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Collaborative document - Build Systems Course and Hackathon, October 2024

Icebreaker question

What is favourit editor / integrated development environment for editing code?

  • vi / vim / neovim: ooooooooo
  • emacs: oooo
  • nano:
  • Visual Studio Code: ooooooooo
  • Zed:

CMake & ENCCS

:::info Links for the Build Systems Course:

Links for ENCCS

On Dardel, load the CMake module with

ml PDC/23.12
ml cmake/3.27.7
cmake --version

Download the exercise files by cloning the git repo

git clone https://github.com/ENCCS/intro-cmake.git
cd intro-cmake/content/code/

Questions:

  • I can't understand what is module scope in add_library. Shared is loading the library at run time with generated .so file, static is linking the library into the code, but what is module?

    • From CMake's docs: "A plugin that may not be linked by other targets, but may be dynamically loaded at runtime using dlopen-like functionality." So an addon plugin that might be present or not when program is being run.

  • When writing ${outer_${inner_variable}_variable} is the inner_variable then part of the variable name of the outer variable?

    • Yes. Dereference the variable ${inner_variable} you will get a string. This string is one part of the out_STRING_variable.

  • Based on cmake help, if you don't specify scope in add_library command, it might use static or shared. How can I check what scope is used after running a command like add_library(target lib_file.cpp)?

    • By default CMake checks what is the value for BUILD_SHARED_LIBS-variable. Default is static. So unless you have set the variable, you can assume it is static.
    • 👍

  • is account.so library supposed to be used outside of the bank target? If not, it should be account.a?

    • no it is a static library and only be used to generate the bank executable.
      • I mean that if one installs bank exe, one would also need to install account.so, although only bank is ever gonna use account.so?
        • In the code example, the answer is YES.
        • But if you have two sources and want to use account.so, you can
          • set it to be interfaced if these two source files are located in the folder-subfolder structure
          • if the account.so has to be private, you can have another copy of the library and link it to another source file.

  • One can set properties of any target also in sub-dir CMakeLists.txt, i.e. basically anywhere? Sounds like a bad idea? I probably interpreted that incorrectly :) Can you clarify?

    • For PUBLIC properties, for example the -ffast-math, you can declare it anywhere as it is PUBLIC. In the code example you can set it in the root CMakeLists.txt or the leaf CMakeLists.txt
    • For PRIVATE properties, you have to declare it in the account folder using the command 
      target_sources(account
  PRIVATE
account.cpp
)
    • Then include add the add_subdirectory(account) to tell CMake where to find the static library and what library should be used 
      target_link_libraries(bank
  PRIVATE
    account
  )

  • I tested setting (in account/CMakeLists.txt)

    target_compile_options(bank
    PUBLIC
    "-ffast-math"
)
    • and it failed, which is good :) One cannot set target properties, even PUBLIC ones, in random places. The initial misconception was that only targets can have properties.
      • in random place I mean that you can use this PUBLIC property (-ffast-math) multiple times to generate the executable/libraries.
      • In the account/CMakeLists.txt, the compiler flag is only used to generate the account library 
        target_compile_options(account
  PUBLIC
    "-ffast-math"
 )
      • for the target_compile_options(bank PUBLIC "-ffast-math"), if you put it in the root CMake script, it should work. In this command, the compiler is used to obtain the bank executable.
      • Still not quite understanding the scope of properties. Adding 
        get_property(foo
  TARGET    account
  PROPERTY  target_compile_options
)

message("foo: ${foo}")
        to the root level CMakeLists.txt does not have access to account target's target_compile_options PUPLIC property (prints empty value). And no errors either.
        • this means that the you did not set the variable foo, and you did not get anything when running the get_property command.
        • let me debug it and paste the result in minutes

  • Regarding graphviz on Dardel, seems it is currently not installed.

    • Might be a module?
    • Check with ml spider graphviz ?
    • The hits for graphviz with spider are old builds which likely do not work. I am trying to install a newer version with Spack, using the specificationspack install [email protected]%[email protected] +ghostscript +gtkplus +qt +x. This will run for an hour so, as Spack builds up a large number of dependency programs and libraries.
    • ... and have a go at it also with EasyBuild.
    • Update: a new build of Graphviz with EasyBuild, but was not functional in runtime. The build attempts with Spack failed so far.

Exercise 07

See https://enccs.github.io/intro-cmake/probing/ and scroll down to "Exercise 07"

Exercise 08, 09

See https://enccs.github.io/intro-cmake/probing/ and scroll down to "Exercise 08", "Exercise 09"

  • How do I test runtime capabilities of MPI programs, which usually require some sort of custom launcher, srun, mpirun, aprun, etc?
    • Good question. On a shared server / supercomputer you typically cannot run srun, mpirun, aprun directly. You need first to request a node or a number of cores on a node(s). On Dardel you can use for instance salloc -n 4 -t 1:00:00 -A edu24.build -p shared which will provide you with 4 cores on a shared CPU node (which has 128 cores in total) for 1 hour.
    • How do I tell cmake (check_c_source_runs) to run the program with srun myapp instead of ./myapp ?
    • check the code example here https://enccs.github.io/intro-cmake/tips-and-tricks/#options-and-flow-control to see how to enable MPI when you build the system.
    • additional topics can be found at the CMake documents: https://cmake.org/cmake/help/latest/module/FindMPI.html
  • I have OpenMP3.1 only (that is what is fount at least). How can I fix that? I did load gcc/14.2.0 at UPPMAX.
    • If I recall correctly, OpenMP support largely depends on the compiler. There isn't much to do than trying to use a newer compiler.
    • If you had that new compiler loaded, then maybe Cmake found your system compiler instead for some reason? Maybe someone else knows better?
    • Yeah gcc/14 should have even openMP5.0 so I'm missing somewhere stating the right gcc version. cc should be gcc somewhere.
      • CMAKE_CXX_COMPILER= is the key!
      • This works from commandline: $ cmake -D CMAKE_CXX_COMPILER=gcc -S. -Bbuild
      • OpenMP 4.5 is available for gcc >=6.X.X !