RCCL

ROCm Communication Collectives Library

Introduction

RCCL (pronounced "Rickle") is a stand-alone library of standard collective communication routines for GPUs, implementing all-reduce, all-gather, reduce, broadcast, reduce-scatter, gather, scatter, and all-to-all. There is also initial support for direct GPU-to-GPU send and receive operations. It has been optimized to achieve high bandwidth on platforms using PCIe, xGMI as well as networking using InfiniBand Verbs or TCP/IP sockets. RCCL supports an arbitrary number of GPUs installed in a single node or multiple nodes, and can be used in either single- or multi-process (e.g., MPI) applications.

The collective operations are implemented using ring and tree algorithms and have been optimized for throughput and latency. For best performance, small operations can be either batched into larger operations or aggregated through the API.

Requirements

ROCm supported GPUs
ROCm stack installed on the system (HIP runtime & HCC or HIP-Clang)

Quickstart RCCL Build

RCCL directly depends on HIP runtime, plus the HCC C++ compiler or the HIP-Clang compiler which are part of the ROCm software stack. For ROCm installation instructions, see https://github.com/RadeonOpenCompute/ROCm.

The root of this repository has a helper script 'install.sh' to build and install RCCL on Ubuntu with a single command. It does not take a lot of options and hard-codes configuration that can be specified through invoking cmake directly, but it's a great way to get started quickly and can serve as an example of how to build/install.

./install.sh -- builds library including unit tests
./install.sh -i -- builds and installs the library to /opt/rocm/rccl; installation path can be changed with --prefix argument (see below.)
./install.sh -d -- installs all necessary dependencies for RCCL. Should be re-invoked if the build folder is removed.
./install.sh -h -- shows help
./install.sh -t -- builds library including unit tests
./install.sh -r -- runs unit tests (must be already built)
./install.sh -p -- builds RCCL package
./install.sh -s -- builds RCCL as a static library (default: shared)
./install.sh -hcc -- builds RCCL with hcc compiler; note that hcc is now deprecated. (default:hip-clang)
./install.sh --prefix -- specify custom path to install RCCL to (default:/opt/rocm)

Manual build

To build the library :

$ git clone https://github.com/ROCmSoftwarePlatform/rccl.git
$ cd rccl
$ mkdir build
$ cd build
$ CXX=/opt/rocm/bin/hipcc cmake -DCMAKE_PREFIX_PATH=/opt/rocm/ ..
$ make -j

You may substitute an installation path of your own choosing by passing CMAKE_INSTALL_PREFIX. For example:

$ CXX=/opt/rocm/bin/hipcc cmake -DCMAKE_PREFIX_PATH=/opt/rocm/ -DCMAKE_INSTALL_PREFIX=$PWD/rccl-install ..

Note: ensure rocm-cmake is installed, apt install rocm-cmake.

To build the RCCL package and install package :

Assuming you have already cloned this repository and built the library as shown in the previous section:

$ cd rccl/build
$ make package
$ sudo dpkg -i *.deb

RCCL package install requires sudo/root access because it creates a directory called "rccl" under /opt/rocm/. This is an optional step and RCCL can be used directly by including the path containing librccl.so.

Enabling peer-to-peer transport

In order to enable peer-to-peer access on machines with PCIe-connected GPUs, the HSA environment variable HSA_FORCE_FINE_GRAIN_PCIE=1 is required to be set, on top of requiring GPUs that support peer-to-peer access and proper large BAR addressing support.

Tests

There are unit tests implemented with the Googletest framework in RCCL. The unit tests require Googletest 1.10 or higher to build and execute properly (installed with the -d option to install.sh). To invoke the unit tests, go to the build folder, then the test subfolder, and execute the appropriate unit test executable(s).

Unit test names are now of the format:

CollectiveCall.[Type of test]

Filtering of unit tests should be done with environment variable and by passing the --gtest_filter command line flag, for example:

UT_DATATYPES=ncclBfloat16 UT_REDOPS=prod ./UnitTests --gtest_filter="AllReduce.C*"

will run only AllReduce correctness tests with float16 datatype. A list of available filtering environment variables appears at the top of every run. See "Running a Subset of the Tests" at https://chromium.googlesource.com/external/github.com/google/googletest/+/HEAD/googletest/docs/advanced.md for more information on how to form more advanced filters.

There are also other performance and error-checking tests for RCCL. These are maintained separately at https://github.com/ROCmSoftwarePlatform/rccl-tests. See the rccl-tests README for more information on how to build and run those tests.

NPKit

RCCL integrates NPKit, a profiler framework that enables collecting fine-grained trace events in RCCL components, especially in giant collective GPU kernels.

Please check NPKit sample workflow for RCCL as a fully automated usage example. It also provides good templates for the following manual instructions.

To manually build RCCL with NPKit enabled, pass -DNPKIT_FLAGS="-DENABLE_NPKIT -DENABLE_NPKIT_...(other NPKit compile-time switches)" with cmake command. All NPKit compile-time switches are declared in the RCCL code base as macros with prefix ENABLE_NPKIT_, and they control which information will be collected. Also note that currently NPKit only supports collecting non-overlapped events on GPU, and -DNPKIT_FLAGS should follow this rule.

To manually run RCCL with NPKit enabled, environment variable NPKIT_DUMP_DIR needs to be set as the NPKit event dump directory. Also note that currently NPKit only supports 1 GPU per process.

To manually analyze NPKit dump results, please leverage npkit_trace_generator.py.

Library and API Documentation

Please refer to the Library documentation for current documentation.

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ROCmSoftwarePlatform / rccl

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