Galois: C++ library for multi-core and multi-node parallelization


CircleCI Build Status

Galois is a C++ library designed to ease parallel programming, especially for applications with irregular parallelism (e.g., irregular amount of work in parallel sections, irregular memory accesses and branching patterns). It implements an implicitly parallel programming model, where the programmer replaces serial loop constructs (e.g. for and while) and serial data structures in their algorithms with parallel loop constructs and concurrent data structures provided by Galois to express their algorithms. Galois is designed so that the programmer does not have to deal with low-level parallel programming constructs such as threads, locks, barriers, condition variables, etc.

Highlights include: - Parallel for_each loop that handles dependencies between iterations, as well as dynamic work creation, and a do_all loop for simple parallelism. Both provide load balancing and excellent scalability on multi-socket systems - A concurrent graph library designed for graph analytics algorithms as well as other domains such as irregular meshes. - Scalable concurrent containers such as bag, vector, list, etc.

Galois is released under the BSD-3-Clause license.

Building Galois

You can checkout the latest release by typing (in a terminal):

git clone -b release-5.0

The master branch will be regularly updated, so you may try out the latest development code as well by checking out master branch:

git clone


Galois builds, runs, and has been tested on GNU/Linux. Even though Galois may build on systems similar to Linux, we have not tested correctness or performance, so please beware.

At the minimum, Galois depends on the following software:

  • A modern C++ compiler compliant with the C++-17 standard (gcc >= 7, Intel >= 19.0.1, clang >= 7.0)
  • CMake (>= 3.13)
  • Boost library (>= 1.58.0, we recommend building/installing the full library)
  • libllvm (>= 7.0 with RTTI support)
  • libfmt (>= 4.0)

Here are the dependencies for the optional features:

  • Linux HUGE_PAGES support (please see Performance will most likely degrade without HUGE_PAGES enabled. Galois uses 2MB huge page size and relies on the kernel configuration to set aside a large amount of 2MB pages. For example, our performance testing machine (4x14 cores, 192GB RAM) is configured to support up to 65536 2MB pages: Shell cat /proc/meminfo | fgrep Huge AnonHugePages: 104448 kB HugePages_Total: 65536 HugePages_Free: 65536 HugePages_Rsvd: 0 HugePages_Surp: 0 Hugepagesize: 2048 kB

  • libnuma support. Performance may degrade without it. Please install libnuma-dev on Debian like systems, and numactl-dev on Red Hat like systems.

  • Doxygen (>= 1.8.5) for compiling documentation as webpages or latex files
  • PAPI (>= ) for profiling sections of code
  • Vtune (>= 2017 ) for profiling sections of code
  • MPICH2 (>= 3.2) if you are interested in building and running distributed system applications in Galois
  • CUDA (>= 8.0 and < 11.0) if you want to build GPU or distributed heterogeneous applications. Note that versions >= 11.0 use an incompatible CUB module and will fail to execute.
  • Eigen (3.3.1 works for us) for some matrix-completion app variants

Compiling and Testing Galois

We use CMake to streamline building, testing and installing Galois. In the following, we will highlight some common commands.

Let's assume that SRC_DIR is the directory where the source code for Galois resides, and you wish to build Galois in some BUILD_DIR. Run the following commands to set up a build directory:

SRC_DIR=`pwd` # Or top-level Galois source dir

mkdir -p $BUILD_DIR

You can also set up a Debug build by running the following instead of the last command above:


Galois applications are in lonestar directory. In order to build a particular application:

make -C $BUILD_DIR/lonestar/<app-dir-name> -j
# or alternatively
make -C $BUILD_DIR <app-executable-name> -j
# or
cmake --build $BUILD_DIR <app-executable-name> --parallel

You can also build everything by running make -j in the top-level of build directory, but that may take a lot of time.

Setting the BUILD_SHARED_LIBS to ON when calling CMake will make the core runtime library be built as a shared object instead of a static library.

The tests for the core runtime will be built by default when you run make with no target specified. They can be also built explicitly with:

make -C $BUILD_DIR/test

We provide a few sample inputs that can be downloaded by running:

make -C $BUILD_DIR input

make input will download a tarball of inputs and extract it to $BUILD_DIR/inputs/small_inputs directory. The tarball is downloaded to $BUILD_DIR/inputs

Most of the Galois apps have corresponding tests. These tests depend on downloading the reference inputs and building the corresponding apps and test binaries. Once the reference inputs have been downloaded and everything has been built, the tests for the core library and all the apps can be run by running:

make test
# or alternatively

in the build directory.

Running Galois Applications

Graph Format

Many Galois/Lonestar applications work with graphs. We store graphs in a binary format called galois graph file (.gr file extension). Other formats such as edge-list or Matrix-Market can be converted to .gr format with graph-convert tool provided in galois. You can build graph-convert as follows:

make graph-convert
./tools/graph-convert/graph-convert --help

Other applications, such as Delaunay Mesh Refinement may read special file formats or some may even generate random inputs on the fly.


All Lonestar applications take a -t command-line option to specify the number of threads to use. All applications run a basic sanity check (often insufficient for correctness) on the program output, which can be turned off with the -noverify option. You can specify -help command-line option to print all available options.

Upon successful completion, each application will produce some stats regarding running time of various sections, parallel loop iterations and memory usage, etc. These stats are in CSV format and can be redirected to a file using -statFile option. Please refer to the manual for details on stats.

Running LonestarGPU applications

Please refer to lonestar/analytics/gpu/ and lonestar/scientific/gpu/ for more details on compiling and running LonestarGPU applications.

Running Distributed Galois

Please refer to lonestar/analytics/distributed/ for more details on running distributed benchmarks.


Galois documentation is produced using doxygen, included in this repository, which includes a tutorial, a user's manual and API documentation for the Galois library.

Users can build doxygen documentation in the build directory using:

make doc
your-fav-browser html/index.html &

See online documentation at:

Source-Tree Organization

  • libgalois contains the source code for the shared-memory Galois library, e.g., runtime, graphs, worklists, etc.
  • lonestar contains the Lonestar benchmark applications and tutorial examples for Galois
  • libdist contains the source code for the distributed-memory and heterogeneous Galois library
  • lonestardist contains the source code for the distributed-memory and heterogeneous benchmark applications. Please refer to lonestardist/ for instructions on building and running these apps.
  • tools contains various helper programs such as graph-converter to convert between graph file formats and graph-stats to print graph properties

Using Galois as a library

There are two common ways to use Galois as a library. One way is to copy this repository into your own CMake project, typically using a git submodule. Then you can put the following in your CMakeLists.txt:

add_subdirectory(galois EXCLUDE_FROM_ALL)
add_executable(app ...)
target_link_libraries(app Galois::shmem)

The other common method is to install Galois outside your project and import it as a package.

If you want to install Galois, assuming that you wish to install it under INSTALL_DIR:

make install

Then, you can put something like the following in CMakeLists.txt:

find_package(Galois REQUIRED)
add_executable(app ...)
target_link_libraries(app Galois::shmem)

If you are not using CMake, the corresponding basic commands (although the specific commands vary by system) are:

c++ -std=c++14 app.cpp -I$INSTALL_DIR/include -L$INSTALL_DIR/lib -lgalois_shmem

Third-Party Libraries and Licensing

Galois includes some third party libraries that do not use the same license as Galois. This includes the bliss library (located in lonestar/include/Mining/bliss) and Modern GPU (located in libgpu/moderngpu). Please be aware of this when using Galois.

Contact Us

For bugs, please raise an issue on GiHub. Questions and comments are also welcome at the Galois users mailing list: [email protected]. You may subscribe here.

If you find a bug, it would help us if you sent (1) the command line and program inputs and outputs and (2) a core dump, preferably from an executable built with the debug build.

You can enable core dumps by setting ulimit -c unlimited before running your program. The location where the core dumps will be stored can be determined with cat /proc/sys/kernel/core_pattern.

To create a debug build, assuming you will build Galois in BUILD_DIR and the source is in SRC_DIR:

make -C $BUILD_DIR

A simple way to capture relevant debugging details is to use the script command, which will record your terminal input and output. For example,

script debug-log.txt
ulimit -c unlimited
cat /proc/sys/kernel/core_pattern
make -C $BUILD_DIR <my-app> VERBOSE=1
my-app with-failing-input

This will generate a file debug-log.txt, which you can send to the mailing list:[email protected] for further debugging or supply when opening a GitHub issue.

Repo Not Found