I think it should be disabled in parallel. Matrix slicing is very useful during development, testing and debugging, but introduces more troubles than uses in parallel.
Kent On 2 January 2011 07:47, Johan Hake <[email protected]> wrote: > Hello! > > As pointed out by Garth in a couple of commit messages lately we need to > consider the slicing interface for vectors and matrices in PyDOLFIN. > > Now that the parallel interface are starting to get more and more robust > (great work Garth!) we need to consider what: > > v[6], v[4:], v[index_array] # returning a scalar and a vector > A[5,6], A[5:,6], A[5:,6:] # returning a scalar, a vector and a matrix > > should return for serial and parallel structures. > > I guess Garth comments that the Matrix slicing "is just a toy" has some > merits > as it uses a dense representation of the Matrix (even though it is erasing > zero entries). Regardless if we decide to discard the Matrix slicing or not > we > need to decide how off process indices should be treated. > > As it is now, a user need to check whether an index is owned by the local > processor or not, by using GenericVector::owns_index, before he access the > data. > > An other scenario would be to always allow a user to write: > > v[5], v[4:] and v[index_array] > > and then return for example None (for single value access) and a reduced > vector (only including local values) for the two other alternatives. Then > the > user need to deal with the result knowing how we treat this in parallel. > > Matrix slicing is more tricky especially now when GenericMatrix::resize is > removed. There are probably ways of getting around this by building a > SparsityPattern based on the slice. But I am not sure it makes any senses > to > do a matrix slice for a parallel structure, as it is only the rows that are > local. Then we need to decide if the Matrix slicing should be: > > 1) removed > 2) reimplemented but disabled in parallel > 3) reimplemented for both serial and parallel cases using for example > a similar approach as sketched above for vectors, that is always > returning something. > > Best regards, > > Johan > > > _______________________________________________ > Mailing list: https://launchpad.net/~dolfin<https://launchpad.net/%7Edolfin> > Post to : [email protected] > Unsubscribe : https://launchpad.net/~dolfin<https://launchpad.net/%7Edolfin> > More help : https://help.launchpad.net/ListHelp >
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