Mpi Send / Receive Comm Time with IO Streams

I'm currently benchmarking Mpi like software on a cluster and I ran into a strange discrepency in communication time.
For simplicity here, I'll be referring to simple synchronous send / receive commands in which 8 bytes of data are sent or received during each command.
In a two compartment case, the compartments are connected via a Socket and the input/output streams are pulled from the socket. In the most simple case, compartment 1 sends a double value to compartment 2, and this repeats 1e6 times. In doing this the average communication time per send / receive operation is about 1 microsecond.
In the next more complex case, compartment 1 sends a double value to compartment 2, then compartment 2 sends that double value back to compartment 1.
This is where communication time increases to 10 microseconds (on the same Compute Node) or up to 100 microseconds (inbetween different Copute Nodes).
At first I was doing the send and receive on the same socket using it's input and outputstreams. To make sure that wasn't a bottleneck, I separated the 2way communication into two different sockets, but that large communication time still existed. (No significant change).
With one-way communication, even with the blocking send/receive methods, communication is still remarkebly fast (1 us). So what I'm trying to figure out is: why does this increase 100 times when doing a send & receive, then the reverse operation?

Looks like low entropy could be the cause. Here are some useful links:
Re: JDBC Connection Reset when using many processes on 64 bit system
http://serverfault.com/questions/298698/low-entropy-on-centos-5-after-new-mount
http://www.ghidinelli.com/2011/01/11/cold-fusion-slow-start-messagebrokerservlet
http://www.usn-it.de/index.php/2009/02/20/oracle-11g-jdbc-driver-hangs-blocked-by-devrandom-entropy-pool-empty/
How would I set up the server so that rngd feeds /dev/random with entropy from /dev/urandom, such that this modification sticks upon reboot (so I don't need to remember to execute some command line statement upon every reboot)?
Edited by: 918175 on Mar 5, 2012 4:49 PM

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