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Legion is a parallel programming model for distributed, heterogeneous machines.


The Legion team uses this repository for active development, so please make sure you're using the right branch for your needs:

  • stable - This is the default branch if you clone the repository. It is generally about a month behind the master branch, allowing us to get some mileage on larger changes before foisting them on everybody. Most users of Legion should use this branch, although you should be prepared to try the master branch if you run into issues. Updates are moved to the stable branch roughly monthly, although important bug fixes may be applied directly when needed. Each batch of updates is given a "version" number, and CHANGES.txt lists the major changes.
  • master - This is the "mainline" used by the Legion team, and contains changes and bug fixes that may not have made it into the stable branch yet. If you are a user of "bleeding-edge" Legion functionality, you will probably need to be using this branch.
  • lots of other feature branches - These exist as necessary for larger changes, and users will generally want to steer clear of them. :)


Legion is a programming model and runtime system designed for decoupling the specification of parallel algorithms from their mapping onto distributed heterogeneous architectures. Since running on the target class of machines requires distributing not just computation but data as well, Legion presents the abstraction of logical regions for describing the structure of program data in a machine independent way. Programmers specify the partitioning of logical regions into subregions, which provides a mechanism for communicating both the independence and locality of program data to the programming system. Since the programming system has knowledge of both the structure of tasks and data within the program, it can aid the programmer in host of problems that are commonly the burden of the programmer:

  • Discovering/verifying correctness of parallel execution: determining when two tasks can be run in parallel without a data race is often difficult. Legion provides mechanisms for creating both implicit and explicit parallel task launches. For implicit constructs Legion will automatically discover parallelism. For explicit constructs, Legion will notify the programmer if there are potential data races between tasks intended to be run in parallel.
  • Managing communication: when Legion determines that there are data dependencies between two tasks run in different locations, Legion will automatically insert the necessary copies and apply the necessary constraints so the second task will not run until its data is available. We describe how tasks and data are placed in the next paragraph on mapping Legion programs.

The Legion programming model is designed to abstract computations in a way that makes them portable across many different potential architectures. The challenge then is to make it easy to map the abstracted computation of the program onto actual architectures. At a high level, mapping a Legion program entails making two kinds of decisions:

  1. For each task: select a processor on which to run the task.
  2. For each logical region a task needs: select a memory in which to create a physical instance of the logical region for the task to use.

To facilitate this process Legion introduces a novel runtime 'mapping' interface. One of the NON-goals of the Legion project was to design a programming system that was magically capable of making intelligent mapping decisions. Instead the mapping interface provides a declarative mechanism for the programmer to communicate mapping decisions to the runtime system without having to actually write any code to perform the mapping (e.g. actually writing the code to perform a copy or synchronization). Furthermore, by making the mapping interface dynamic, it allows the programmer to make mapping decisions based on information that may only be available at runtime. This includes decisions based on:

  • Program data: some computations are dependent on data (e.g. is our irregular graph sparse or dense in the number of edges).
  • System data: which processors or nodes are currently up or down, or which are running fast or slow to conserve power.
  • Execution data: profiling data that is fed back to the mapper about how a certain mapping performed previously. Alternatively which processors are currently over- or under- loaded.

All of this information is made available to the mapper via various mapper calls, some of which query the mapping interface while others simply are communicating information to the mapper.

One very important property of the mapping interface is that no mapping decisions are capable of impacting the correctness of the program. Consequently, all mapping decisions made are only performance decisions. Programmers can then easily tune a Legion application by modifying the mapping interface implementation without needing to be concerned with how their decisions impact correctness. Ultimately, this makes it possible in Legion to explore whole spaces of mapping choices (which tasks run on CPUs or GPUs, or where data gets placed in the memory hierarchy) simply by enumerating all the possible mapping decisions and trying them.

To make it easy to get a working program, Legion provides a default mapper implementation that uses heuristics to make mapping decisions. In general these decision are good, but they are certain to be sub-optimal across all applications and architectures. All calls in the mapping interface are C++ virtual functions that can be overridden, so programmers can extend the default mapper and only override the mapping calls that are impacting performance. Alternatively a program can implement the mapping interface entirely from scratch.

For more details on the Legion programming model and its current implementation we refer to you to our Supercomputing paper.


This repository includes the following contents:

  • 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:


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 or newer (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)


Legion is currently compiled with each application. To try a Legion application, just call make in the directory in question. The LG_RT_DIR variable is used to locate the Legion runtime directory. For example:

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

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.
  • USE_GASNET=<0,1>: enables GASNet support (see installation instructions).
  • USE_LLVM=<0,1>: enables LLVM support.
  • USE_HDF=<0,1>: enables HDF5 support.

Build Flags

In addition to Makefile variables, compilation is influenced by a number of build flags. These flags may be added to variables in the environment (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.

Developing Programs

To start a new Legion application, make a new directory and copy apps/Makefile.template into your directory under the name Makefile. Fill in the appropriate fields at the top of the Makefile with the filenames needed for your application.

Most Legion APIs are described in legion.h; a smaller number are described in the various header files in the runtime/realm directory. The default mapper is available in default_mapper.h.


Legion has a number of tools to aid in debugging programs.

Extended Correctness Checks

Compile with DEBUG=1 PRIVILEGE_CHECKS=1 BOUNDS_CHECKS=1" make and rerun the application. This enables dynamic checks for privilege and out-of-bounds errors in the application. (These checks are not enabled by default because they are relatively expensive.) If the application runs without terminating with an error, then continue on to Legion Spy.

Legion Spy

Legion provides a task-level visualization tool called Legion Spy. This captures the logical and physical dependence graphs. These may help, for example, as a sanity check to ensure that the correct sequence of tasks is being launched (and the tasks have the correct dependencies). Legion Spy also has a self-checking mode which can validate the correctness of the runtime's logical and physical dependence algorithms.

To capture a trace, invoke the application with -lg:spy -logfile spy_%.log. (No special compile-time flags are required.) This will produce a log file per node. Call the post-processing script to render PDF files of the dependence graphs:

./app -lg:spy -logfile spy_%.log
$LG_RT_DIR/../tools/ -dez spy_*.log

To run Legion Spy's self-checking mode, Legion must be built with the flag USE_SPY=1. Following this, the application can be run again, and the script used to validate (or render) the trace.

DEBUG=1 USE_SPY=1 make
./app -lg:spy -logfile spy_%.log
$LG_RT_DIR/../tools/ -lpa spy_*.log
$LG_RT_DIR/../tools/ -dez spy_*.log


Legion contains a task-level profiler. No special compile-time flags are required. However, it is recommended to build with DEBUG=0 make to avoid any undesired performance issues.

To profile an application, run with -lg:prof <N> where N is the number of nodes to be profiled. (N can be less than the total number of nodes---this profiles a subset of the nodes.) Use the -lg:prof_logfile <logfile> flag to save the output from each node to a separate file. The argument to the -lg:prof_logfile flag follows the same format as for -logfile, except that a % (to be replaced by the node number) is mandatory. Finally, pass the resulting log files to

DEBUG=0 make
./app -lg:prof <N> -lg:prof_logfile prof_%.gz
$LG_RT_DIR/../tools/ prof_*.gz

This will generate a subdirectory called legion_prof under the current directory, including a file named index.html. Open this file in a browser.

Other Features

  • Inorder Execution: Users can force the high-level runtime to execute all tasks in program order by passing -lg:inorder flag on the command-line.

  • Dynamic Independence Tests: Users can request the high-level runtime perform dynamic independence tests between regions and partitions by passing -lg:dynamic flag on the command-line.