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

yes

allows for hybrid parallelization

mesh generation

no

mesh distribution

yes

using metis

uniform refinement

yes

refine the netgen-mesh

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