This is a package for the psana framework used to analyze LCLS data. Documentation about data analysis at LCLS starts here:

https://confluence.slac.stanford.edu/display/PSDM/LCLS+Data+Analysis

and more details about the software environment, psana framework and how packages fit into it and are built is here:

https://confluence.slac.stanford.edu/display/PSDM/Analysis+Environment+Details https://confluence.slac.stanford.edu/display/PSDM/Packages+and+Releases https://confluence.slac.stanford.edu/display/PSDM/SConsTools

The purpose of this package is to solve a problem users have when using MPI to analyze events from shared memory. When reading from shared memory, each MPI rank can get a different number of events. This makes intermittent collective communication such as gather or reduce more complicated. The complication is that one rank may only get enough data to participate in say 5 of these gathers, while another will particpate in 6 of these gathers. The solution presented here is to always update the communicator used for collective communication before a gather or reduce, allowing ranks which process fewer events to drop out of the collective communication.