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Queues

Venus's queueing system is very basic. There is only one queue. Jobs may run for up to 72 hours and priority is done by fairshare. There's an example job script in /info/slurm .

Types of node

venus has two types of node with different CPU types. The better nodes are assigned to jobs first. To ensure you get assigned only the better nodes you can request only to have 'westmere' nodes:

sbatch -n12 --constraint=westmere

Because the two types of node have different CPU types it's possible to generate very optimized code that can't run on the older ones. If you get an 'illegal instruction' error, that's probably what's happened.

The older nodes have much less RAM than the newer ones.

Parallel work

Venus is a cluster system and so message passing libraries are used to make code run in parallel. There are several different libraries installed. Shared memory parallelization (OpenMP, provided by the autoparallelizing options on some of the compilers) can also be used for up to twelve cores but not further.

The available message passing libraries are MPI implementations: currently we have OpenMPI in various versions.

To change your parallel environment you need to load the appropriate module. To switch you'd edit your .bash_profile file and change the module add line.

Compile your code with the MPI compilers. These are usually called mpicc, mpicxx, mpif77 and mpif90.

To run MPI code you normally use a command such as mpirun to launch your program. On venus there is a queueing system which assigns work to compute nodes, and so the MPI library has to interact with that which makes things more complicated. You use a special MPI launcher program to start your job. The launcher doesn't take any arguments but works out the correct number of cpus and which nodes to use from the queueing system. You must use the appropriate one for your MPI library, and you must have your environment configured for the library you want to use when you submit a job, as otherwise the job won't be able to find its library files. Here's a list of which launcher command goes with which library:

MPI Command name
OpenMPI mpirun

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