[Myrinet] problem about the bandwidth of myrinet2k

hi: 
our group is working on a user level message subsystem for lanai9.2 & myrinet2k. and our host driver and LCP driver can run now. 
But we have some environments we did not understand. Our systems are two nodes connected by myrinet 2k with lanai9.2 and a switch with 32 ports. one node includes 2 AMD athlon CPU, 512M DDR mem and a lanai9.2 card inserted in a 64bit/66Mhz PCI slot. our softwares connect each other using the port number as route info directly. 
From our test, if our software in one node sends messages to other node that is not connected on the switch(send message to a not existed node), the max bandwidth of sending 4KB messages will be 250-255MB/s(very exciting). The send side just send 4kB messages 10000 times, then we get the results. 
But if our software in a node sends messages to a node that is connected on the switch(send message to a existed and connected node), the max bandwidth of sending 4KB message only be 104MB/s. In order to reach the max bandwidth, on sending side, we send 100 or 200 4kB messages to receiving side, then wait receiving side send a ack message. If get the ack message, sending side send another 100 or 200 4kB messages to receiving side. we repeat the process 100 times, then get the results. 
We don't know why? Can you give us some reasons? 
Wait your answer! 

chen yu 










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