-blocking usage.

New NCCL Environment Plugin

• The env plugin allows users to set NCCL environment variables, for
  example, after loading them from a centralized database.
• The NCCL_ENV_PLUGIN variable can be used to let NCCL load an external
  environment plugin.

New NCCL Examples on GitHub

• The NCCL examples directory provides users and developers with
  practical code samples that highlight NCCL’s core features.
• It covers basic operations like communicator initialization,
  point-to-point communication, and collective operations, as well as
  advanced features such as user buffer registration, symmetric memory,
  and the device API.

Device API improvements

• Adds ncclFindWindow API.
• Adds new ncclBarrierSession to provide hybrid synchronization
  functionality.
• Makes multimem available with as few as two ranks.
• Removes distance (NCCL_P2P_LEVEL) considerations from determining the
  availability of symmetric memory.

Enhanced NCCL RAS output

• Extends RAS subsystem with JSON format to support machine-parsable
  metrics collection.
• Enables structured data export for monitoring tools, dashboards, and
  automated analysis systems.

Github Pull Requests resolved

• Fast Init - CPU Optimizations for NCCL Initialization Large Scale.
  (PR #1789)
• Fast Init - Improve Bootstrap AllGather by 2x at large scale by
  sending bootstrap information bidirectionally. (PR #1791)
• Fixes spurious failures when PyTorch is statically linked with
  NCCL-2.28.3 because error is not drained, but rather gets propagated
  into the next CUDA kernel invocation. (PR #1864)

Other notable improvements

• Fixes multicast object leaks in case of failed NVLS user buffer
  registrations, which could lead to crashes. Avoids such registration
  attempts in case of the use of incompatible memory allocators.
• Fixes potential data corruption with built-in symmetric kernels for
  small messages with size granularity under 8 bytes or when multiple
  symmetric operations were aggregated in a group.
• Generalizes the existing point-to-point scheduling to the case of
  un-even GPU count per node.
• Fixes a crash when network plugin assignment fails.
• Fixes a large performance issue with NCCL_CROSS_NIC=0 and certain
  split mask settings, where NCCL cannot find a viable ring.
• Fixes crash when NCCL is compiled with recent CUDA versions but
  running on hosts with certain specific older CUDA drivers.

NCCL v2.28.3-1 Release

See the NCCL 2.28.3 Release Notes for more information

Device API (Experimental)

• Introduces device-side APIs to integrate NCCL communication directly into application kernels.
• Supports LSA (Load/Store Access) for CUDA P2P communication over NVLink and some PCIe platforms.
• Supports Multimem for hardware multicast using NVLink SHARP.
• Adds initial framework for GIN (GPU-Initiated Networking), currently under development.
• Introduces device communicators created using ncclDevCommCreate.
• Enables device-side communication operations with synchronization (ncclLsaBarrierSession) and memory accessors (ncclGetLsaPointer, ncclGetLsaMultimemPointer).
• Experimental APIs - signatures and functionality may evolve in future releases.
• No ABI compatibility is guaranteed — applications must be recompiled with each new NCCL release.

Symmetric memory improvements

• Support for aggregating symmetric operations using ncclGroupStart/End APIs.
• Reimplement symmetric kernels using device API.

New Host APIs

• Introduce new host collective APIs: ncclAlltoAll, ncclScatter, ncclGather.

CE (Copy Engine) Collectives

• Reduce SM utilization for alltoall, scatter, gather, and allgather within a single (MN)NVL domain.
• Free up SM capacity for the application to do computation at the same time.
• To enable the feature for ncclAllGather, ncclAlltoAll, ncclGather, ncclScatter, register buffers into symmetric windows and use the NCCL_CTA_POLICY_ZERO flag in the communicator config_t.

NCCL Inspector Plugin

• Introduces an Inspector plugin for always-on performance monitoring.
• Produces structured JSON output with metadata, execution time, bandwidth, and optional event traces for each NCCL operation.
• Enables integration with analysis tools such as Performance Exporter to visualize NCCL performance bottlenecks.
• Lightweight to enable via environment variables NCCL_PROFILER_PLUGIN and NCCL_INSPECTOR_ENABLE.

CMake support (Experiemental)

• Adds a CMake build system as an alternative to existing Makefiles.
• Known issues: pkg.build and Device API currently do not work with CMake.
• The known issues will be addressed in a future release.

Decreased max CTA count from 32 to 16 on Blackwell

• SM overhead is decreased by 50% with this improvement.
• This may cause some perf drop on Blackwell because of the reduced SM usage.
• If the extra SM capacity is not desired, two options are available to restore to previous behavior: 1) Setting NCCL_MIN_CTAS=32 NCCL_MAX_CTAS=32 environment variables; 2) setting communicator config to over-write max CTA count to 32.
• Based on community feedback, future versions may consider different trade-offs between performance and SM overhead.

Plugins

• Network
  • App-aware Network plugin. NCCL passes information about communication operations to be executed on the network end point. This allows for better tuning of network end points and their use in the plugins.
  • Improve handling of physical and virtual network devices and load/unload.
  • Network plugin version 11 - add explicit context and communication ID support for per communicator init/finalize.
  • Add Multi-Request Net API. Using this will help NCCL to anticipate multiple send/recv requests and optimize for it. See maxMultiRequestSize field in ncclNetProperties_v11_t.
• Profiler
  • Add support for API events (group, collective, and p2p) and for tracking kernel launches in the profiler plugin.
  • Add Inspector Profiler Plugin (see section above).
  • Add a hook to Google’s CoMMA profiler on github.
• Tuner
  • Expose NCCL tuning constants at tuner initialization via ncclTunerConstants_v5_t.
  • Add NVL Domain Information API.
• Support multiple plugin types from a single shared object.

New Parameterization and ncclConfig changes:

• Add new option NCCL_MNNVL_CLIQUE_ID=-2 which will use rack serial number to partition the MNNVL clique. This will limit NVLink domains to GPUs within a single rack.
• Add NCCL_NETDEVS_POLICY to control how NET devices are assigned to GPUs. The default (AUTO) is the policy used in previous versions.
• Add NCCL_SINGLE_PROC_MEM_REG_ENABLE control variable to enable NVLS UB registration in the “one process, multiple ranks” case as opt in.
• Move nChannelsPerNetPeer into ncclConfig. NCCL_NCHANNELS_PER_NET_PEER can override the value in ncclConfig.
• Enable PxN over C2C by default
  • PxN over C2C will improve performance for Grace-Blackwell platforms by allowing NCCL to leverage the NIC attached to a peer GPU over NVLINK, C2C, and PCIe.
  • This behavior can be overridden by setting NCCL_PXN_C2C=0.

Other Improvements:

• Allow FP8 support for non-reductive operations on pre sm90 devices. (See pytorch/pytorch#151594 (comment))
• Fix NVLS+CollNet and temporarily disables COLLNET_CHAIN for >8 GPUs.
• Only consider running interfaces for socket traffic. NCCL will not attempt to use interfaces that do not have the IFF_RUNNING bit. (#1798)
• Modernize mutex management. Convert to std::mutex and std::lock_guard.
• Remove sm35 and sm50 GENCODE targets which have long been deprecated and were causing issues with the latest NCCL release builds.
• Improved NVLS/NVLSTree tuning prediction to improve algorithm and protocol selection.
• NVLSTree Tuning Fixes. Update tuning data for H100, GB200-NV72.
• Respond better to RoCE link flaps. Instead of reporting an “unknown event” it will now report “GID table changed”.
• Move libvirt bridge interface to the end of possible interfaces so that they are considered last. These interfaces are usually virtual bridges to relay traffic to containers running on the host and cannot be used for traffic to a remote node and are therefore unsuitable.