[petsc-users] petsc4py and MPI.COMM_SELF.Spawn

[Barry Smith]

  Always cut and paste the full error message:

   Based on the fact that import petsc4py doesn't work this means that either petsc4py did not build successfully or the environment is not correct; for example is PYTHONPATH set properly?


> On Feb 28, 2017, at 6:17 PM, Rodrigo Felicio <Rodrigo.Felicio at iongeo.com> wrote:
> 
> Dear All,
> 
> I am new to both pestc (petsc4py) and MPI. I am prototyping  an application that acts on data belonging to thousands of different spatial locations, so I decided to use mpi4py to parallelize my computations over said  set of points. At each point, however, I have to solve  linear systems of the type A x = b, using least-squares. As matrices A are sparse and large, I would like use petsc LSQR solver to obtain x. Trouble is that  I cannot get mpi4py to work with petsc4py when using MPI.Spawn(), so that I could  solve those linear systems using threads spawned from my main algorithm. I am not sure if this is a problem with my petsc/mpi installation, or if there is some inherent incompatibility between mpi4py and petsc4py on spawned threads.  Anyway, I would be really  thankful to anyone who could shed some light on this issue. For illustration,  the child and parent codes found on the pestc4py spawning demo folder fail for me if I just include the petsc4py initialization in either of them. In fact, just the introduction of the line " from petsc4py import PETSc" is enough to make the code to hang and issue, upon keyboard termination, error msgs of the type "spawned process group was unable to connect back to parent port" .
> 
> Kind regards
> Rodrigo
> 
> ------------------------
> CHILD code:
> -------------------------
> 
> # inclusion of petsc4py lines causes code to hang
> import petsc4py
> petsc4py.init(sys.argv)
> from petsc4py import PETSc
> from mpi4py import MPI
> from array import array
> 
> master = MPI.Comm.Get_parent()
> nprocs = master.Get_size()
> myrank = master.Get_rank()
> 
> n  = array('i', [0])
> master.Bcast([n, MPI.INT], root=0)
> n = n[0]
> 
> h = 1.0 / n
> s = 0.0
> for i in range(myrank+1, n+1, nprocs):
>    x = h * (i - 0.5)
>    s += 4.0 / (1.0 + x**2)
> pi = s * h
> 
> pi = array('d', [pi])
> master.Reduce(sendbuf=[pi, MPI.DOUBLE],
>              recvbuf=None,
>              op=MPI.SUM, root=0)
> 
> master.Disconnect()
> 
> 
> ---------------------
> MASTER CODE:
> ----------------------
> from mpi4py import MPI
> from array import array
> from math import pi as PI
> from sys import argv
> 
> cmd = 'cpi-worker-py.exe'
> if len(argv) > 1: cmd = argv[1]
> print("%s -> %s" % (argv[0], cmd))
> 
> worker = MPI.COMM_SELF.Spawn(cmd, None, 5)
> 
> n  = array('i', [100])
> worker.Bcast([n,MPI.INT], root=MPI.ROOT)
> 
> pi = array('d', [0.0])
> worker.Reduce(sendbuf=None,
>              recvbuf=[pi, MPI.DOUBLE],
>              op=MPI.SUM, root=MPI.ROOT)
> pi = pi[0]
> 
> worker.Disconnect()
> 
> print('pi: %.16f, error: %.16f' % (pi, abs(PI-pi)))
> 
> 
> 
> 
> 
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> 
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