Skip to content

Latest commit

 

History

History
140 lines (119 loc) · 8.78 KB

FAQ.md

File metadata and controls

140 lines (119 loc) · 8.78 KB
Raw

Frequently Asked Questions

1. I have installed OpenCoarrays with OpenMPI but I'm having trouble running with more than a few images, why?

OpenMPI requires oversubscribed jobs (more MPI ranks/coarray images than logical CPU cores available) to pass the --oversubscribe flag to mpirun. To run a Coarray program with more images than available logical CPU cores, please pass --oversubscribe as an argument to the cafrun wrapper script. cafrun will pass any additional flags through to the underlying run-time launcher (i.e., mpirun). Here is an example invocation of cafrun demonstrating this procedure:

cafrun -np 32 --oversubscribe ./a.out <arg1> <arg2>

2. I have installed OpenCoarrays with a recent version of OpenMPI, but my Coarray Fortran programs won't run when launched with cafrun, why?

Recent versions of OpenMPI require a hostfile to be used, specifying the number of "slots" (i.e., logical CPU cores) available on each node or host on the system. If you are running OpenCoarrays on a production HPC system, your system administrator should have configured OpenMPI with a default hosts file or should provide instructions on how to generate your own. If you are running OpenCoarrays on a shared memory machine, such as a laptop, desktop, or workstation, then you may have to provide your own hostfile. On a shared memory machine, this hostfile should look something like:

hostname.local 8

where hostname is the hostname of the machine you are running on and 8 is the number of logical CPU cores. On a Macbook Pro (late 2013) with a 4 core Intel core i7 CPU with two hyper threads per core, 8 is the number of logical CPU cores. When OpenCoarrays is configured and installed, if OpenMPI is detected a hostfile is created in the build directory for use when running the test suite. (This hostfile corresponds to the machine/node used to configure OpenCoarrays using CMake.) If you installed OpenCoarrays via the install.sh script, then it will be located at ./prerequisites/builds/opencoarrays/<version>/hostfile or if you invoked CMake yourself it should be in the top level of your build directory at /path/to/build/dir/hostfile. (You must run CMake first, to create this file.)

3. install.sh is trying to download and install GCC/GFortran and its prerequisites, but I want to use GCC version X already present on my system, how can I do this?

install.sh defaults to trying to install the most recent stable version of GFortran and GCC that has the best functionality with OpenCoarrays. This is usually the latest stable version of GCC/GFortran, however, sometimes regressions are present and a slightly older version is set as the default until most of the serious regressions are resolved. If you know what you are doing and wish to use a different Fortran/C compiler, you can explicitly pass arguments to install.sh to specify which compilers to use. Please see the usage information for install.sh by invoking it with the --help flag. For example, the -f or --with-fortran flag is passed with the path to the desired Fortran compiler, the -c or --with-c flag is passed with the path to the desired C compiler and the -C or --with-cxx flag is passed with the path to the desired C++ compiler. Alternatively the "Developer/quick-start instructions" (located in the [INSTALL] file) may be followed to perform the installation using CMake directly, without calling install.sh. The compilers are specified using the FC, CC and CXX environment variables, and all prerequisites (GFortran, CMake, and a suitable MPI implementation) are assumed to be already installed on the system. If you pass one compiler-specification argument (e.g., -f), it is best to pass all three (e.g., -f, -c, and -C) to ensure consistency.

4. How can I uninstall OpenCoarrays?

After installing OpenCoarrays, you can enter the build directory (prerequisites/builds/opencoarrays/<version> if installed via install.sh) and run make uninstall. A script or additional flag to install.sh is planned to automate this process, but has yet to be implemented. When OpenCoarrays is installed a file manifest is written in this build directory, so you can examine which files get installed and their locations on your system.

5. OpenCoarrays was built with MPICH as the MPI back end, but I am running into bugs, what should I do?

There is a bug in MPICH that, as of this writing, is patched, but is not yet in a stable release. This bug effects the failed images functionality, which can be disabled during configuration. If you encounter issues using OpenCoarrays with MPICH, reinstalling OpenCoarrays without failed image support may resolve the issue. To do so you will have to perform a manual CMake configuration and installation as described in the "Developer/quick start" installation guide documented in the file [INSTALL]. Please pass the -DCAF_ENABLE_FAILED_IMAGES=FALSE flag to CMake when configuring OpenCoarrays. If this doesn't resolve your issue, please file a new issue.

6. How can I pass additional flags through to the underlying parallel run-time or compiler?

The caf compiler wrapper script and the cafrun Coarray Fortran program launch script will both pass additional flags through to the underlying compiler and parallel run-time job launcher, respectively. Specify the additional flags to be passed to GFortran or mpirun after any flags specific to caf or cafrun such as the -s flag to show the underlying command, or the -np <N> flag to specify the number of images in the cafrun script and before any files such as Fortran source files or Coarray Fortran executables.

7. When install.sh builds the GCC compilers, it takes forever (hours). How can I speed up the build?

To increase the odds of success, install.sh defaults to a GCC bootstrap build, which builds a minimal compiler to build the ultimate compiler (not every version of GCC can build every other version of GCC). For a much faster build process that has a somewhat higher chance of failing, pass the --disable-bootstrap or -z argument and use more threads by passing, for example, --num-threads -4 or -j 4 to use four threads. In combination, these two recommendations can decrease the GCC build time from several hours to 15 or fewer minutes.