Performance degradation of GM!

Hi:
   It's a little too silent here, and may need some noise:-)
   We've tested the performance of GM-1.4 on our new cluster platform
(linux2.4.4 + Myrinet2000 + 64bit 66HMz +4dual xeon SMP). The test
results for one-way communication performance is about the same with the 
results reported in Myrinet website. But for two-way communication, the 
performance degrades greatly.
   For two-way communication between two pairs nodes in the same direction
(two source processes on the same node communicate with other two processes
residing on another node), each way communication achieves about 50% of the 
performance of one-way communication (bandwidth). Which can be explained 
by the physical link performance limits and each way communication shares 50%
of the communication capacity of one link. But for two-way communication 
between two pairs nodes in reverse direction, each way achieves 
only about 80% of the performance of one-way communication. As is said that 
Myrinet supports full-duplex link communication, then what's the cause for the 
performance degradation of two-way reverse communication ?
   Any explain would be appreciated !


Best regards
Geoge