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A Data-Centric Parallel Programming System


Getting Started


To get started with Legion, you’ll need:

  • Linux, macOS, or another Unix
  • A C++ 11 (or newer) compiler (GCC, Clang, Intel, or PGI) and GNU Make
  • Optional: CMake 3.16 or newer
  • Optional: Python 3.5 (used for profiling/debugging tools)
  • Optional: CUDA 10.0 or newer (for NVIDIA GPUs)
  • Optional: GASNet (for networking, see installation instructions)
  • Optional: LLVM 7-14 (for dynamic code generation)
  • Optional: HDF5 (for file I/O)


Download Legion from Github:

git clone

To test, find an example you’d like to try and run make. For example:

export LG_RT_DIR="$PWD/legion/runtime"
cd legion/examples/circuit


The top-level contents of the repository include:

  • tutorial: Source code for the tutorials.
  • examples: Larger examples for advanced programming techniques.
  • apps: Several complete Legion applications.
  • language: The Regent programming language compiler and examples.
  • runtime: The core runtime components:
    • legion: The Legion runtime itself (see legion.h).
    • realm: The Realm low-level runtime (see realm.h).
    • mappers: Several mappers, including the default mapper (see default_mapper.h).
  • tools: Miscellaneous tools:

The rest of this page covers how to begin using the Legion runtime.

Makefile Variables

The Legion Makefile includes several variables which influence the build. These may either be set in the environment (e.g. DEBUG=0 make) or at the top of each application’s Makefile.

  • DEBUG=<0,1>: controls optimization level and enables various dynamic checks which are too expensive for release builds.
  • OUTPUT_LEVEL=<level_name>: controls the compile-time logging level.
  • USE_CUDA=<0,1>: enables CUDA support. If enabled, CUDA (or CUDA_TOOLKITHOME) should be set to the CUDA install location (e.g. /usr/local/cuda).
  • USE_GASNET=<0,1>: enables GASNet support (see installation instructions). If enabled, GASNET (or GASNET_ROOT) should be set to the GASNet installation location, and CONDUIT must be set to the desired GASNet conduit (e.g. ibv, gemini, aries).
  • USE_LLVM=<0,1>: enables LLVM support. If enabled, and an llvm-config binary is not in your path, specify its location with LLVM_CONFIG.
  • USE_HDF=<0,1>: enables HDF5 support. If enabled, and HDF5 is not installed in the standard include/library directories, specify the install location using HDF_ROOT.

Build Flags

In addition to Makefile variables, compilation is influenced by a number of build flags. These flags may be added to the environment variable CC_FLAGS (or again set inside the Makefile).

Command-Line Flags

Legion and Realm accept command-line arguments for various runtime parameters. Below are some of the more commonly used flags:

  • -level <category>=<int>: sets logging level for category
  • -logfile <filename>: directs logging output to filename
  • -ll:cpu <int>: CPU processors to create per process
  • -ll:gpu <int>: GPU processors to create per process
  • -ll:util <int>: utility processors to create per process
  • -ll:csize <int>: size of CPU DRAM memory per process (in MB)
  • -ll:gsize <int>: size of GASNet global memory available per process (in MB)
  • -ll:rsize <int>: size of GASNet registered RDMA memory available per process (in MB)
  • -ll:fsize <int>: size of framebuffer memory for each GPU (in MB)
  • -ll:zsize <int>: size of zero-copy memory for each GPU (in MB)
  • -lg:window <int>: maximum number of tasks that can be created in a parent task window
  • -lg:sched <int>: minimum number of tasks to try to schedule for each invocation of the scheduler

The default mapper also has several flags for controlling the default mapping. See for more details.


Now that a working version of Legion has been established we recommend that users follow the tutorials to begin using Legion.