Container Images

Spack Environments are a great tool to create container images, but preparing one that is suitable for production requires some more boilerplate than just:

COPY spack.yaml /environment
RUN spack -e /environment install

Additional actions may be needed to minimize the size of the container, or to update the system software that is installed in the base image, or to set up a proper entrypoint to run the image. These tasks are usually both necessary and repetitive, so Spack comes with a command to generate recipes for container images starting from a spack.yaml.

A Quick Introduction

Consider having a Spack environment like the following:

spack:
  specs:
  - gromacs+mpi
  - mpich

Producing a Dockerfile from it is as simple as moving to the directory where the spack.yaml file is stored and giving the following command:

$ spack containerize > Dockerfile

The Dockerfile that gets created uses multi-stage builds and other techniques to minimize the size of the final image:

# Build stage with Spack pre-installed and ready to be used
FROM spack/ubuntu-bionic:latest as builder

# What we want to install and how we want to install it
# is specified in a manifest file (spack.yaml)
RUN mkdir /opt/spack-environment \
&&  (echo "spack:" \
&&   echo "  specs:" \
&&   echo "  - gromacs+mpi" \
&&   echo "  - mpich" \
&&   echo "  concretization: together" \
&&   echo "  config:" \
&&   echo "    install_tree: /opt/software" \
&&   echo "  view: /opt/view") > /opt/spack-environment/spack.yaml

# Install the software, remove unnecessary deps
RUN cd /opt/spack-environment && spack env activate . && spack install --fail-fast && spack gc -y

# Strip all the binaries
RUN find -L /opt/view/* -type f -exec readlink -f '{}' \; | \
    xargs file -i | \
    grep 'charset=binary' | \
    grep 'x-executable\|x-archive\|x-sharedlib' | \
    awk -F: '{print $1}' | xargs strip -s

# Modifications to the environment that are necessary to run
RUN cd /opt/spack-environment && \
    spack env activate --sh -d . >> /etc/profile.d/z10_spack_environment.sh

# Bare OS image to run the installed executables
FROM ubuntu:18.04

COPY --from=builder /opt/spack-environment /opt/spack-environment
COPY --from=builder /opt/software /opt/software
COPY --from=builder /opt/view /opt/view
COPY --from=builder /etc/profile.d/z10_spack_environment.sh /etc/profile.d/z10_spack_environment.sh

ENTRYPOINT ["/bin/bash", "--rcfile", "/etc/profile", "-l"]

The image itself can then be built and run in the usual way, with any of the tools suitable for the task. For instance, if we decided to use docker:

$ spack containerize > Dockerfile
$ docker build -t myimage .
[ ... ]
$ docker run -it myimage

The various components involved in the generation of the recipe and their configuration are discussed in details in the sections below.

Spack Images on Docker Hub

Docker images with Spack preinstalled and ready to be used are built on Docker Hub at every push to develop or to a release branch. The OS that are currently supported are summarized in the table below:

Supported operating systems
Operating System Base Image Spack Image
Ubuntu 16.04 ubuntu:16.04 spack/ubuntu-xenial
Ubuntu 18.04 ubuntu:18.04 spack/ubuntu-bionic
CentOS 6 centos:6 spack/centos6
CentOS 7 centos:7 spack/centos7

All the images are tagged with the corresponding release of Spack:

_images/dockerhub_spack.png

with the exception of the latest tag that points to the HEAD of the develop branch. These images are available for anyone to use and take care of all the repetitive tasks that are necessary to setup Spack within a container. The container recipes generated by Spack use them as default base images for their build stage, even though handles to use custom base images provided by users are available to accommodate complex use cases.

Creating Images From Environments

Any Spack Environment can be used for the automatic generation of container recipes. Sensible defaults are provided for things like the base image or the version of Spack used in the image. If a finer tuning is needed it can be obtained by adding the relevant metadata under the container attribute of environments:

spack:
  specs:
  - gromacs+mpi
  - mpich

  container:
    # Select the format of the recipe e.g. docker,
    # singularity or anything else that is currently supported
    format: docker

    # Sets the base images for the stages where Spack builds the
    # software or where the software gets installed after being built..
    images:
      os: "centos:7"
      spack: develop

    # Whether or not to strip binaries
    strip: true

    # Additional system packages that are needed at runtime
    os_packages:
      final:
      - libgomp

    # Extra instructions
    extra_instructions:
      final: |
        RUN echo 'export PS1="\[$(tput bold)\]\[$(tput setaf 1)\][gromacs]\[$(tput setaf 2)\]\u\[$(tput sgr0)\]:\w $ "' >> ~/.bashrc

    # Labels for the image
    labels:
      app: "gromacs"
      mpi: "mpich"

A detailed description of the options available can be found in the Configuration Reference section.

Setting Base Images

The images subsection is used to select both the image where Spack builds the software and the image where the built software is installed. This attribute can be set in two different ways and which one to use depends on the use case at hand.

Use Official Spack Images From Dockerhub

To generate a recipe that uses an official Docker image from the Spack organization to build the software and the corresponding official OS image to install the built software, all the user has to do is specify:

  1. An operating system under images:os
  2. A Spack version under images:spack

Any combination of these two values that can be mapped to one of the images discussed in Spack Images on Docker Hub is allowed. For instance, the following spack.yaml:

spack:
  specs:
  - gromacs+mpi
  - mpich

  container:
    images:
      os: centos:7
      spack: 0.15.4

uses spack/centos7:0.15.4 and centos:7 for the stages where the software is respectively built and installed:

# Build stage with Spack pre-installed and ready to be used
FROM spack/centos7:0.15.4 as builder

# What we want to install and how we want to install it
# is specified in a manifest file (spack.yaml)
RUN mkdir /opt/spack-environment \
&&  (echo "spack:" \
&&   echo "  specs:" \
&&   echo "  - gromacs+mpi" \
&&   echo "  - mpich" \
&&   echo "  concretization: together" \
&&   echo "  config:" \
&&   echo "    install_tree: /opt/software" \
&&   echo "  view: /opt/view") > /opt/spack-environment/spack.yaml
[ ... ]
# Bare OS image to run the installed executables
FROM centos:7

COPY --from=builder /opt/spack-environment /opt/spack-environment
COPY --from=builder /opt/software /opt/software
COPY --from=builder /opt/view /opt/view
COPY --from=builder /etc/profile.d/z10_spack_environment.sh /etc/profile.d/z10_spack_environment.sh

ENTRYPOINT ["/bin/bash", "--rcfile", "/etc/profile", "-l"]

This method of selecting base images is the simplest of the two, and we advise to use it whenever possible. There are cases though where using Spack official images is not enough to fit production needs. In these situations users can manually select which base image to start from in the recipe, as we’ll see next.

Use Custom Images Provided by Users

Consider, as an example, building a production grade image for a CUDA application. The best strategy would probably be to build on top of images provided by the vendor and regard CUDA as an external package.

Spack doesn’t currently provide an official image with CUDA configured this way, but users can build it on their own and then configure the environment to explicitly pull it. This requires users to:

  1. Specify the image used to build the software under images:build
  2. Specify the image used to install the built software under images:final

A spack.yaml like the following:

spack:
  specs:
  - gromacs@2019.4+cuda build_type=Release
  - mpich
  - fftw precision=float
  packages:
    cuda:
      buildable: False
      externals:
      - spec: cuda%gcc
        prefix: /usr/local/cuda

  container:
    images:
      build: custom/cuda-10.1-ubuntu18.04:latest
      final: nvidia/cuda:10.1-base-ubuntu18.04

produces, for instance, the following Dockerfile:

# Build stage with Spack pre-installed and ready to be used
FROM custom/cuda-10.1-ubuntu18.04:latest as builder

# What we want to install and how we want to install it
# is specified in a manifest file (spack.yaml)
RUN mkdir /opt/spack-environment \
&&  (echo "spack:" \
&&   echo "  specs:" \
&&   echo "  - gromacs@2019.4+cuda build_type=Release" \
&&   echo "  - mpich" \
&&   echo "  - fftw precision=float" \
&&   echo "  packages:" \
&&   echo "    cuda:" \
&&   echo "      buildable: false" \
&&   echo "      externals:" \
&&   echo "      - spec: cuda%gcc" \
&&   echo "        prefix: /usr/local/cuda" \
&&   echo "  concretization: together" \
&&   echo "  config:" \
&&   echo "    install_tree: /opt/software" \
&&   echo "  view: /opt/view") > /opt/spack-environment/spack.yaml

# Install the software, remove unnecessary deps
RUN cd /opt/spack-environment && spack env activate . && spack install --fail-fast && spack gc -y

# Strip all the binaries
RUN find -L /opt/view/* -type f -exec readlink -f '{}' \; | \
    xargs file -i | \
    grep 'charset=binary' | \
    grep 'x-executable\|x-archive\|x-sharedlib' | \
    awk -F: '{print $1}' | xargs strip -s

# Modifications to the environment that are necessary to run
RUN cd /opt/spack-environment && \
    spack env activate --sh -d . >> /etc/profile.d/z10_spack_environment.sh

# Bare OS image to run the installed executables
FROM nvidia/cuda:10.1-base-ubuntu18.04

COPY --from=builder /opt/spack-environment /opt/spack-environment
COPY --from=builder /opt/software /opt/software
COPY --from=builder /opt/view /opt/view
COPY --from=builder /etc/profile.d/z10_spack_environment.sh /etc/profile.d/z10_spack_environment.sh

ENTRYPOINT ["/bin/bash", "--rcfile", "/etc/profile", "-l"]

where the base images for both stages are completely custom.

This second mode of selection for base images is more flexible than just choosing an operating system and a Spack version, but is also more demanding. Users may need to generate by themselves their base images and it’s also their responsibility to ensure that:

  1. Spack is available in the build stage and set up correctly to install the required software
  2. The artifacts produced in the build stage can be executed in the final stage

Therefore we don’t recommend its use in cases that can be otherwise covered by the simplified mode shown first.

Singularity Definition Files

In addition to producing recipes in Dockerfile format Spack can produce Singularity Definition Files by just changing the value of the format attribute:

$ cat spack.yaml
spack:
  specs:
  - hdf5~mpi
  container:
    format: singularity

$ spack containerize > hdf5.def
$ sudo singularity build hdf5.sif hdf5.def

The minimum version of Singularity required to build a SIF (Singularity Image Format) image from the recipes generated by Spack is 3.5.3.

Configuration Reference

The tables below describe all the configuration options that are currently supported to customize the generation of container recipes:

General configuration options for the container section of spack.yaml
Option Name Description Allowed Values Required
format The format of the recipe docker or singularity Yes
images:os Operating system used as a base for the image See Supported operating systems Yes, if using constrained selection of base images
images:spack Version of Spack use in the build stage Valid tags for base:image Yes, if using constrained selection of base images
images:build Image to be used in the build stage Any valid container image Yes, if using custom selection of base images
images:final Image to be used in the build stage Any valid container image Yes, if using custom selection of base images
strip Whether to strip binaries true (default) or false No
os_packages:command Tool used to manage system packages apt, yum Only with custom base images
os_packages:update Whether or not to update the list of available packages True or False (default: True) No
os_packages:build System packages needed at build-time Valid packages for the current OS No
os_packages:final System packages needed at run-time Valid packages for the current OS No
extra_instructions:build Extra instructions (e.g. RUN, COPY, etc.) at the end of the build stage Anything understood by the current format No
extra_instructions:final Extra instructions (e.g. RUN, COPY, etc.) at the end of the final stage Anything understood by the current format No
labels Labels to tag the image Pairs of key-value strings No
Configuration options specific to Singularity
Option Name Description Allowed Values Required
singularity:runscript Content of %runscript Any valid script No
singularity:startscript Content of %startscript Any valid script No
singularity:test Content of %test Any valid script No
singularity:help Description of the image Description string No

Best Practices

MPI

Due to the dependency on Fortran for OpenMPI, which is the spack default implementation, consider adding gfortran to the apt-get install list.

Recent versions of OpenMPI will require you to pass --allow-run-as-root to your mpirun calls if started as root user inside Docker.

For execution on HPC clusters, it can be helpful to import the docker image into Singularity in order to start a program with an external MPI. Otherwise, also add openssh-server to the apt-get install list.

CUDA

Starting from CUDA 9.0, Nvidia provides minimal CUDA images based on Ubuntu. Please see their instructions. Avoid double-installing CUDA by adding, e.g.

packages:
  cuda:
    externals:
    - spec: "cuda@9.0.176%gcc@5.4.0 arch=linux-ubuntu16-x86_64"
      prefix: /usr/local/cuda
    buildable: False

to your spack.yaml.

Users will either need nvidia-docker or e.g. Singularity to execute device kernels.

Docker on Windows and OSX

On Mac OS and Windows, docker runs on a hypervisor that is not allocated much memory by default, and some spack packages may fail to build due to lack of memory. To work around this issue, consider configuring your docker installation to use more of your host memory. In some cases, you can also ease the memory pressure on parallel builds by limiting the parallelism in your config.yaml.

config:
  build_jobs: 2