Pitfall due to additional superstep after the iteration has stopped

Currently, after an iteration has exceeded the maximum number of iterations, all tasks are started again for an additional superstep during which they are stopped. This works if a tasks only waits for dynamic input. However, in the case where one has a task, e.g. a coGroup operation, which gets dynamic and static input the execution is not blocked. This can then lead to erroneous behaviour which the user is not aware of.

I had this problem implementing ALS. Here one has a loop which gets as dynamic input matrix columns and as static input matrix entries. The columns and the entries are used to construct a matrix which represents a system of linear equations. If the set of columns are empty, then the matrix is singular and thus not solvable. During the additional superstep the task won't receive any columns but would still try to solve the now singular matrix.

It would be good to finish the iteration without initiating this additional superstep.

---------------- Imported from GitHub ----------------
Url: https://github.com/stratosphere/stratosphere/issues/909
Created by: tillrohrmann
Labels:
Created at: Thu Jun 05 17:50:17 CEST 2014
State: open