Difference between revisions of "Parallel processing"

Revision as of 18:14, 12 March 2007

The historical need in numerical models to reduce computational time became a priority to the Mohid development team as an operational hydrodynamic and water quality model to the Tagus Estuary, in Lisbon, Portugal, was implemented using the Mohid Water model full capabilities. Thus, parallel processing has been implemented in Mohid Water in 2003, by using MPICH, a free portable implementation of MPI, the standard for message-passing libraries.

Currently, and due to the use of the new Intel Fortran compiler both Mohid Water and Mohid Land have parallelization features using OpenMP.

Parallel processing via MPI

The Mohid Water ability to run nested models was accomplished by creating a linked list of all the models and by attributing to each one a father-son identification, through which the models communicate. The first stage for introducing parallel processing in Mohid was to add the possibility of launching a process by each model to run, and then, using MPICH, establish communication between models. This enables each sub-model to run in a different processor (even if the processor belongs to a different computer, as long as it is in the same network) and in parallel, instead of running all in the same processor and each model having to wait for the others to perform their calculations.

Parallel processing as it is presently implemented in Mohid, could not be achieved without object oriented programming philosophy, as each model is an instance of class Model and no changes, exception made to the implementation of the MPI communications calls needed to be added. Using this feature, computational speed was improved (varying from application to application), as now the whole model will take the same time as the slowest model to run plus the time to communicate with the other processes. Here, the network communication speed plays an important role, as it can become limiting. However, the amount of information passing between models, depending of course on the memory allocated for each model, has not yet proven to be big enough to make a 100 Mbps network connection time limiting.

Domain Composition is an ongoing project, in a very early stage, aimed at decoupling a domain into several subdomain that communicate among them (2-way) via MPI.

Find here information on how to setup a MOHID simulation using MPI and on compiling Mohid with MPI.

Parallel processing via OpenMP

Parallel processing using OpenMP is currently being implemented in Mohid by defining directives to optimize loops. These directives are defined as comments in the code and therefore need special compilation options. See more on compiling Mohid with OpenMP.