# Known issues in parallel

(Difference between revisions)
 Revision as of 11:31, 10 March 2012Zaleski (Talk | contribs) (→A two-box simulation gives different results with one or two pids - updated Gerris version)← Previous diff Revision as of 11:41, 10 March 2012Zaleski (Talk | contribs) (→A two-box simulation gives different results with one or two pids - added scheme=none)Next diff → Line 1: Line 1: == A two-box simulation gives different results with one or two pids == == A two-box simulation gives different results with one or two pids == - [[user:zaleski|We]] run a simplified version of the [http://gerris.dalembert.upmc.fr/gerris/examples/examples/cylinder.html#htoc3 Benard Von-Karman test case]. The modified simulation file is + [[user:zaleski|We]] run a simplified version of the [http://gerris.dalembert.upmc.fr/gerris/examples/examples/cylinder.html#htoc3 Benard Von-Karman test case] for zero time steps. Only the approximate projection + is performed. The modified simulation file is

Line 8:                                                                                                                                                                                                                                                                                                                                                                      Line 9:
Refine 6                                                                                                                                                                                                                                                                                                                                                             Refine 6
Solid (x*x + y*y - 0.0625*0.0625)                                                                                                                                                                                                                                                                                                                                    Solid (x*x + y*y - 0.0625*0.0625)
+                                                                                                                                                                   scheme = none
+                                                                                                                                                                   }
+
OutputTime { istep = 1 } stderr                                                                                                                                                                                                                                                                                                                                      OutputTime { istep = 1 } stderr
OutputProjectionStats { istep = 1 } stderr                                                                                                                                                                                                                                                                                                                           OutputProjectionStats { istep = 1 } stderr

## A two-box simulation gives different results with one or two pids

We run a simplified version of the Benard Von-Karman test case for zero time steps. Only the approximate projection is performed. The modified simulation file is

2 1  GfsSimulation GfsBox GfsGEdge {} {
Time { iend = 0 }
Refine 6
Solid (x*x + y*y - 0.0625*0.0625)
scheme = none
}

OutputTime { istep = 1 } stderr
OutputProjectionStats { istep = 1 } stderr
OutputSimulation { start = 0.1 step = 0.1} simulation.gfs {
variables = U,V,P
}
}

GfsBox { id=1 pid=0
left = Boundary {
BcDirichlet U 1
}
}
GfsBox { id=2 pid=1 right = BoundaryOutflow }
1 2 right

We use the following version of Gerris

% gerris2D -V
gerris: using 2D libgfs version 1.3.2 (120310-112425)
compiled with flags:  -DBSD_SOURCE -D_DARWIN_C_SOURCE -D_DARWIN_C_SOURCE
MPI:          yes
pkg-config:   yes
m4:           yes

Here is the result of running without mpi on MacOS 10.7.3 on a MacBook Pro with a four-core intel i7 system.

% gerris2D twobox-twopid.gfs
step:       0 t:      0.00000000 dt:  1.000000e-01 cpu:      0.12000000 real:      0.12236900
Approximate projection
niter:   13
residual.bias:   -1.000e-01 -1.984e-04
residual.first:   5.020e-02  9.960e-05    1.6
residual.second:  5.668e-01  1.330e-04    1.9
residual.infty:   6.400e+00  6.251e-04      2

On the other hand, if we run the same simulation with mpi (openmpi installed with macports) and two pids, this is the result:

% mpirun -np 2 gerris2D twobox-twopid.gfs
step:       0 t:      0.00000000 dt:  1.000000e-01 cpu:      0.04000000 real:      0.03555900
Approximate projection
niter:    4
residual.bias:   -1.000e-01 -7.446e-05
residual.first:   5.020e-02  3.839e-05      6
residual.second:  5.668e-01  4.914e-05     10
residual.infty:   6.400e+00  2.713e-04     12

The two results are different: the pre-iteration Projection statistics (first column) are the same but the post-iteration Projection statistics are different. However, since there are the same number of boxes, the mpi communication should send the same information that is exchanged between boxes in the non-mpi run. Thus something is amiss in the way information is exchanged between boxes.

Using larger number of boxes and pids (typically 24), we found cases where the non-mpi runs converge but the mpi run do not converge, i.e. the residual is not reduced below the required minimum of 0.001 .

We have also run the same test case on an Ubuntu system. Here are the results. 1. non-mpi

% gerris2D cylinder.gfs
step:       0 t:      0.00000000 dt:  5.263158e-03 cpu:      0.19000000 real:      0.20179100
Approximate projection
niter:   13
residual.bias:   -5.020e-02 -9.960e-05
residual.first:   5.020e-02  9.960e-05    1.6
residual.second:  5.668e-01  1.330e-04    1.9
residual.infty:   6.400e+00  6.251e-04      2

2. mpi

% mpirun -np 2 gerris2D cylinder.gfs
step:       0 t:      0.00000000 dt:  5.263158e-03 cpu:      0.05000000 real:      0.05292300
Approximate projection
niter:    4
residual.bias:   -5.020e-02 -3.737e-05
residual.first:   5.020e-02  3.839e-05      6
residual.second:  5.668e-01  4.914e-05     10
residual.infty:   6.400e+00  2.713e-04     12

Notice the results are the same, except for the cpu times and residual bias.