layout: documentation title: Building gem5 doc: gem5 documentation parent: building_extras permalink: /documentation/general_docs/building authors: Bobby R. Bruce

Building gem5

Supported operating systems and environments

gem5 has been designed with a Linux environment in mind. We test regularly on Ubuntu 18.04 and Ubuntu 20.04 to ensure gem5 functions well in these environments. Though any Linux based OS should function if the correct dependencies are installed. We ensure that gem5 is compilable with both gcc and clang (see Dependencies below for compiler version information).

Mac OS should work when compiling using the clang compiler, with all other dependencies installed. However, at present, we do not officially test our builds on Mac OS. We therefore cannot guarantee the same stability for those wishing to compile and run gem5 in Mac OS as we can in Linux-based systems. In later versions of gem5, we hope to more effectively support Mac OS through improved testing.

As of gem5 20, building and running gem5 in both Python 2 and Python 3 are supported. When testing gem5 we primarily test using Ubuntu 18.04 with Python 2, and Ubuntu 20.04 with Python 3. We would therefore advice, if possible, for users to utilize Python 2 in Ubuntu 18.04 and Python 3 in Ubuntu 20.04. Though we shall eventually drop support for Python 2, (due to its retirement), we do not intend to do so until we receive ample evidence our community has migrated to Python 3.

If running gem5 in a suitable OS/environment is not possible, we have provided pre-prepared Docker images which may be used to compile and run gem5. Please see our Docker section below for more information on this.


  • git : gem5 uses git for version control.
  • gcc 4.8+: gcc is used to compiled gem5. Version 4.8+ must be used. We support up to gcc Version 9. Please note that support for gcc 4 may be dropped in future versions of gem5.
    • Clang: Clang can also be used. At present, we support Clang 3.9 to Clang 9 (inclusive).
  • SCons : gem5 uses SCons as its build environment.
  • Python : gem5 relies on Python development libraries. As of gem5 version 20, gem5 can be compiled and run in environments using either Python 2.7 or Python 3
  • protobuf 2.1+ (Optional): The protobuf library is used for trace generation and playback.
  • Boost (Optional): The Boost library is a set of general purpose C++ libraries. It is a necessary dependency if you wish to use the SystemC implementation.

Setup on Ubuntu 18.04

If compiling gem5 on Ubuntu 18.04, or related Linux distributions, you may install all these dependencies using APT:

sudo apt install build-essential git m4 scons zlib1g zlib1g-dev \
    libprotobuf-dev protobuf-compiler libprotoc-dev libgoogle-perftools-dev \
    python-dev python-six python libboost-all-dev

This will create an environment which uses Python 2.

Setup on Ubuntu 20.04

If compiling gem5 on Ubuntu 20.04, or related Linux distributions, you may install all these dependencies using API:

sudo apt install build-essential git m4 scons zlib1g zlib1g-dev \
    libprotobuf-dev protobuf-compiler libprotoc-dev libgoogle-perftools-dev \
    python3-dev python3-six python-is-python3 libboost-all-dev

This will create an environment which uses Python 3.


For users struggling to setup an environment to build and run gem5, we provide the following Docker Images:

Ubuntu 18.04 with all optional dependencies (Python 2): (source Dockerfile).

Ubuntu 18.04 with the minimum set of dependencies (Python 2): (source Dockerfile).

Ubuntu 20.04 with all optional dependencies (Python 3): (source Dockerfile).

To obtain a docker image:

docker pull <image>

E.g., for Ubuntu 18.04 with all optional dependencies:

docker pull

Then, to work within this enviornment, we suggest using the following:

docker run -u $UID:$GID --volume <gem5 directory>:/gem5 --rm -it <image>

Where <gem5 directory> is the full path of the gem5 in your file system, and <image> is the image pulled (e.g.,

From this environment, you will be able to build and run gem5 from the /gem5 directory.

Getting the code

git clone

Building with SCons

gem5‘s build system is based on SCons, an open source build system implemented in Python. You can find more information about scons at The main scons file is called SConstruct and is found in the root of the source tree. Additional scons files are named SConscript and are found throughout the tree, usually near the files they’re associated with.

Within the root of the gem5 directory, gem5 can be built with SCons using:

scons build/{ISA}/gem5.{variant} -j {cpus}

where {ISA} is the target (guest) Instruction Set Architecture, and {variant} specifies the compilation settings. For most intents and purposes opt is a good target for compilation. The -j flag is optional and allows for parallelization of compilation with {cpus} specifying the number of threads. A single-threaded compilation from scratch can take up to 2 hours on some systems. We therefore strongly advise allocating more threads if possible.

The valid ISAs are:

  • ARCH
  • ARM
  • NULL
  • MIPS
  • X86

The valid build variants are:

  • debug has optimizations turned off. This ensures that variables won‘t be optimized out, functions won’t be unexpectedly inlined, and control flow will not behave in surprising ways. That makes this version easier to work with in tools like gdb, but without optimizations this version is significantly slower than the others. You should choose it when using tools like gdb and valgrind and don't want any details obscured, but other wise more optimized versions are recommended.
  • opt has optimizations turned on and debugging functionality like asserts and DPRINTFs left in. This gives a good balance between the speed of the simulation and insight into what's happening in case something goes wrong. This version is best in most circumstances.
  • fast has optimizations turned on and debugging functionality compiled out. This pulls out all the stops performance wise, but does so at the expense of run time error checking and the ability to turn on debug output. This version is recommended if you're very confident everything is working correctly and want to get peak performance from the simulator.
  • prof is similar to but also includes instrumentation that allows it to be used with the gprof profiling tool. This version is not needed very often, but can be used to identify the areas of gem5 that should be focused on to improve performance.
  • perf also includes instrumentation, but does so using google perftools, allowing it to be profiled with google-pprof. This profiling version is complementary to, and can probably replace it for all Linux-based systems.

These versions are summarized in the following table.

Build variantOptimizationsRun time debugging supportProfiling support

For example, to build gem5 on 4 threads with opt and targeting x86:

scons build/X86/gem5.opt -j 4


Once compiled, gem5 can then be run using:

./build/{ISA}/gem5.{variant} [gem5 options] {simulation script} [script options]

Running with the --help flag will display all the available options:

  gem5.opt [gem5 options] [script options]

gem5 is copyrighted software; use the --copyright option for details.

--version               show program's version number and exit
--help, -h              show this help message and exit
--build-info, -B        Show build information
--copyright, -C         Show full copyright information
--readme, -R            Show the readme
--outdir=DIR, -d DIR    Set the output directory to DIR [Default: m5out]
--redirect-stdout, -r   Redirect stdout (& stderr, without -e) to file
--redirect-stderr, -e   Redirect stderr to file
--stdout-file=FILE      Filename for -r redirection [Default: simout]
--stderr-file=FILE      Filename for -e redirection [Default: simerr]
                        Port (e.g., gdb) listener mode (auto: Enable if
                        running interactively) [Default: auto]
                        Port listeners will only accept connections over the
                        loopback device
--interactive, -i       Invoke the interactive interpreter after running the
--pdb                   Invoke the python debugger before running the script
--path=PATH[:PATH], -p PATH[:PATH]
                        Prepend PATH to the system path when invoking the
--quiet, -q             Reduce verbosity
--verbose, -v           Increase verbosity

Statistics Options
--stats-file=FILE       Sets the output file for statistics [Default:
--stats-help            Display documentation for available stat visitors

Configuration Options
--dump-config=FILE      Dump configuration output file [Default: config.ini]
--json-config=FILE      Create JSON output of the configuration [Default:
--dot-config=FILE       Create DOT & pdf outputs of the configuration
--dot-dvfs-config=FILE  Create DOT & pdf outputs of the DVFS configuration
                        [Default: none]

Debugging Options
                        Create breakpoint(s) at TICK(s) (kills process if no
                        debugger attached)
--debug-help            Print help on debug flags
                        Sets the flags for debug output (-FLAG disables a
--debug-start=TICK      Start debug output at TICK
--debug-end=TICK        End debug output at TICK
--debug-file=FILE       Sets the output file for debug [Default: cout]
--debug-ignore=EXPR     Ignore EXPR sim objects
                        Remote gdb base port (set to 0 to disable listening)

Help Options
--list-sim-objects      List all built-in SimObjects, their params and default


The EXTRAS scons variable can be used to build additional directories of source files into gem5 by setting it to a colon delimited list of paths to these additional directories. EXTRAS is a handy way to build on top of the gem5 code base without mixing your new source with the upstream source. You can then manage your new body of code however you need to independently from the main code base.