NCCLbpf -- eBPF-based Policy Execution for NCCL

NCCLbpf brings verified eBPF policy execution to NCCL (NVIDIA Collective Communication Library). It uses bpftime, a userspace eBPF runtime, to load and execute eBPF programs inside NCCL's plugin system. Policies run on every collective operation to govern algorithm/protocol selection, channel allocation, and transport-layer behavior -- all with static verification and process-level isolation.

This is a research prototype targeting the eBPF Workshop at SOSP 2026.

Architecture

NCCLbpf consists of two NCCL plugins and a library of eBPF policy programs:

+------------------+     +---------------------------+
|  NCCL Runtime    |     |  eBPF Policy Programs     |
|                  |     |  (noop, size_aware,        |
|  Tuner v5 hook --+---->|   slo_enforcer, ...)      |
|  Profiler v6 hook+---->|                           |
|  Net v11 hook ---+---->+---------------------------+
|                  |              |
+------------------+     bpftime (LLVM JIT + verifier)

Tuner+Profiler Plugin (src/nccl-policy-plugin/) -- Implements NCCL's Tuner v5 and Profiler v6 interfaces in a single shared library. On each getCollInfo call, it executes an eBPF policy program that receives a context (message size, collective type, rank count, profiler-fed telemetry) and returns a packed action (algorithm, protocol, channel count). The profiler adapter writes runtime latency data into shared eBPF maps, closing the telemetry loop.

Net Plugin (src/nccl-net-ebpf-plugin/) -- Wraps NCCL's built-in Socket transport (Net v11 interface). Executes eBPF hooks on init, listen, connect, accept, isend, irecv, and finalize events. Designed for transport-layer observability and policy enforcement.

eBPF Policies (src/ebpf-policies/) -- Verified eBPF programs compiled with clang -target bpf. Includes:

• noop.bpf.c -- Passthrough (no override), used for overhead measurement
• size_aware.bpf.c / size_aware_v2-v5 -- Size-based algorithm/protocol selection
• adaptive_channels.bpf.c -- Dynamic channel count adjustment using telemetry maps
• slo_enforcer.bpf.c -- SLO-driven policy using config maps and telemetry feedback
• ring_simple_all.bpf.c -- Forces RING/SIMPLE for all sizes
• bad_*.bpf.c -- Intentionally unsafe programs for verifier testing (div-by-zero, OOB access, stack overflow, infinite loop, etc.)

Prerequisites

• NVIDIA GPU with CUDA toolkit
• clang/LLVM (for BPF compilation, tested with LLVM 15+)
• cmake (>= 3.20), pkg-config
• libelf-dev, zlib1g-dev
• Pre-built bpftime (at build-bpftime/; see the bpftime repository for build instructions)
• MPI implementation (for running nccl-tests)

Build

1. Initialize submodules

git submodule update --init --recursive

2. Build NCCL

make -C nccl -j$(nproc) src.build BUILDDIR=$(pwd)/nccl/build

3. Build bpftime

Follow the instructions in the bpftime repository. The build output is expected at build-bpftime/. You can override this with -DBPFTIME_BUILD_DIR=<path> when running cmake.

4. Build the tuner+profiler plugin

cmake -S src/nccl-policy-plugin -B src/nccl-policy-plugin/build
cmake --build src/nccl-policy-plugin/build -j$(nproc)

This produces src/nccl-policy-plugin/build/libnccl-policy.so and compiles all eBPF policy objects into src/nccl-policy-plugin/build/ebpf-policies/.

5. Build the net plugin

cmake -S src/nccl-net-ebpf-plugin -B src/nccl-net-ebpf-plugin/build
cmake --build src/nccl-net-ebpf-plugin/build -j$(nproc)

This produces src/nccl-net-ebpf-plugin/build/libnccl-net-ebpf.so and compiles net_trace.bpf.o.

Usage

Run with the tuner+profiler plugin

LD_LIBRARY_PATH=nccl/build/lib \
NCCL_TUNER_PLUGIN=src/nccl-policy-plugin/build/libnccl-policy.so \
NCCL_POLICY_BPF_PATH=src/ebpf-policies/size_aware_v5.bpf.o \
mpirun -np 2 nccl-tests/build/all_reduce_perf_mpi -b 1M -e 128M -g 1

Run with the net plugin

LD_LIBRARY_PATH=nccl/build/lib \
NCCL_NET_PLUGIN=src/nccl-net-ebpf-plugin/build/libnccl-net-ebpf.so \
NCCL_NET_EBPF_BPF_PATH=src/nccl-net-ebpf-plugin/build/net_trace.bpf.o \
NCCL_NET=Socket NCCL_P2P_DISABLE=1 NCCL_SHM_DISABLE=1 \
mpirun -np 2 nccl-tests/build/all_reduce_perf_mpi -b 128M -e 128M -g 1

Environment Variables

Tuner+Profiler Plugin

Variable	Description
NCCL_TUNER_PLUGIN	Path to libnccl-policy.so
NCCL_POLICY_BPF_PATH	Path to the eBPF policy .bpf.o file to load
NCCL_POLICY_VERIFY_MODE	Verifier behavior: strict (default, reject unsafe), warning (log but allow), none (skip verification)

Net Plugin

Variable	Description
NCCL_NET_PLUGIN	Path to libnccl-net-ebpf.so
NCCL_NET_EBPF_BPF_PATH	Path to the net eBPF program .bpf.o file
NCCL_NET_EBPF_VERIFY_MODE	Verifier behavior: strict (default), warning/warn, none

Directory Structure

src/
  nccl-policy-plugin/    Tuner v5 + Profiler v6 plugin (C++)
  nccl-net-ebpf-plugin/  Net v11 plugin wrapping Socket transport (C++)
  ebpf-policies/         eBPF policy programs and shared headers
docs/
  paper/                 Workshop paper (LaTeX)
bpftime/                 Submodule: userspace eBPF runtime (bpftime)
nccl/                    Submodule: NVIDIA NCCL 2.29.7
build-bpftime/           Pre-built bpftime libraries (not checked in)

Key Dependencies

• bpftime -- Userspace eBPF runtime with LLVM JIT compilation, static verification, and shared-memory map support. Statically linked into both plugins.
• NCCL -- NVIDIA Collective Communication Library. Plugins use the Tuner v5, Profiler v6, and Net v11 plugin interfaces.

License

MIT