On Wed, Aug 11, 2010 at 11:38:12AM +0200, Ola Skavhaug wrote: > On Mon, Aug 9, 2010 at 4:19 PM, Anders Logg <l...@simula.no> wrote: > > On Mon, Aug 09, 2010 at 03:02:18PM +0100, Garth N. Wells wrote: > >> On Mon, 2010-08-09 at 15:54 +0200, Ola Skavhaug wrote: > >> > On Mon, Aug 9, 2010 at 3:39 PM, Garth N. Wells <gn...@cam.ac.uk> wrote: > >> > > On Mon, 2010-08-09 at 15:32 +0200, Ola Skavhaug wrote: > >> > >> Hi, > >> > >> > >> > >> when running DOLFIN in parallel, data reduction would be a nice thing > >> > >> to have. Typically, it would be good to have something like this: > >> > >> > >> > >> mesh = UnitSquare(2000, 2000) > >> > >> reduced_mesh = UnitSquare(200, 200) > >> > >> > >> > >> V = FunctionSpace(mesh, "CG", 1) > >> > >> reduced_V = FunctionSpace(reduced_mesh, "CG", 1) > >> > >> > >> > >> u = Function(V) > >> > >> > >> > >> ... > >> > >> > >> > >> reduced_u = interpolate(u, reduced_V) > >> > >> f << reduced_u > >> > >> > >> > >> However, the problem is that the automatic partitioning of the meshes > >> > >> constructs two different non-overlapping meshes on each local node. I > >> > >> really don't want to allow extrapolation in this case. I would be > >> > >> happy to contribute fixing this problem; does anybody see how to > >> > >> attack it? > >> > >> > >> > > > >> > > We just need to have the program handle correctly the case in which a > >> > > point is not found in the domain. It shouldn't be hard to fix (it may > >> > > be > >> > > just a case of removing an error message ans stressing in the code > >> > > comment that points must reside on the same process). > >> > > > >> > > Start by looking in the Function::interpolate functions. > >> > > >> > Isn't the issue here that since the local meshes don't overlap, we > >> > need to fetch the values from one or more neighboring processes? So, > >> > during interpolation in parallel, all processes send a list of > >> > coordinates for the values needed that are not local, and then all > >> > processes send and receive these values. Or is this too simple for > >> > more advanced elements? > >> > > >> > >> The first step of finding the cell which contains a point doesn't depend > >> on the element. We don't have this in place for points which reside on > >> other processes. The next step would be for the value of function to be > >> evaluated and communicated. > > > > I suggest that when a point is found which is not inside the domain > > and MPI::num_processes > 1, the point is added to a list of points > > which are not inside the domain. Then all the missing points are > > distributed to all processes and each process goes through the list of > > points to see which points it can handle. Then the results are > > communicated back. I think this is the same as what Ola suggests. > > Yes, this is more or less what I have in mind. Should I add a > structure in Data to hold the points?
In Data? That doesn't sound like an optimal solution. Data is supposed to be a small data structure for local data. We probably need a new data structure for that, or maybe just some std::vector<>s as part of the algorithm that does the interpolation. -- Anders > > The problem with this is that it can potentially lead to quite a lot > > of communication. In the worst case, all points would be missing for > > all processes and then all points would have to be passed around > > between all processes. But I don't see any other way since it's not > > possible for a process to know where a missing point may be located. > > Yes, this might be pretty bad. Also if the two different meshes don't > overlap at all, all points in each mesh are outside the other local > mesh, giving lots of values to send around. But lets not worry too > much about this yet :) > > Ola > > > > > -----BEGIN PGP SIGNATURE----- > > Version: GnuPG v1.4.10 (GNU/Linux) > > > > iEYEARECAAYFAkxgDncACgkQTuwUCDsYZdG6cACdE3u0O50658K3Llr2DNLaHyU6 > > 44UAn1/zql0Uh/oRuIpKnWrW6VnYwdgm > > =98Vv > > -----END PGP SIGNATURE----- > > > > > > >
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