Tutorial setup

If you have not done the prior sections, you’ll need to start the docker image:

docker run -it ghcr.io/spack/tutorial:sc25

and then set Spack up like this:

git clone --depth=2 --branch=releases/v1.1 https://github.com/spack/spack
. spack/share/spack/setup-env.sh
spack repo update builtin --tag v2025.11.0
spack tutorial -y
spack bootstrap now
spack compiler find

See the Basic Installation Tutorial for full details on setup. For more help, join us in the #tutorial channel on Slack – get an invitation at slack.spack.io

Configuration Tutorial

This tutorial will guide you through various configuration options that allow you to customize Spack’s behavior with respect to software installation. There are many different configuration sections. A partial list of some key configuration sections is provided below.

Spack Configuration Sections

Name	Description
config	General settings (install location, number of build jobs, etc)
concretizer	Specialization of the concretizer behavior (reuse, unification, etc)
Mirrors	Locations where spack can look for stashed source or binary distributions
Packages	Define the compilers that Spack can use, and add rules/preferences for package concretization
Modules	Naming, location and additional configuration of Spack generated modules

The full list of sections can be viewed with spack config list. For further education, we encourage you to explore the Spack documentation on configuration files.

The principle goals of this section of the tutorial are:

1. Introduce the configuration sections and scope hierarchy

2. Demonstrate how to manipulate configurations

3. Show how to configure system assets with Spack (compilers and packages)

As such, we will primarily focus on the compilers and packages configuration sections in this portion of the tutorial.

We will explain this by first covering how to manipulate configurations from the command line and then show how this impacts the configuration file hierarchy. We will then move into compiler and package configurations to help you develop skills for getting the builds you want on your system. Finally, we will give some brief attention to more generalized Spack configurations in the config section.

For all of these features, we will demonstrate how we build up a full configuration file. For some, we will then demonstrate how the configuration affects the install command, and for others we will use the spack spec command to demonstrate how the configuration changes have affected Spack’s concretization algorithm. The provided output is all from a server running Ubuntu version 22.04.

Configuration from the command line

You can run spack config blame [section] at any point in time to see what your current configuration is. If you omit the section, then Spack will dump all the configuration settings to your screen. Let’s go ahead and run this for the concretizer section.

$ spack config blame concretizer

Notice that the spack:concretizer:reuse option is defaulted to true. For this section we’d actually like to turn reuse off so that when we demonstrate package configuration our preferences are weighted higher than available binaries for the concretizer solution selection procedure.

One of the most convenient ways to set configuration options is through the command line.

$ spack config add concretizer:reuse:false

If we rerun spack config blame concretizer we can see that the change was applied.

$ spack config blame concretizer

Notice that the reference file for this option is now different. This indicates the scope where the configuration was set in, and we will discuss how Spack chooses the default scope shortly. For now, it is important to note that the spack config command accepts an optional --scope flag so we can be more precise in the configuration process. This will make more sense after the next section which provides the definition of Spack’s configuration scopes and their hierarchy.

Configuration Scopes

Depending on your use case, you may want to provide configuration settings common to everyone on your team, or you may want to set default behaviors specific to a single user account. Spack provides six configuration scopes to handle this customization. These scopes, in order of decreasing priority, are:

Scope	Directory
Command Line	N/A
Environment	In environment base directory (in spack.yaml)
Custom	Custom directory, specified with --config-scope
User	~/.spack/
Site	$SPACK_ROOT/etc/spack/
System	/etc/spack/
Defaults	$SPACK_ROOT/etc/spack/defaults/

Spack’s default configuration settings reside in $SPACK_ROOT/etc/spack/defaults. These are useful for reference, but should never be directly edited. To override these settings, create new configuration files in any of the higher-priority configuration scopes.

A particular cluster may have multiple Spack installations associated with different projects. To provide settings common to all Spack installations, put your configuration files in /etc/spack. To provide settings specific to a particular Spack installation, you can use the $SPACK_ROOT/etc/spack directory.

For settings specific to a particular user, you will want to add configuration files to the ~/.spack directory. When Spack first checked for compilers on your system, you may have noticed that it placed your compiler configuration in this directory.

Configuration settings can also be placed in a custom location, which is then specified on the command line via --config-scope. An example use case is managing two sets of configurations, one for development and another for production preferences.

Settings specified on the command line have precedence over all other configuration scopes.

YAML Format

Spack configurations are nested YAML dictionaries with a specified schema. The configuration is organized into sections based on theme (e.g., a ‘packages’ section) and the highest-level keys of the dictionary specify the section. Spack generally maintains a separate file for each section, although environments keep them together (in spack.yaml).

When Spack checks its configuration, the configuration scopes are updated as dictionaries in increasing order of precedence, allowing higher precedence files to override lower. YAML dictionaries use a colon “:” to specify key-value pairs. Spack extends YAML syntax slightly to allow a double-colon “::” to specify a key-value pair. When a double-colon is used, instead of adding that section, Spack replaces what was in that section with the new value. For example, look at high-level config:

$ spack config blame config

---                                                   config:
/etc/spack/config.yaml:2                                suppress_gpg_warnings: True
/home/spack/spack/etc/spack/defaults/config.yaml:19     install_tree:
/home/spack/spack/etc/spack/defaults/config.yaml:20       root: $spack/opt/spack
...
/home/spack/spack/etc/spack/defaults/config.yaml:238    aliases:
/home/spack/spack/etc/spack/defaults/config.yaml:239      concretise: concretize
/home/spack/spack/etc/spack/defaults/config.yaml:240      containerise: containerize
/home/spack/spack/etc/spack/defaults/config.yaml:241      rm: remove

We can see overrides in action with:

$ spack config add config:aliases::{}
$ spack config blame config

---                                                   config:
/home/spack/.spack/config.yaml:2                        aliases: {}

The default write scope is the user scope, which overrides the defaults. You can undo this by editing the config section like:

$ spack config edit config

A configuration section appears nearly the same when managed in an environment’s spack.yaml file except that the section is nested 1 level underneath the top-level ‘spack’ key. For example the above config.yaml could be incorporated into an environment’s spack.yaml like so:

spack:
  specs: []
  view: true
  config:
    aliases:: {}

Compiler Configuration

For most tasks, we can use Spack with the compilers auto-detected the first time Spack runs on a system. As discussed in the basic installation tutorial, we can also tell Spack where compilers are located using the spack compiler add command. However, in some circumstances, we want even more fine-grained control over the compilers available. This section will teach you how to exercise that control using the compilers configuration.

We will start by opening the compilers configuration (which lives in the packages section):

$ spack config edit packages

We start with no active environment, so this will open a packages.yaml file for editing (you can also do this with an active environment):

packages:
  gcc:
    externals:
    - spec: [email protected] languages:='c,c++,fortran'
      prefix: /usr
      extra_attributes:
        compilers:
          c: /usr/bin/gcc-10
          cxx: /usr/bin/g++-10
          fortran: /usr/bin/gfortran-10
    - spec: [email protected] languages:='c,c++,fortran'
      prefix: /usr
      extra_attributes:
        compilers:
          c: /usr/bin/gcc
          cxx: /usr/bin/g++
          fortran: /usr/bin/gfortran
  llvm:
    externals:
    - spec: [email protected]+clang~flang~lld~lldb
      prefix: /usr
      extra_attributes:
        compilers:
          c: /usr/bin/clang
          cxx: /usr/bin/clang++

This specifies two versions of the GCC compiler and one version of the Clang compiler with no Flang compiler. Now suppose we have a code that we want to compile with the Clang compiler for C/C++ code, but with gfortran for Fortran components. We can do this by adding creating a toolchain config:

$ spack config edit toolchains

toolchains:
  clang_gfortran:
  - spec: '%[email protected]'
    when: '%c'
  - spec: '%[email protected]'
    when: '%cxx'
  - spec: '%[email protected]'
    when: '%fortran'

We are essentially saying “use Clang for C/C++, and use GCC for Fortran”. You can use this new entry like so:

$ spack spec openblas %clang_gfortran

Note the identifier clang_gfortran is not itself a spec (you don’t version it). You reference it in other specs. Note that without when: '%fortran', you could not use clang_gfortran with packages unless they depended on Fortran (likewise for the when statements on c/cxx).

Compiler flags

Some compilers may require specific compiler flags to work properly in a particular computing environment. Spack provides configuration options for setting compiler flags every time a specific compiler is invoked. These flags become part of the package spec and therefore of the build provenance. As on the command line, the flags are set through the implicit build variables cflags, cxxflags, cppflags, fflags, ldflags, and ldlibs.

Let’s open our compilers configuration file again and add a compiler flag:

packages:
  gcc:
    externals:
    - spec: [email protected] languages:='c,c++,fortran'
      prefix: /usr
      extra_attributes:
        compilers:
          c: /usr/bin/gcc-10
          cxx: /usr/bin/g++-10
          fortran: /usr/bin/gfortran-10
        flags:
          cppflags: -g

We can test this out using the spack spec command to show how the spec is concretized:

$ spack spec zlib%gcc@10
 -   [email protected] cppflags=-g +optimize+pic+shared build_system=makefile arch=linux-ubuntu22.04-x86_64_v3 %c,[email protected]
[+]      ^[email protected] build_system=generic arch=linux-ubuntu22.04-x86_64_v3
[e]      ^[email protected]~binutils+bootstrap~graphite~nvptx~piclibs~profiled~strip build_system=autotools build_type=RelWithDebInfo languages:='c,c++,fortran' patches:=2c18531 arch=linux-ubuntu22.04-x86_64_v3
 -       ^[email protected] cppflags=-g  build_system=generic arch=linux-ubuntu22.04-x86_64_v3
[e]      ^[email protected] build_system=autotools arch=linux-ubuntu22.04-x86_64_v3
 -       ^[email protected] cppflags=-g ~guile build_system=generic arch=linux-ubuntu22.04-x86_64_v3 %[email protected]

We can see that cppflags="-g" has been added to every node in the DAG.

Advanced compiler configuration

Some additional fields not discussed yet, in an example:

packages:
  gcc:
    externals:
    - spec: [email protected] languages:='c,c++,fortran'
      prefix: /usr
      modules:
      - gcc/11.4.0
      extra_attributes:
        compilers:
          c: /usr/bin/gcc
          cxx: /usr/bin/g++
          fortran: /usr/bin/gfortran
        flags:
          cppflags: -g
        extra_rpaths:
        - /a/path/to/somewhere/important
        environment:
          set:
            EG_A_LICENSE_FILE: 1713@license4

The modules field of the compiler was originally designed to support older Cray systems, but can be useful on any system that has compilers that are only usable when a particular module is loaded. Any modules in the modules field of the compiler configuration will be loaded as part of the build environment for packages using that compiler.

The environment field of the compiler configuration is used for compilers that require environment variables to be set during build time. For example, if your Intel compiler suite requires the INTEL_LICENSE_FILE environment variable to point to the proper license server. In addition to set, environment also supports unset, prepend_path, and append_path.

The extra_rpaths field of the compiler configuration is used for compilers that do not rpath all of their dependencies by default. Since compilers are often installed externally to Spack, Spack is unable to manage compiler dependencies and enforce rpath usage. This can lead to packages not finding link dependencies imposed by the compiler properly. For compilers that impose link dependencies on the resulting executables that are not rpath’ed into the executable automatically, the extra_rpaths field of the compiler configuration tells Spack which dependencies to rpath into every executable created by that compiler. The executables will then be able to find the link dependencies imposed by the compiler.

Configuring Package Preferences

Package preferences in Spack are managed through the packages configuration section. First, we will look at the default packages.yaml file.

$ spack config --scope=defaults:base edit packages

# -------------------------------------------------------------------------
# This file controls default concretization preferences for Spack.
#
# Settings here are versioned with Spack and are intended to provide
# sensible defaults out of the box. Spack maintainers should edit this
# file to keep it current.
#
# Users can override these settings by editing the following files.
#
# Per-spack-instance settings (overrides defaults):
#   $SPACK_ROOT/etc/spack/packages.yaml
#
# Per-user settings (overrides default and site settings):
#   ~/.spack/packages.yaml
# -------------------------------------------------------------------------
packages:
  all:
    providers:
      awk: [gawk]
      armci: [armcimpi]
      blas: [openblas, amdblis]
      c: [gcc, llvm, intel-oneapi-compilers]
      cxx: [gcc, llvm, intel-oneapi-compilers]
      D: [ldc]
      daal: [intel-oneapi-daal]
      elf: [elfutils]
      fftw-api: [fftw, amdfftw]
      flame: [libflame, amdlibflame]
      fortran: [gcc, llvm, intel-oneapi-compilers]
      fortran-rt: [gcc-runtime, intel-oneapi-runtime]
      fuse: [libfuse]
      gl: [glx, osmesa]
      glu: [mesa-glu, openglu]
      golang: [go, gcc]
      go-or-gccgo-bootstrap: [go-bootstrap, gcc]
      iconv: [libiconv]
      ipp: [intel-oneapi-ipp]
      java: [openjdk, jdk]
      jpeg: [libjpeg-turbo, libjpeg]
      lapack: [openblas, amdlibflame]
      libc: [glibc, musl]
      libgfortran: [gcc-runtime]
      libglx: [mesa+glx]
      libifcore: [intel-oneapi-runtime]
      libllvm: [llvm]
      lua-lang: [lua, lua-luajit-openresty, lua-luajit]
      luajit: [lua-luajit-openresty, lua-luajit]
      mariadb-client: [mariadb-c-client, mariadb]
      mkl: [intel-oneapi-mkl]
      mpe: [mpe2]
      mpi: [openmpi, mpich]
      mysql-client: [mysql, mariadb-c-client]
      opencl: [pocl]
      onedal: [intel-oneapi-dal]
      pbs: [openpbs, torque]
      pil: [py-pillow]
      pkgconfig: [pkgconf, pkg-config]
      qmake: [qt-base, qt]
      rpc: [libtirpc]
      scalapack: [netlib-scalapack, amdscalapack]
      sycl: [hipsycl]
      szip: [libaec, libszip]
      tbb: [intel-tbb]
      unwind: [libunwind]
      uuid: [util-linux-uuid, libuuid]
      wasi-sdk: [wasi-sdk-prebuilt]
      xkbdata-api: [xkeyboard-config, xkbdata]
      xxd: [xxd-standalone, vim]
      yacc: [bison, byacc]
      ziglang: [zig]
      zlib-api: [zlib-ng+compat, zlib]
    permissions:
      read: world
      write: user
  cce:
    buildable: false
  cray-fftw:
    buildable: false
  cray-libsci:
    buildable: false
  cray-mpich:
    buildable: false
  cray-mvapich2:
    buildable: false
  cray-pmi:
    buildable: false
  egl:
    buildable: false
  essl:
    buildable: false
  fj:
    buildable: false
  fujitsu-mpi:
    buildable: false
  fujitsu-ssl2:
    buildable: false
  glibc:
    buildable: false
  hpcx-mpi:
    buildable: false
  iconv:
    prefer: [libiconv]
  mpt:
    buildable: false
  musl:
    buildable: false
  spectrum-mpi:
    buildable: false
  xl:
    buildable: false

This sets the default preferences for providers of virtual packages. We can edit this to change provider preferences and also to create a preference for compilers. To illustrate how this works, suppose we want to change the preferences to prefer the Clang compiler and to prefer MPICH over OpenMPI. Currently, we prefer GCC and OpenMPI.

$ spack spec hdf5
 -   [email protected]~cxx~fortran~hl~ipo~java~map+mpi+shared~subfiling~szip~threadsafe+tools api=default build_system=cmake build_type=Release generator=make arch=linux-ubuntu22.04-x86_64 %[email protected]
 -       ^[email protected]~doc+ncurses+ownlibs~qtgui build_system=generic build_type=Release arch=linux-ubuntu22.04-x86_64 %c,[email protected]
 -           ^[email protected]~gssapi~ldap~libidn2~librtmp~libssh~libssh2+nghttp2 build_system=autotools libs:=shared,static tls:=openssl arch=linux-ubuntu22.04-x86_64 %c,[email protected]
 -               ^[email protected] build_system=autotools arch=linux-ubuntu22.04-x86_64 %c,[email protected]
 -                   ^[email protected] build_system=autotools arch=linux-ubuntu22.04-x86_64 %[email protected]
 -               ^[email protected]~docs+shared build_system=generic certs=mozilla arch=linux-ubuntu22.04-x86_64 %c,[email protected]
 -                   ^ca-certificates-mozilla@2025-05-20 build_system=generic arch=linux-ubuntu22.04-x86_64 
 -           ^[email protected]~symlinks+termlib abi=none build_system=autotools patches:=7a351bc arch=linux-ubuntu22.04-x86_64 %c,[email protected]
 -       ^[email protected] build_system=generic arch=linux-ubuntu22.04-x86_64 
[e]      ^[email protected]~binutils+bootstrap~graphite~nvptx~piclibs~profiled~strip build_system=autotools build_type=RelWithDebInfo languages:='c,c++,fortran' arch=linux-ubuntu22.04-x86_64 
 -       ^[email protected] build_system=generic arch=linux-ubuntu22.04-x86_64 
[e]      ^[email protected] build_system=autotools arch=linux-ubuntu22.04-x86_64 
 -       ^[email protected]~guile build_system=generic arch=linux-ubuntu22.04-x86_64 %[email protected]
 -       ^[email protected]+atomics~cuda~debug+fortran~gpfs~internal-hwloc~internal-libevent~internal-pmix~ipv6~java~lustre~memchecker~openshmem~rocm~romio+rsh~static~two_level_namespace+vt+wrapper-rpath build_system=autotools fabrics:=none romio-filesystem:=none schedulers:=none arch=linux-ubuntu22.04-x86_64 %c,cxx,[email protected]
 -           ^[email protected] build_system=autotools arch=linux-ubuntu22.04-x86_64 
 -               ^[email protected]+sigsegv build_system=autotools arch=linux-ubuntu22.04-x86_64 %c,[email protected]
 -                   ^[email protected] build_system=autotools arch=linux-ubuntu22.04-x86_64 %[email protected]
 -           ^[email protected] build_system=autotools arch=linux-ubuntu22.04-x86_64 %[email protected]
 -           ^[email protected]~cairo~cuda~gl~level_zero~libudev+libxml2~nvml~opencl+pci~rocm build_system=autotools libs:=shared,static arch=linux-ubuntu22.04-x86_64 %c,[email protected]
 -               ^[email protected] build_system=autotools arch=linux-ubuntu22.04-x86_64 %[email protected]
 -                   ^[email protected] build_system=autotools arch=linux-ubuntu22.04-x86_64 
 -               ^[email protected]~http+pic~python+shared build_system=autotools arch=linux-ubuntu22.04-x86_64 %[email protected]
 -                   ^[email protected] build_system=autotools libs:=shared,static arch=linux-ubuntu22.04-x86_64 %[email protected]
 -                   ^[email protected]~pic build_system=autotools libs:=shared,static arch=linux-ubuntu22.04-x86_64 %[email protected]
 -           ^[email protected]+openssl build_system=autotools arch=linux-ubuntu22.04-x86_64 %[email protected]
 -           ^[email protected] build_system=autotools arch=linux-ubuntu22.04-x86_64 %[email protected]
 -               ^[email protected] build_system=autotools patches:=440b954 arch=linux-ubuntu22.04-x86_64 %[email protected]
 -                   ^[email protected]+bzip2+curses+git~libunistring+libxml2+pic+shared+tar+xz build_system=autotools arch=linux-ubuntu22.04-x86_64 %c,[email protected]
 -                       ^[email protected] build_system=autotools zip=pigz arch=linux-ubuntu22.04-x86_64 %[email protected]
 -                           ^[email protected] build_system=makefile arch=linux-ubuntu22.04-x86_64 %[email protected]
 -                           ^[email protected]+programs build_system=makefile compression:=none libs:=shared,static arch=linux-ubuntu22.04-x86_64 %c,[email protected]
 -           ^[email protected] build_system=autotools arch=linux-ubuntu22.04-x86_64 %[email protected]
 -           ^[email protected]+gssapi build_system=autotools arch=linux-ubuntu22.04-x86_64 %c,[email protected]
 -               ^[email protected]+shared build_system=autotools arch=linux-ubuntu22.04-x86_64 %c,[email protected]
 -                   ^[email protected]~color build_system=autotools arch=linux-ubuntu22.04-x86_64 %c,[email protected]
 -               ^[email protected] build_system=autotools arch=linux-ubuntu22.04-x86_64 %[email protected]
 -               ^[email protected]~obsolete_api build_system=autotools arch=linux-ubuntu22.04-x86_64 %[email protected]
 -           ^[email protected]+cpanm+opcode+open+shared+threads build_system=generic arch=linux-ubuntu22.04-x86_64 %[email protected]
 -               ^[email protected]+cxx~docs+stl build_system=autotools patches:=26090f4,b231fcc arch=linux-ubuntu22.04-x86_64 %c,[email protected]
 -               ^[email protected]~debug~pic+shared build_system=generic arch=linux-ubuntu22.04-x86_64 %[email protected]
 -               ^[email protected] build_system=autotools arch=linux-ubuntu22.04-x86_64 %[email protected]
 -                   ^[email protected] build_system=autotools patches:=21f0a03 arch=linux-ubuntu22.04-x86_64 %[email protected]
 -           ^[email protected]~munge~python build_system=autotools arch=linux-ubuntu22.04-x86_64 %[email protected]
 -           ^[email protected] build_system=autotools arch=linux-ubuntu22.04-x86_64 %[email protected]
 -               ^[email protected]+lex~nls build_system=autotools arch=linux-ubuntu22.04-x86_64 %c,[email protected]
 -       ^[email protected] build_system=autotools arch=linux-ubuntu22.04-x86_64 %[email protected]
 -       ^[email protected]+compat+new_strategies+opt+pic+shared build_system=autotools arch=linux-ubuntu22.04-x86_64 %c,[email protected]

Let’s override these default preferences in an environment. When you have an activated environment, you can edit the associated configuration with spack config edit (you don’t have to provide a section name):

$ spack env create config-env
$ spack env activate config-env
$ spack config edit

Warning

You will get exactly the same effects if you make these changes without using an environment, but you must delete the associated packages.yaml file after the config tutorial or the commands you run in later tutorial sections will not produce the same output (because they weren’t run with the configuration changes made here)

spack:
  specs: []
  view: true
  concretizer:
    unify: true
  packages:
    all:
      prefer:
      - "%llvm"
    mpi:
      require: mpich

We see if we retry that we now get what we want without getting any more specific on the command line.

$ spack spec hdf5
 -   [email protected]~cxx~fortran~hl~ipo~java~map+mpi+shared~subfiling~szip~threadsafe+tools api=default build_system=cmake build_type=Release generator=make arch=linux-ubuntu22.04-x86_64 %[email protected]
 -       ^[email protected]~doc+ncurses+ownlibs~qtgui build_system=generic build_type=Release arch=linux-ubuntu22.04-x86_64 %c,[email protected]
 -           ^[email protected]~gssapi~ldap~libidn2~librtmp~libssh~libssh2+nghttp2 build_system=autotools libs:=shared,static tls:=openssl arch=linux-ubuntu22.04-x86_64 %c,[email protected]
 -               ^[email protected] build_system=autotools arch=linux-ubuntu22.04-x86_64 %c,[email protected]
 -                   ^[email protected] build_system=autotools arch=linux-ubuntu22.04-x86_64 %[email protected]
 -               ^[email protected]~docs+shared build_system=generic certs=mozilla arch=linux-ubuntu22.04-x86_64 %c,[email protected]
 -                   ^ca-certificates-mozilla@2025-05-20 build_system=generic arch=linux-ubuntu22.04-x86_64 
 -                   ^[email protected]+cpanm+opcode+open+shared+threads build_system=generic arch=linux-ubuntu22.04-x86_64 %[email protected]
 -                       ^[email protected]+cxx~docs+stl build_system=autotools patches:=26090f4,b231fcc arch=linux-ubuntu22.04-x86_64 %c,[email protected]
 -           ^[email protected]~symlinks+termlib abi=none build_system=autotools patches:=7a351bc arch=linux-ubuntu22.04-x86_64 %c,[email protected]
 -       ^[email protected] build_system=generic arch=linux-ubuntu22.04-x86_64 
[e]      ^[email protected] build_system=autotools arch=linux-ubuntu22.04-x86_64 
 -       ^[email protected]~guile build_system=generic arch=linux-ubuntu22.04-x86_64 %[email protected]
[e]      ^[email protected]+clang~cuda~flang+gold+libomptarget~libomptarget_debug~link_llvm_dylib~lld~lldb+llvm_dylib+lua~mlir+polly~python~split_dwarf~z3 build_system=cmake build_type=Release compiler-rt=runtime generator=ninja libcxx=runtime libunwind=runtime openmp=runtime patches:=1f42874,25bc503,6379168,8248141,b216cff,cb8e645 shlib_symbol_version=none targets:=all version_suffix=none arch=linux-ubuntu22.04-x86_64 
 -       ^[email protected]~argobots~cuda+fortran+hwloc+hydra~level_zero+libxml2+pci~rocm+romio~slurm~vci~verbs+wrapperrpath~xpmem build_system=autotools datatype-engine=auto device=ch4 netmod=ofi pmi=default arch=linux-ubuntu22.04-x86_64 %[email protected] %c,[email protected]
 -           ^[email protected] build_system=autotools patches:=440b954 arch=linux-ubuntu22.04-x86_64 %[email protected]
 -               ^[email protected]+bzip2+curses+git~libunistring+libxml2+pic+shared+tar+xz build_system=autotools arch=linux-ubuntu22.04-x86_64 %c,[email protected]
 -                   ^[email protected]~debug~pic+shared build_system=generic arch=linux-ubuntu22.04-x86_64 %[email protected]
 -                   ^[email protected] build_system=autotools zip=pigz arch=linux-ubuntu22.04-x86_64 %[email protected]
 -                       ^[email protected] build_system=makefile arch=linux-ubuntu22.04-x86_64 %[email protected]
 -                       ^[email protected]+programs build_system=makefile compression:=none libs:=shared,static arch=linux-ubuntu22.04-x86_64 %c,[email protected]
[e]          ^[email protected]~binutils+bootstrap~graphite~nvptx~piclibs~profiled~strip build_system=autotools build_type=RelWithDebInfo languages:='c,c++,fortran' arch=linux-ubuntu22.04-x86_64 
 -           ^[email protected] build_system=generic arch=linux-ubuntu22.04-x86_64 
 -           ^[email protected]~cairo~cuda~gl~level_zero~libudev+libxml2~nvml~opencl+pci~rocm build_system=autotools libs:=shared,static arch=linux-ubuntu22.04-x86_64 %c,[email protected]
 -           ^[email protected]~cuda~debug~kdreg~level_zero~uring build_system=autotools fabrics:=sockets,tcp,udp arch=linux-ubuntu22.04-x86_64 %[email protected]
 -           ^[email protected] build_system=autotools arch=linux-ubuntu22.04-x86_64 %[email protected]
 -               ^[email protected] build_system=autotools arch=linux-ubuntu22.04-x86_64 
 -           ^[email protected]~http+pic~python+shared build_system=autotools arch=linux-ubuntu22.04-x86_64 %[email protected]
 -               ^[email protected] build_system=autotools libs:=shared,static arch=linux-ubuntu22.04-x86_64 %[email protected]
 -               ^[email protected]~pic build_system=autotools libs:=shared,static arch=linux-ubuntu22.04-x86_64 %[email protected]
 -           ^[email protected]~cuda~level_zero~rocm build_system=autotools arch=linux-ubuntu22.04-x86_64 %[email protected]
 -               ^[email protected] build_system=autotools arch=linux-ubuntu22.04-x86_64 
 -               ^[email protected] build_system=autotools arch=linux-ubuntu22.04-x86_64 %[email protected]
 -               ^[email protected] build_system=autotools arch=linux-ubuntu22.04-x86_64 %[email protected]
 -               ^[email protected]+sigsegv build_system=autotools arch=linux-ubuntu22.04-x86_64 %c,[email protected]
 -                   ^[email protected] build_system=autotools arch=linux-ubuntu22.04-x86_64 %[email protected]
 -               ^[email protected]+bz2+ctypes+dbm~debug+libxml2+lzma~optimizations+pic+pyexpat+pythoncmd+readline+shared+sqlite3+ssl~tkinter+uuid+zlib build_system=generic arch=linux-ubuntu22.04-x86_64 %c,[email protected]
 -                   ^[email protected]+libbsd build_system=autotools arch=linux-ubuntu22.04-x86_64 %c,[email protected]
 -                       ^[email protected] build_system=autotools arch=linux-ubuntu22.04-x86_64 %[email protected]
 -                           ^[email protected] build_system=autotools arch=linux-ubuntu22.04-x86_64 %[email protected]
 -                   ^[email protected] build_system=autotools arch=linux-ubuntu22.04-x86_64 %[email protected]
 -                   ^[email protected] build_system=autotools arch=linux-ubuntu22.04-x86_64 %c,[email protected]
 -                   ^[email protected] build_system=autotools patches:=21f0a03 arch=linux-ubuntu22.04-x86_64 %[email protected]
 -                   ^[email protected]+column_metadata+dynamic_extensions+fts~functions+rtree build_system=autotools arch=linux-ubuntu22.04-x86_64 %[email protected]
 -                   ^[email protected] build_system=autotools arch=linux-ubuntu22.04-x86_64 %[email protected]
 -       ^[email protected] build_system=autotools arch=linux-ubuntu22.04-x86_64 %[email protected]
 -       ^[email protected]+compat+new_strategies+opt+pic+shared build_system=autotools arch=linux-ubuntu22.04-x86_64 %c,[email protected]

Variant preferences

As we’ve seen throughout this tutorial, HDF5 builds with MPI enabled by default in Spack. If we were working on a project that would routinely need serial HDF5, that might get annoying quickly, having to type hdf5~mpi all the time. Instead, we’ll update our config to force disable it:

spack:
  specs: []
  view: true
  concretizer:
    unify: true
  packages:
    all:
      prefer:
      - "%llvm"
    mpi:
      require: mpich
    hdf5:
      require:
      - spec: "~mpi"

Note if you define require under all and hdf5, you must reintroduce any requirements in hdf5.

$ spack spec hdf5
 -   [email protected]~cxx~fortran~hl~ipo~java~map~mpi+shared~szip~threadsafe+tools api=default build_system=cmake build_type=Release generator=make arch=linux-ubuntu22.04-x86_64 %[email protected]
 -       ^[email protected]~doc+ncurses+ownlibs~qtgui build_system=generic build_type=Release arch=linux-ubuntu22.04-x86_64 %c,[email protected]
 -           ^[email protected]~gssapi~ldap~libidn2~librtmp~libssh~libssh2+nghttp2 build_system=autotools libs:=shared,static tls:=openssl arch=linux-ubuntu22.04-x86_64 %c,[email protected]
 -               ^[email protected] build_system=autotools arch=linux-ubuntu22.04-x86_64 %c,[email protected]
 -                   ^[email protected] build_system=autotools arch=linux-ubuntu22.04-x86_64 %[email protected]
 -                       ^[email protected] build_system=autotools libs:=shared,static arch=linux-ubuntu22.04-x86_64 %[email protected]
 -               ^[email protected]~docs+shared build_system=generic certs=mozilla arch=linux-ubuntu22.04-x86_64 %c,[email protected]
 -                   ^ca-certificates-mozilla@2025-05-20 build_system=generic arch=linux-ubuntu22.04-x86_64
 -                   ^[email protected]+cpanm+opcode+open+shared+threads build_system=generic arch=linux-ubuntu22.04-x86_64 %[email protected]
 -                       ^[email protected]+cxx~docs+stl build_system=autotools patches:=26090f4,b231fcc arch=linux-ubuntu22.04-x86_64 %c,[email protected]
 -                       ^[email protected]~debug~pic+shared build_system=generic arch=linux-ubuntu22.04-x86_64 %[email protected]
 -                       ^[email protected] build_system=autotools arch=linux-ubuntu22.04-x86_64 %[email protected]
 -                           ^[email protected] build_system=autotools patches:=21f0a03 arch=linux-ubuntu22.04-x86_64 %[email protected]
 -           ^[email protected]~symlinks+termlib abi=none build_system=autotools patches:=7a351bc arch=linux-ubuntu22.04-x86_64 %c,[email protected]
 -       ^[email protected] build_system=generic arch=linux-ubuntu22.04-x86_64
[e]      ^[email protected] build_system=autotools arch=linux-ubuntu22.04-x86_64
 -       ^[email protected]~guile build_system=generic arch=linux-ubuntu22.04-x86_64 %[email protected]
[e]      ^[email protected]+clang~cuda~flang+gold+libomptarget~libomptarget_debug~link_llvm_dylib~lld~lldb+llvm_dylib+lua~mlir+polly~python~split_dwarf~z3 build_system=cmake build_type=Release compiler-rt=runtime generator=ninja libcxx=runtime libunwind=runtime openmp=runtime patches:=1f42874,25bc503,6379168,8248141,b216cff,cb8e645 shlib_symbol_version=none targets:=all version_suffix=none arch=linux-ubuntu22.04-x86_64
 -       ^[email protected] build_system=autotools arch=linux-ubuntu22.04-x86_64 %[email protected]
 -       ^[email protected]+compat+new_strategies+opt+pic+shared build_system=autotools arch=linux-ubuntu22.04-x86_64 %c,[email protected]

In general, every attribute that we can set for all packages we can set separately for an individual package.

External packages

The packages configuration file also controls when Spack will build against an externally installed package. Spack has a spack external find command that can automatically discover and register externally installed packages. This works for many common build dependencies, but it’s also important to know how to do this manually for packages that Spack cannot yet detect.

On these systems, we have a pre-installed curl. Let’s tell Spack about this package and where it can be found:

spack:
  specs: []
  view: true
  concretizer:
    unify: true
  packages:
    all:
      prefer:
      - "%llvm"
    mpi:
      require: mpich
    hdf5:
      require:
      - spec: "~mpi"
    curl:
      externals:
      - spec: [email protected] %[email protected]
        prefix: /usr
      buildable: false

Here, we’ve told Spack that Curl 7.81.0 is installed on our system. We’ve also told it the installation prefix where Curl can be found. We don’t know exactly which variants it was built with, but that’s okay. Finally, we set buildable: false to require that Spack not try to build its own.

$ spack spec hdf5
 -   [email protected]~cxx~fortran~hl~ipo~java~map~mpi+shared~szip~threadsafe+tools api=default build_system=cmake build_type=Release generator=make arch=linux-ubuntu22.04-x86_64 %[email protected]
 -       ^[email protected]~doc+ncurses+ownlibs~qtgui build_system=generic build_type=Release arch=linux-ubuntu22.04-x86_64 %c,[email protected]
[e]          ^[email protected]~gssapi~ldap~libidn2~librtmp~libssh~libssh2+nghttp2 build_system=autotools libs:=shared,static tls:=openssl arch=linux-ubuntu22.04-x86_64
[e]          ^[email protected]+clang~cuda~flang+gold+libomptarget~libomptarget_debug~link_llvm_dylib~lld~lldb+llvm_dylib+lua~mlir+polly~python~split_dwarf~z3 build_system=cmake build_type=Release compiler-rt=runtime generator=ninja libcxx=runtime libunwind=runtime openmp=runtime patches:=1f42874,25bc503,6379168,8248141,b216cff,cb8e645 shlib_symbol_version=none targets:=all version_suffix=none arch=linux-ubuntu22.04-x86_64
 -           ^[email protected]~symlinks+termlib abi=none build_system=autotools patches:=7a351bc arch=linux-ubuntu22.04-x86_64 %c,[email protected]
 -       ^[email protected] build_system=generic arch=linux-ubuntu22.04-x86_64
[e]      ^[email protected]~binutils+bootstrap~graphite~nvptx~piclibs~profiled~strip build_system=autotools build_type=RelWithDebInfo languages:='c,c++,fortran' arch=linux-ubuntu22.04-x86_64
 -       ^[email protected] build_system=generic arch=linux-ubuntu22.04-x86_64
[e]      ^[email protected] build_system=autotools arch=linux-ubuntu22.04-x86_64
 -       ^[email protected]~guile build_system=generic arch=linux-ubuntu22.04-x86_64 %[email protected]
 -       ^[email protected] build_system=autotools arch=linux-ubuntu22.04-x86_64 %[email protected]
 -       ^[email protected]+compat+new_strategies+opt+pic+shared build_system=autotools arch=linux-ubuntu22.04-x86_64 %c,[email protected]

This gets slightly more complicated with virtual dependencies. Suppose we don’t want to build our own MPI, but we now want a parallel version of HDF5. Well, fortunately, we have MPICH installed on these systems.

Instead of manually configuring an external for MPICH like we did for Curl we will use the spack external find command. For packages that support this option, this is a useful way to avoid typos and get more accurate external specs.

$ spack external find mpich
==> The following specs have been detected on this system and added to /home/spack1/spack/var/spack/environments/config-env/spack.yaml
[email protected]

To express that we don’t want any other MPI installed, we can use the virtual mpi package as a key. While we’re editing the spack.yaml file, make sure to configure HDF5 to be able to build with MPI again:

spack:
  specs: []
  view: true
  concretizer:
    unify: true
  packages:
    all:
      prefer:
      - "%llvm"
    mpi:
      require: mpich
      buildable: false
    curl:
      externals:
      - spec: [email protected] %[email protected]
        prefix: /usr
      buildable: false
    mpich:
      externals:
      - spec: [email protected]+hydra device=ch4 netmod=ofi
        prefix: /usr

If you run this as-is, you’ll notice Spack still hasn’t built hdf5 with our external mpich. The concretizer has instead turned off mpi support in hdf5. To debug this, we will force Spack to use hdf5+mpi.

$ spack spec hdf5+mpi
==> Error: failed to concretize `hdf5+mpi` for the following reasons:
     1. cannot satisfy a requirement for package 'mpich'.
     2. hdf5: '+mpi' conflicts with '^[email protected]:4.0.3'
     3. hdf5: '+mpi' conflicts with '^[email protected]:4.0.3'
        required because conflict is triggered when +mpi
          required because hdf5+mpi requested explicitly
        required because conflict constraint ^[email protected]:4.0.3
          required because mpich available as external when satisfying mpich@=4.0+hydra device=ch4 netmod=ofi
          required because hdf5+mpi requested explicitly

In this case, we cannot use the external mpich. The version is incompatible with hdf5. At this point, the best option is to give up and let Spack build mpi for us. The alternative is to try to find a version of hdf5 which doesn’t have this conflict.

By configuring most of our package preferences in packages.yaml, we can cut down on the amount of work we need to do when specifying a spec on the command line. In addition to compiler and variant preferences, we can specify version preferences as well. Except for specifying dependencies via ^, anything that you can specify on the command line can be specified in packages.yaml with the exact same spec syntax.

Installation permissions

The packages configuration also controls the default permissions to use when installing a package. You’ll notice that by default, the installation prefix will be world-readable but only user-writable.

Let’s say we need to install converge, a licensed software package. Since a specific research group, fluid_dynamics, pays for this license, we want to ensure that only members of this group can access the software. We can do this like so:

packages:
  converge:
    permissions:
      read: group
      group: fluid_dynamics

Now, only members of the fluid_dynamics group can use any converge installations.

At this point we want to discard the configuration changes we made in this tutorial section, so we can deactivate the environment:

$ spack env deactivate

Warning

If you do not deactivate the config-env environment, then specs will be concretized differently in later tutorial sections and your results will not match.

High-level Config

In addition to compiler and package settings, Spack allows customization of several high-level settings. These settings are managed in the config section (in config.yaml when stored as an individual file outside of an environment). You can see the default settings by running:

$ spack config --scope defaults edit config

# -------------------------------------------------------------------------
# This is the default spack configuration file.
#
# Settings here are versioned with Spack and are intended to provide
# sensible defaults out of the box. Spack maintainers should edit this
# file to keep it current.
#
# Users can override these settings by editing the following files.
#
# Per-spack-instance settings (overrides defaults):
#   $SPACK_ROOT/etc/spack/config.yaml
#
# Per-user settings (overrides default and site settings):
#   ~/.spack/config.yaml
# -------------------------------------------------------------------------
config:
  # This is the path to the root of the Spack install tree.
  # You can use $spack here to refer to the root of the spack instance.
  install_tree:
    root: $spack/opt/spack
    projections:
      all: "{architecture.platform}-{architecture.target}/{name}-{version}-{hash}"
    # install_tree can include an optional padded length (int or boolean)
    # default is False (do not pad)
    # if padded_length is True, Spack will pad as close to the system max path
    # length as possible
    # if padded_length is an integer, Spack will pad to that many characters,
    # assuming it is higher than the length of the install_tree root.
    # padded_length: 128


  # Locations where templates should be found
  template_dirs:
    - $spack/share/spack/templates

  # Directory where licenses should be located
  license_dir: $spack/etc/spack/licenses

  # Temporary locations Spack can try to use for builds.
  #
  # Recommended options are given below.
  #
  # Builds can be faster in temporary directories on some (e.g., HPC) systems.
  # Specifying `$tempdir` will ensure use of the default temporary directory
  # (i.e., ``$TMP` or ``$TMPDIR``).
  #
  # Another option that prevents conflicts and potential permission issues is
  # to specify `$user_cache_path/stage`, which ensures each user builds in their
  # home directory.
  #
  # A more traditional path uses the value of `$spack/var/spack/stage`, which
  # builds directly inside Spack's instance without staging them in a
  # temporary space.  Problems with specifying a path inside a Spack instance
  # are that it precludes its use as a system package and its ability to be
  # pip installable.
  #
  # In Spack environment files, chaining onto existing system Spack
  # installations, the $env variable can be used to download, cache and build
  # into user-writable paths that are relative to the currently active
  # environment.
  #
  # In any case, if the username is not already in the path, Spack will append
  # the value of `$user` in an attempt to avoid potential conflicts between
  # users in shared temporary spaces.
  #
  # The build stage can be purged with `spack clean --stage` and
  # `spack clean -a`, so it is important that the specified directory uniquely
  # identifies Spack staging to avoid accidentally wiping out non-Spack work.
  build_stage:
    - $tempdir/$user/spack-stage
    - $user_cache_path/stage
  # - $spack/var/spack/stage

  # Directory in which to run tests and store test results.
  # Tests will be stored in directories named by date/time and package
  # name/hash.
  test_stage: $user_cache_path/test

  # Cache directory for already downloaded source tarballs and archived
  # repositories. This can be purged with `spack clean --downloads`.
  source_cache: $spack/var/spack/cache


  ## Directory where spack managed environments are created and stored
  # environments_root: $spack/var/spack/environments


  # Cache directory for miscellaneous files, like the package index.
  # This can be purged with `spack clean --misc-cache`
  misc_cache: $user_cache_path/cache


  # Abort downloads after this many seconds if not data is received.
  # Setting this to 0 will disable the timeout.
  connect_timeout: 30


  # If this is false, tools like curl that use SSL will not verify
  # certificates. (e.g., curl will use use the -k option)
  verify_ssl: true


  # This is where custom certs for proxy/firewall are stored.
  # It can be a path or environment variable. To match ssl env configuration
  # the default is the environment variable SSL_CERT_FILE
  ssl_certs: $SSL_CERT_FILE


  # Suppress gpg warnings from binary package verification
  # Only suppresses warnings, gpg failure will still fail the install
  # Potential rationale to set True: users have already explicitly trusted the
  # gpg key they are using, and may not want to see repeated warnings that it
  # is self-signed or something of the sort.
  suppress_gpg_warnings: false


  # If set to true, Spack will always check checksums after downloading
  # archives. If false, Spack skips the checksum step.
  checksum: true


  # If set to true, Spack will fetch deprecated versions without warning.
  # If false, Spack will raise an error when trying to install a deprecated version.
  deprecated: false


  # If set to true, `spack install` and friends will NOT clean
  # potentially harmful variables from the build environment. Use wisely.
  dirty: false


  # The language the build environment will use. This will produce English
  # compiler messages by default, so the log parser can highlight errors.
  # If set to C, it will use English (see man locale).
  # If set to the empty string (''), it will use the language from the
  # user's environment.
  build_language: C


  # When set to true, concurrent instances of Spack will use locks to
  # avoid modifying the install tree, database file, etc. If false, Spack
  # will disable all locking, but you must NOT run concurrent instances
  # of Spack.  For filesystems that don't support locking, you should set
  # this to false and run one Spack at a time, but otherwise we recommend
  # enabling locks.
  locks: true

  # The default url fetch method to use.
  # If set to 'curl', Spack will require curl on the user's system
  # If set to 'urllib', Spack will use python built-in libs to fetch
  url_fetch_method: urllib

  # The maximum number of jobs to use for the build system (e.g. `make`), when
  # the -j flag is not given on the command line. Defaults to 16 when not set.
  # Note that the maximum number of jobs is limited by the number of cores
  # available, taking thread affinity into account when supported.
  # For instance:
  # - With `build_jobs: 16` and 4 cores available `spack install` will run `make -j4`
  # - With `build_jobs: 16` and 32 cores available `spack install` will run `make -j16`
  # - With `build_jobs: 2` and 4 cores available `spack install -j6` will run `make -j6`
  build_jobs: 16

  # The maximum number of concurrent package builds a single Spack instance will run,
  # when the `-p` / `--concurrent-packages`  flag is not given on the command line.
  # Defaults to 1 when not set.
  # Generally, big builds like LLVM are going to perform better with a higher -j, and
  # builds with lots of dependencies may build better with higher -p. It is currently
  # up to the user to balance between -j (build_jobs) and -p (concurrent_packages)
  # on `spack install`, but this will be less of an issue in the future when we
  # introduce support for the gmake job server and allow more dynamic parallelism.
  # This, like `build_jobs`, is also limited by available cores.
  # Note: This option has no effect on windows, as parallel builds are disabled on
  # windows due to a lack of filesystem locks.
  concurrent_packages: 1


  # If set to true, Spack will use ccache to cache C compiles.
  ccache: false

  # How long to wait to lock the Spack installation database. This lock is used
  # when Spack needs to manage its own package metadata and all operations are
  # expected to complete within the default time limit. The timeout should
  # therefore generally be left untouched.
  db_lock_timeout: 60


  # How long to wait when attempting to modify a package (e.g. to install it).
  # This value should typically be 'null' (never time out) unless the Spack
  # instance only ever has a single user at a time, and only if the user
  # anticipates that a significant delay indicates that the lock attempt will
  # never succeed.
  package_lock_timeout: null


  # Control how shared libraries are located at runtime on Linux. See the
  # the Spack documentation for details.
  shared_linking:
    # Spack automatically embeds runtime search paths in ELF binaries for their
    # dependencies. Their type can either be "rpath" or "runpath". For glibc, rpath is
    # inherited and has precedence over LD_LIBRARY_PATH; runpath is not inherited
    # and of lower precedence. DO NOT MIX these within the same install tree.
    type: rpath


    # (Experimental) Embed absolute paths of dependent libraries directly in ELF
    # binaries to avoid runtime search. This can improve startup time of
    # executables with many dependencies, in particular on slow filesystems.
    bind: false

    # Controls the handling of missing dynamic libraries after installation.
    # Options are ignore (default), warn, or error. If set to error, the
    # installation fails if installed binaries reference dynamic libraries that
    # are not found in their specified rpaths.
    missing_library_policy: ignore


  # Set to 'false' to allow installation on filesystems that doesn't allow setgid bit
  # manipulation by unprivileged user (e.g. AFS)
  allow_sgid: true

  # Whether to show status information during building and installing packages.
  # This gives information about Spack's current progress as well as the current
  # and total number of packages. Information is shown both in the terminal
  # title and inline.
  install_status: true

  # Number of seconds a buildcache's index.json is cached locally before probing
  # for updates, within a single Spack invocation. Defaults to 10 minutes.
  binary_index_ttl: 600

  flags:
    # Whether to keep -Werror flags active in package builds.
    keep_werror: 'none'

  # A mapping of aliases that can be used to define new commands. For instance,
  # `sp: spec -I` will define a new command `sp` that will execute `spec` with
  # the `-I` argument. Aliases cannot override existing commands.
  aliases:
    concretise: concretize
    containerise: containerize
    rm: remove

As you can see, many of the directories Spack uses can be customized. For example, you can tell Spack to install packages to a prefix outside of the $SPACK_ROOT hierarchy. Module files can be written to a central location if you are using multiple Spack instances. If you have a fast scratch file system, you can run builds from this file system with the following config.yaml:

config:
  build_stage:
  - /scratch/$user/spack-stage

Note

It is important to distinguish the build stage directory from other directories in your scratch space to ensure spack clean does not inadvertently remove unrelated files. This can be accomplished by including a combination of spack and or stage in each path as shown in the default settings and documented examples. See Basic Settings for details.

On systems with compilers that absolutely require environment variables like LD_LIBRARY_PATH, it is possible to prevent Spack from cleaning the build environment with the dirty setting:

config:
  dirty: true

However, this is strongly discouraged, as it can pull unwanted libraries into the build.

One last setting that may be of interest to many users is the ability to customize the parallelism of Spack builds. By default, Spack installs all packages in parallel with the number of jobs equal to the number of cores on the node (up to a maximum of 16). For example, on a node with 16 cores, this will look like:

$ spack install --no-cache --verbose --overwrite --yes-to-all zlib
==> Installing zlib
==> Executing phase: 'install'
==> './configure' '--prefix=/home/user/spack/opt/spack/linux-ubuntu22.04-x86_64/gcc-11.3.0/zlib-1.2.12-fntvsj6xevbz5gyq7kfa4xg7oxnaolxs'
...
==> 'make' '-j16'
...
==> 'make' '-j16' 'install'
...
[+] /home/user/spack/opt/spack/linux-ubuntu22.04-x86_64/gcc-11.3.0/zlib-1.2.12-fntvsj6xevbz5gyq7kfa4xg7oxnaolxs

As you can see, we are building with all 16 cores on the node. If you are on a shared login node, this can slow down the system for other users. If you have a strict ulimit or restriction on the number of available licenses, you may not be able to build at all with this many cores. To limit the number of cores our build uses, set build_jobs like so:

$ spack config edit config

config:
  build_jobs: 2

If we uninstall and reinstall zlib-ng, we see that it now uses only 2 cores:

$ spack install --no-cache --verbose --overwrite --yes-to-all zlib-ng
==> Installing zlib
==> Executing phase: 'install'
==> './configure' '--prefix=/home/user/spack/opt/spack/linux-ubuntu22.04...
...
==> 'make' '-j2'
...
==> 'make' '-j2' 'install'
...
[+] /home/user/spack/opt/spack/linux-ubuntu22.04...

Obviously, if you want to build everything in serial for whatever reason, you would set build_jobs to 1.

Last, we’ll unset concretizer:reuse:false since we’ll want to enable concretizer reuse for the rest of this tutorial.

$ spack config rm concretizer:reuse

Warning

If you do not do this step, the rest of the tutorial will not reuse binaries!

Conclusion

In this tutorial, we covered basic Spack configuration using compilers.yaml, packages.yaml, and config.yaml. Spack has many more configuration files, including modules.yaml, which will be covered in the Module Files Tutorial. For more detailed documentation on Spack’s many configuration settings, see the configuration section of Spack’s main documentation.

For examples of how other sites configure Spack, see https://github.com/spack/spack-configs. If you use Spack at your site and want to share your config files, feel free to submit a pull request!