Ecosyste.ms: Awesome

An open API service indexing awesome lists of open source software.

Awesome Lists | Featured Topics | Projects

https://github.com/StanfordLegion/legion

The Legion Parallel Programming System
https://github.com/StanfordLegion/legion

Last synced: 7 days ago
JSON representation

The Legion Parallel Programming System

Awesome Lists containing this project

README 

        
# Legion 



[Legion](http://legion.stanford.edu) is a parallel programming model

for distributed, heterogeneous machines.



## Branches



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](https://github.com/StanfordLegion/legion/blob/stable/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.  :)



## Overview



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.



http://theory.stanford.edu/~aiken/publications/papers/sc12.pdf



## Contents



This repository includes the following contents:



  * `tutorial`: Source code for the [tutorials](http://legion.stanford.edu/tutorial/).

  * `examples`: Larger examples for advanced programming techniques.

  * `apps`: Several complete Legion applications.

  * `language`: The [Regent programming language](http://regent-lang.org/) 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:

      * `legion_spy.py`: A [visualization tool](http://legion.stanford.edu/debugging/#legion-spy) for task dependencies.

      * `legion_prof.py`: A task-level [profiler](http://legion.stanford.edu/profiling/#legion-prof).



## Dependencies



To get started with Legion, you'll need:



  * Linux, macOS, or another Unix

  * A C++ 17 (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 tools and Python bindings)

      * Note: Python 3.8 or newer is required for `tools/legion_prof.py`

  * *Optional*: Rust 1.74 or newer (used for Rust profiler)

  * *Optional*: CUDA 10.0 or newer (for NVIDIA GPUs)

  * *Optional*: CUDA 11.7 or newer if using Legion's built-in complex reduction operators

  * *Optional*: [GASNet](https://gasnet.lbl.gov/) (for networking, see

     [installation instructions](http://legion.stanford.edu/gasnet/))

  * *Optional*: LLVM 7-14 (for dynamic code generation)

  * *Optional*: HDF5 (for file I/O)



## Installing



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:



```bash

git clone https://github.com/StanfordLegion/legion.git

export LG_RT_DIR="$PWD/legion/runtime"

cd legion/examples/circuit

make

./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=`: controls the compile-time [logging

    level](http://legion.stanford.edu/debugging/#logging-infrastructure).

  * `USE_CUDA=<0,1>`: enables CUDA support.

  * `USE_GASNET=<0,1>`: enables GASNet support (see [installation instructions](http://legion.stanford.edu/gasnet/)).

  * `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).



  * `USE_SPY=1`: enables [Legion Spy](http://legion.stanford.edu/debugging/#legion-spy).

  * `PRIVILEGE_CHECKS=1`: enables [extra privilege checks](http://legion.stanford.edu/debugging/#privilege-checks).

  * `BOUNDS_CHECKS=1`: enables [dynamic bounds checks](http://legion.stanford.edu/debugging/#bounds-checks).



## Command-Line Flags



Legion and Realm accept command-line arguments for various runtime

parameters. Below are some of the more commonly used flags:



  * `-level =`:

    sets [logging level](http://legion.stanford.edu/debugging/#logging-infrastructure) for `category`

  * `-logfile `:

    directs [logging output](http://legion.stanford.edu/debugging/#logging-infrastructure) to `filename`

  * `-ll:cpu `: CPU processors to create per process

  * `-ll:gpu `: GPU processors to create per process

  * `-ll:util `: utility processors to create per process

  * `-ll:csize `: size of CPU DRAM memory per process (in MB)

  * `-ll:gsize `: size of GASNET global memory available per process (in MB)

  * `-ll:rsize `: size of GASNET registered RDMA memory available per process (in MB)

  * `-ll:fsize `: size of framebuffer memory for each GPU (in MB)

  * `-ll:zsize `: size of zero-copy memory for each GPU (in MB)

  * `-lg:window `: maximum number of tasks that can be created in a parent task window

  * `-lg:sched `: 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 `default_mapper.cc` 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`.



## Debugging



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:



```bash

./app -lg:spy -logfile spy_%.log

$LG_RT_DIR/../tools/legion_spy.py -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.



```bash

DEBUG=1 USE_SPY=1 make

./app -lg:spy -logfile spy_%.log

$LG_RT_DIR/../tools/legion_spy.py -lpa spy_*.log

$LG_RT_DIR/../tools/legion_spy.py -dez spy_*.log

```



## Profiling



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 ` 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 ` 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 `legion_prof.py`.



```bash

DEBUG=0 make

./app -lg:prof  -lg:prof_logfile prof_%.gz

$LG_RT_DIR/../tools/legion_prof.py 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.