Hello. I"m using MPICH2 1.5. My system contains master and 16 slaves. System uses number of communicators. The single communicator used for below scenario: Each slave sends non-stop 2Kbyte data buffer using MPI_Isend and waits using MPI_Wait. Master starts with MPI_Irecv to each slave Then in endless loop: MPI_Waitany and MPI_Irecv on rank returned by MPI_Waitany. Another communicator used for broadcast communication (commands between master + slaves), but it's not used in parallel with previous communicator, only before or after data transfer. The system executed on two computers linked by 1Gbit/s Ethernet. Master executed on first computer, all slaves on other one. Network traffic is ~800Mbit/s. After 1-2 minutes of execution, master process starts to increase it's memory allocation and network traffic becomes low. This memory allocation & network traffic slow down continues until fail of MPI, without core file save. My program doesn't allocate memory. Can you please explain this behaviour. How can I cause MPI to stop sending slaves if Master can't serve such traffic, instead of memory allocation and fail? Thank you, Anatoly. P.S. On my stand alone test, I simulate similar behaviour, but with single thread on each process (master & hosts). When I start stand alone test, master stops slaves until it completes accumulated data processing and MPI doesn't increase memory allocation. When Master is free slaves continue to send data.