Parallel computing with NGS-Py

There are several options to run NGS-Py in parallel, either in a shared-memory, or distributed memory paradigm.

Shared memory parallelisation

NGSolve shared memory parallelisation is based on a the task-stealing paradigm. On entering a parallel execution block, worker threads are created. The master thread executes the algorithm, and whenever a parallelized function is executed, it creates tasks. The waiting workers pick up and process these tasks. Since the threads stay alive for a longer time, these paradigm allows to parallelize also very small functions, practically down to the range of 10 micro seconds.

The task parallelization is also available in NGS-Py. By the with TaskManager statement one creates the threads to be used in the following code-block. At the end of the block, the threads are stopped.

with TaskManager():
    a = BilinearForm(fespace)
    a += SymbolicBFI(u*v)
    a.Assemble()

Here, the assembling operates in parallel. The finite element space provides a coloring such that elements of the same color can be processed simultaneously. Also helper functions such as sparse matrix graph creation uses parallel loops.

The default number of threads is the (logical) number of cores. It can be overwritten by the environment variable NGS_NUM_THREADS, or by calling the python function

SetNumThreads ( num_threads )

Another typical example for parallel execution are equation solvers. Here is a piece of code of the conjugate gradient solver from NGS-Py:

with TaskManager():

  ...
  for it in range(maxsteps):
      w.data = mat * s
      wd = wdn
      as_s = InnerProduct (s, w)
      alpha = wd / as_s
      u.data += alpha * s
      d.data += (-alpha) * w

The master thread executes the algorithm. In matrix - vector product function calls, and also in vector updates and innner products tasks are created and picked up by workers.

Distributed memory

The distributed memory paradigm requires to build Netgen as well as NGSolve with MPI - support, which must be enabled during the cmake configuration step.

Many ngsolve features can be used in the MPI-parallel version, some features are work in progress, some others may take for longer. The following list shows what is available:

Distributed parallel features

feature	y/n	comment
TaskManager	yes	allows for hybrid parallelization
mesh generation	no
mesh distribution	yes	using metis
uniform refinement	yes	refine the netgen-mesh
adaptive refinement	no
finite element spaces	yes	all spaces should work
Forms, Gridfunction	yes
periodic boundaries	no
discontinuous Galerkin	yes	apply operator
diagonal preconditioner	yes
multigrid preconditioner	no
BDDC preconditioner	yes
direct solvers	yes	MUMPs, Masterinverse
pickling	in progress