Ofer Filiba

My final project is "Parallelization of molecular dynamics'', supervised by ``Dr Joan Adler''. I thank P.Pine and J. Halupovich for assistance and RBNI for funding for the computer resources.

This project originated from the need to write a more efficient code for simulating vibrations of nanotubes (for more details please see papers by Polina Pine). Thus the scalar Molecular Dynamics code used by P. Pine was partly parallelized with MPI and run for unclamped nanotubes.

Molecular dynamics

Molecular dynamics is a way to simulate a physical behavior on the atomic level through a computer program. In order for the simulation to yield correct results, it's important to choose wisely a relevant potential. This simulation is based on the potential which was published by Donald.W Brenner in Physical Review B. Here is a short summary of the BRENNER potential.

 

About MPI (Message Passing Interface):

First thing about MPI is that it's not a programming language, but rather a tool that controls the distribution of some (computational) work among a group of processors. Through this interface each processor is assigned specific task/s and communicates with the other processors when necessary.

The core idea behind parallel computation is that each processor receives copy of the whole code. Inside the code lines there are instructions for each processor what to do and how and when to communicate with other processors. This is much like a screenplay of a show where each actor (processor) gets the whole screenplay but knows exactly what to say in each step of the show.

 

About "Tamnun"

TAMNUN is a computer cluster of 1056 cores that has been purchased (arrived at end of March 2012) by the Technion from the SGI-TNN company and is available for the use of all researchers at the Technion.  This new cluster was funded by the Russell Berrie Nanotechnology Institute and the Minerva Foundation.

 

Compiling on Tamnun

For "Fortran" source code, the compilation command is "ifort" for a serial code and "mpiifort" for parallel code.

Here is an example of the command that I used to compile the parallel code: compile.

Notes:

       i.            The addition of "2> log.txt" at the end of the command is very useful when the compiler generate a long list of errors: all the errors are written into a single text file.

     ii.            The "-I" operator is used to link other relevant code in other libraries.

 

Running a job on Tamnun

Running a job (serial/ parallel) on an HPC is carried out through a queuing system. The execution is done through "qsub" command, after which comes the name of the shell file. The shell file is a script that specifies everything for a successful run of the program.

For example:

·        Running a serial job:

qsub scalar_job.sh

·        Running a parallel job:

qsub parallel_job.sh

 

Source code

Those are the code lines that I've modified (See the text files):

·        Main.f

·        Pred (inside pred_corr.f)

Other parts of the code weren't changed and are represented in a general flowchart of the serial code

 

Parallelization scheme

Here is a schematic diagram of how the data was distributed and gathered

 

Goal of the project

The aim of my project was to parallelize a specific part of the serial code, which calculates the first part of the predictor-corrector algorithm.

 

The predictor-corrector algorithm

The predictor-corrector algorithm is used to solve the classical Newtonian equation of motion (which is a second-order differential equation). This algorithm is divided into three parts:

·        Prediction

·        Evaluation

·        Correction

At the first two steps of the algorithm the positions and velocities of the atoms are predicted and evaluated at some future time interval (t+Dt). The calculation is based on the positions and the velocities at time some past time (t+i∙Dt), where i = 0,...,k-2, and k being the order of the predictor part (here we are using third order predictor).
At the third part of the algorithm the calculated values are corrected from the information based on the force (Newton first law of motion) at the some future time (t+Dt).

*for more information about the predictor-corrector algorithm you can see Polin Pine's site.

Results

Four samples were chosen and generated through a computer program "Chrial".

Two samples were generated according to chiral vector (m,n)=(7,7) and consisted of 50/100 periods (splines) of 28 atoms each, Thus making two samples of 1400 and 2800 atoms.

The other two samples were generated according to chiral vector (m,n)=(7,0) and consisted of 40/80 periods (splines) of 28 atoms each, Thus making two samples of 1120 and 2240 atoms.

Click here to see the wall time distribution of for subroutine PRED.