Known issues in parallel
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| == 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#htoc3Benard 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/example/cylinder.html#htoc3|Benard Von-Karman test case]. The modified simulation file is |
| <pre> | <pre> | ||
Revision as of 09:04, 10 March 2012
A two-box simulation gives different results with one or two pids
We run a simplified version of the Von-Karman test case. The modified simulation file is
2 1 GfsSimulation GfsBox GfsGEdge {} {
Time { iend = 0 }
Refine 6
Solid (x*x + y*y - 0.0625*0.0625)
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 (120229-075733 + local changes) compiled with flags: -DBSD_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: 5.263158e-03 cpu: 0.12000000 real: 0.12951800
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: 5.263158e-03 cpu: 0.03000000 real: 0.03374700
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.
