tcp performance

[Zongsheng Zhang]

Hi, *,

For testing throughput of a TCP connection, the following topology is used:
     Host-A ---GB Ethernet--- Dummynet ---GB Ethernet--- Host-B

Host-A/B use FreeBSD v6.0. Sysctl parameters of Host-A/B are:
kern.ipc.nmbclusters=32768
net.inet.tcp.inflight.enable=0
net.inet.tcp.sendspace=2097152  # 2M
net.inet.tcp.recvspace=2097152  # 2M

When RTT in Dummynet is set to 0ms, the throughput (A--B) is about
900Mbps. The buffer size is enough to fill a link bandwidth=800Mbps, and
RTT=20ms. However, if RTT is set to 20ms, the throughput is only about
500Mbps.

Are there other parameters which are necessary to adjust? Does anyone
have suggestion for high throughput?

Thanks in advance.

-- 
Zongsheng Zhang
zhang at ist.osaka-u.ac.jp