wave_mpi, a C++ code which solves the 1D wave equation in parallel, using MPI.
This program solves the 1D wave equation of the form:
Utt = c^2 Uxxover the spatial interval [X1,X2] and time interval [T1,T2], with initial conditions:
U(X,T1) = U_T1(X), Ut(X,T1) = UT_T1(X),and boundary conditions of Dirichlet type:
U(X1,T) = U_X1(T),The value C represents the propagation speed of waves.
U(X2,T) = U_X2(T).
The program uses the finite difference method, and marches forward in time, solving for all the values of U at the next time step by using the values known at the previous two time steps.
Central differences may be used to approximate both the time and space derivatives in the original differential equation.
Thus, assuming we have available the approximated values of U at the current and previous times, we may write a discretized version of the wave equation as follows:
Uxx(X,T) = ( U(X+dX,T ) - 2 U(X,T) + U(X-dX,T, ) ) / dX^2 Utt(X,T) = ( U(X, T+dT) - 2 U(X,T) + U(X, T-dt) ) / dT^2If we multiply the first term by C^2 and solve for the single unknown value U(X,T+dt), we have:
U(X,T+dT) = ( C^2 * dT^2 / dX^2 ) * U(X+dX,T) + 2 * ( 1 - C^2 * dT^2 / dX^2 ) * U(X, T) + ( C^2 * dT^2 / dX^2 ) * U(X-dX,T) - U(X, T-dT)(Equation to advance from time T to time T+dT, except for FIRST step!)
However, on the very first step, we only have the values of U for the initial time, but not for the previous time step. In that case, we use the initial condition information for dUdT which can be approximated by a central difference that involves U(X,T+dT) and U(X,T-dT):
dU/dT(X,T) = ( U(X,T+dT) - U(X,T-dT) ) / ( 2 * dT )and so we can estimate U(X,T-dT) as
U(X,T-dT) = U(X,T+dT) - 2 * dT * dU/dT(X,T)If we replace the "missing" value of U(X,T-dT) by the known values on the right hand side, we now have U(X,T+dT) on both sides of the equation, so we have to rearrange to get the formula we use for just the first time step:
U(X,T+dT) = 1/2 * ( C^2 * dT^2 / dX^2 ) * U(X+dX,T) + ( 1 - C^2 * dT^2 / dX^2 ) * U(X, T) + 1/2 * ( C^2 * dT^2 / dX^2 ) * U(X-dX,T) + dT * dU/dT(X, T )(Equation to advance from time T to time T+dT for FIRST step.)
It should be clear now that the quantity ALPHA = C * dT / dX will affect the stability of the calculation. If it is greater than 1, then the middle coefficient (1-C^2 dT^2 / dX^2) is negative, and the sum of the magnitudes of the three coefficients becomes unbounded.
We wish to use MPI in order to accelerate this computation. We use the method of domain decomposition - that is, we assume we have P MPI processes, we divide the original interval into P subintervals, and we expect each process to update the data associated with its subinterval.
However, to compute the estimated solution U(X,T+dT) at the next time step requires information about U(X-dX,T) and U(X+dX,T). When process ID tries to make these estimates, it will need one value from process ID-1, and one from process ID+1, before it can make all the updates. MPI allows the processes to communicate this information using messages.
At the end of the complete calculation, we wish to print a table of the solution at the final time. To do this in an organized fashion, we want each process to send its final result to the master process (with ID = 0). Once all the data has been collected, the master process prints it.
The latest versions of MPI no longer support the special C++ MPI bindings, so the examples given here have reverted to using the C MPI bindings.
The computer code and data files described and made available on this web page are distributed under the MIT license
wave_mpi is available in a C version and a C++ version and a FORTRAN90 version.
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