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usage.md

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Usage

AmgXWrapper usage is simple. Here is a simple example usage:

Step 1

In your PETSc application, at any point after PetscInitialize(...), declare an instance of AmgXSolver.

AmgXSolver      solver;

Step 2

Next, at any point you like, initialize this instance.

solver.initialize(comm, mode, configFile);

  1. comm is the preferred MPI communicator. Typically, this is MPI_COMM_WORLD or PETSC_COMM_WORLD.

  2. mode is a std::string indicating AmgX solver mode. The mode string is composed of four letters. The first letter (lowercase) indicates if AmgX solver will run on GPU (d) or CPU (h). The second letter (uppercase) indicates the precision of matrix entries: float (F) or double (D). The third one (uppercase) indicates the precision of the vectors. The last one indicates the integer type of the indices of matrix and vector entries (currently only 32bit integer is supported, so only I is available). For example, mode = dDDI means the AmgX solver will run on GPUs, using double precision floating numbers for matrix and vector entries, and using 32-bit integers for indices.

  3. configFile is a std::string indicating the path to AmgX configuration file. Please see AmgX Reference Manual for writing configuration files. For example, here is a typical solver and preconditioner settings we used in PetIBM simulations.

The return values of all AmgXWrapper functions are PetscErrorCode, so one can call these functions and check the returns in PETSc style:

ierr = solver.initialize(comm, mode, configFile); CHKERRQ(ierr);

It's also to combine the first step and this step together:

AmgXSolver      solver(comm, mode, configFile);

Step 3

After done creating the coefficient matrix using PETSc, upload the matrix to GPUs and bind to the solver with

ierr = solver.setA(A); CHKERRQ(ierr);

A is the coefficient matrix with data type Mat from PETSc.

No matter how many GPUs and how many CPU cores or nodes being used, setA will handle data gathering/scattering automatically.

Step 4

After creating the right-hand-side vector, the system can be solved through:

ierr = solver.solve(lhs, rhs); CHKERRQ(ierr);

lhs is a PETSc Vec type vector holding the initial guess of solution. Also, lhs will holding the final solution in the end. rhs is a PETSc Vec type vector for the right-hand-side. Matrix A, lhs, and rhs must be created with the same MPI communicator.

For time-marching problems, the same linear system may be solved again and again with different right-hand-side values. In this case, simply call solver.solve again with updated right-hand-side vector.

Step 5 (optional)

If interested in the number of iterations used in a solve, use

int     iters;
ierr = solver.getIters(iters); CHKERRQ(ierr);

The iters is the number of iterations in the last solve.

solver.getResidual(...) can be used to obtain the residual at any iteration in the last solve. For example, to get the residual at the second iteration, use

double      res;
ierr = solver.getResidual(2, res); CHKERRQ(ierr);

Step 6

Finalization can be done manually:

ierr = solver.finalize(); CHKERRQ(ierr);

The destructor of the class AmgXSolver also does the same thing. This benefits when using pointer. For example,

AmgXSolver  *solver = new AmgXSolver(comm, mode, configFile);

solver->setA(A);

solver->solve(lhs, rhs);

delete solver;

Note:
If memory leak is a concern, all AmgXSolver instances must be finalized before calling PetscFinalize(). This is because there are some PETSc data in AmgXSolver instances.