I am experiencing a rather unpleasant issue with a simple OpenMPI app. I have 4 nodes communicating with a central node. Performance is good and the application behaves as it should. (i.e. performance steadily decreases as I increase the work size). My problem is that immediately after messages passed between nodes become larger that 128 KB performance drops suddenly in an unexpected way. I have done some research and tried to modify various eager limits, without any success. I am a beginner in OpenMPI and I can't seem to figure out this issue. I am hopping that one of you might shed some light on this situation. My OpenMPI version is 1.5.4 on Ubuntu Server 10.04 64 bit. Any help is welcome.