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Subject: [OMPI users] Strange "All-to-All" behavior
From: Stephan Wolf (wolfst_at_[hidden])
Date: 2013-04-26 18:14:59


I have encountered really bad performance when all the nodes send data
to all the other nodes. I use Isend and Irecv with multiple
outstanding sends per node. I debugged the behavior and came to the
following conclusion: It seems that one sender locks out all other
senders for one receiver. This sender releases the receiver only when
there are no more sends posted or a node with lower rank, wants to
send to this node (deadlock prevention). As a consequence, node 0
sends all its data to all nodes, while all others are waiting, then
node 1 sends all the data, …

What is the rationale behind this behaviour and can I change it by
some MCA parameter?