Running OGS with MPI

The executable OGS for MPI parallel computing is compiled with a special build configuration. If you need to compile the source code, please read Build configuration for MPI and PETSc.

To conduct DDC enabled parallel computing with OGS, following steps are required:

1. Prepare input data

1.1 Partition mesh

For the domain decomposition approach, an application of OGS using METIS as a node-wise mesh topology partitioner, partmesh, is provided to partition meshes by node. An example of how to partition meshes is available in a workflow documentation on how to create a simple parallel model. You can type partmesh --help for the detailed command line options.

The partitioned meshes are written in binary files for a fast parallel reading in OGS.

1.2 Configure PETSc solver in project file

Setting the PETSc solver is the only change the in project file for running parallel OGS with PETSc. For the linear solver, it is done by adding a tag of petsc inside linear_solver tag, e.g:

    <linear_solvers>
        <linear_solver>
            <name>linear_solver</name>
            <eigen>
                <solver_type>SparseLU</solver_type>
                <scaling>true</scaling>
            </eigen>
            <petsc>
                <parameters>-ksp_type bcgs
                    -pc_type jacobi
                    -ksp_rtol 1.e-16 -ksp_atol 1.e-12
                    -ksp_max_it 4000
                </parameters>
            </petsc>
        </linear_solver>
    </linear_solvers>

If the tag of petsc is not given in project file, the default setting of the PETSc linear solver will be taken, which uses the solver type of cg and the preconditioner type of jacobi.

For the simulation of coupled processes with the staggered scheme, a prefix can be used by the tag prefix to set the PETSc solver for each process, individually. For example:

    <linear_solvers>
        <linear_solver>
            <name>linear_solver_T</name>
            <petsc>
                <prefix>T</prefix>
                <parameters>-T_ksp_type bcgs
                            -T_pc_type bjacobi
                            -T_ksp_rtol 1e-16
                            -T_ksp_max_it 3000
                </parameters>
            </petsc>
        </linear_solver>
        <linear_solver>
            <name>linear_solver_H</name>
            <petsc>
                <prefix>H</prefix>
                <parameters>-H_ksp_type bcgs
                            -H_pc_type mg
                            -H_ksp_rtol 1e-12
                            -H_ksp_max_it 4000
                </parameters>
            </petsc>
        </linear_solver>
    </linear_solvers>

The above example shows that once a prefix is given for PETSc linear solver settings, the original prefix of PETSc keyword, -, can be replaced with a new prefix, -[given prefix string]_. In the above example, -, is replaced with -T_ and -H_, respectively.

An introduction and a list of PETSc KSP solvers and preconditioners can be found by this link.

2. Launch MPI OGS

For MPI launcher, either mpiexec or mpirun can be used to run OGS. Preferably, mpiexec is recommended because it is defined in the MPI standard. The number of processes to run of mpiexec must be identical to the number of mesh partitions. For example, if the meshes of a project, foo.prj, are partitioned into 5 partitions, OGS can be launched in MPI as

mpiexec -n 5 ogs foo.prj -o [path to the output directory]

Running PETSc enabled OGS with one compute thread does not need mesh partitioning. However, the MPI launcher mpiexc or mpirun is required, e.g.:

mpiexec -n 1 ogs ...

Additional PETSc command line options can be given as unlabelled arguments at the end of the OGS run command preceded by two minus-signs (... ogs ... -- [PETSc options]).

With PETSc command line options, you can

• monitor KSP solver convergence status, e.g.:

mpiexec -n 5 ogs foo.prj -o output -- -ksp_converged_reason -ksp_monitor_true_residual

• change KSP solver setting, e.g.:

mpiexec -n 5 ogs foo.prj -o output -- -ksp_type gmres -ksp_rtol 1e-16 -ksp_max_it 2000

• or use other PETSc command line options.

For Linux clusters or supercomputers, a computation job has to be submitted to the queue and job management system, which may require a special command to launch the MPI job. A job script for such queue system is required.

The cluster system EVE of UFZ uses SLURM (Simple Linux Utility for Resource Management) to manage computing jobs. Here is an example of a job script for the SLURM system on EVE:

#!/bin/bash
#SBATCH --job-name=ARESH_HM_3D_20
#SBATCH --chdir=/home/wwang/data_D/project/AREHS/HM_3D
#SBATCH --output=/home/wwang/data_D/project/AREHS/HM_3D/output/log_%x_%j.txt
#SBATCH --error=/home/wwang/data_D/project/AREHS/HM_3D/output/err_%x_%j.txt
#SBATCH --time=0-48:00:00
#SBATCH -n 20
#SBATCH --mem-per-cpu=4G

#SBATCH --mail-user=wenqing.wang@ufz.de
#SBATCH --mail-type=BEGIN,END

export MODULEPATH="/software/easybuild-broadwell/modules/all/Core:/software/modulefiles"
module load  foss/2020b   petsc-bilke/3.16.5_foss2020b
module load  OpenMPI/4.0.5  HDF5/1.10.7  GMP/6.2.0

APP="/home/wwang/code/ogs6/exe_petsc/bin/ogs"
PRJ_FILE="/home/wwang/data_D/project/AREHS/HM_3D/simHM_glaciation.prj"
/bin/echo In directory: `pwd`
/bin/echo Number of CPUs: $SLURM_CPUS_PER_TASK
/bin/echo File name: $1

srun $APP $PRJ_FILE -o /home/wwang/data_D/project/AREHS/HM_3D/output

In the job script for EVE, module load foss/2020b must be presented, and srun is a sort of MPI job launcher. If a job fails with an error message about a ‘shared library not found’, you can check the EVE modules specified in the files in the source code directory: scripts/env/eve, and add the corresponding modules to the load list in the job script.

Once the job script is ready, you can

• submit the job by command, sbatch [job script name],
• check the job status by command squeue,
• or cancel the job by command scancel [job ID].

For the detailed syntax of job script of SLURM for EVE, please visit https://wiki.ufz.de/eve/ (user login required).

2a. Use a container to launch MPI OGS

A prebuilt container with ogs (current master) is available at:

• /data/ogs/apptainer/guix/ogs-petsc_head.squashfs

You need to modify your submit script, e.g.:

...
#SBATCH ...

srun apptainer exec /data/ogs/apptainer/guix/ogs-petsc_head.squashfs ogs $PRJ_FILE

3. Check results

There are two output types available, VTK and XDMF. In the project file, output type can be specified in the type tag of the output tag, for example:

    <output>
       <type>VTK</type>
       ...
    </output>

or

    <output>
       <type>XDMF</type>
       ...
    </output>

3.1 VTK output

The results are output in the partitioned VTU files, which are governed by a PVTU file. The data in the ghost cells of VTU files are overlapped. An OGS utility, pvtu2vtu, is available to merge the partition VTU files into one VTU file, meanwhile to eliminate the data overlapping. Here is an example to use that tool:

pvtu2vtu -i foo.pvtu -o foo.vtu

Where the input file name is the name of PVTU file.

If you use the merged mesh together with some meshes that for initial or boundary conditions for a restart simulation, you have to reset the bulk geometry entity ( node, face, or element) IDs of the meshes to the merged mesh by using toolidentifySubdomains. For example, north.vtu, south.vtu, and top.vtu are the meshes for the boundary conditions, their bulk geometry entity IDs can be reset by running the following command:

identifySubdomains -f -m foo.vtu  --  north.vtu  south.vtu  top.vtu

3.2 XDMF output

With XDMF, OGS outputs two files, one XDMF file and one HDF5 file with file name extension of h5. You can use ParaView to open the XDMF file by selecting Xdmf3ReaderS or Xdmf3ReaderT. The XDMF output is highly recommended for running OGS with a large mesh, especially on supercomputers.