If you've read my earlier blog posts about Gigabit Ethernet, you know that I've had my share of difficulty getting good performance out of my GigE network. I've also posted that I've had problems with Vista that I didn't see in Windows XP or Windows Server 2003. Now I know why. Windows Media Player puts a big throttle on GigE network performance, even if it is paused and not playing anything.
The issue was reported at 2CPU.com and a response from Microsoft was reported at ZDNet. The problem is minor on 10/100 Ethernet, but on a GigE Ethernet the network performance can be throttled back to 100Mps levels. Apparently, there's no registry setting that will resolve the problem, the only solution is to shut down Windows Media Player. There are scattered reports that the problem happens with other media players, such as WinAmp, but I haven't confirmed these reports.
There's another problem I also ran into that caused GigE to throttle back to 100Mbps. This was a self-inflicted problem, but it took several weeks to resolve. I have a LinkSys WRT54G router that handles my gateway/firewall/NAT. All of my servers get their IP address with DHCP and the WRT54G is set to always hand out the same IP address to those servers. This was done so that the servers would get the DHCP settings from our provider. Anyway, one of the servers was migrated to a new motherboard with a different MAC address. The WRT54G started handing out a random DHCP address to that node but still reported that node name as having the other, assigned IP address, all of which made the network become schizophrenic over the IP address assigned to that particular node name. The result was that any traffic that was destined for that node ended up going to the gateway, which didn't have a GigE connection, so traffic was throttled back to 100Mbps speeds.
Update 8/28/2007 - Mark Russinovich has posted a detailed analysis of the network slowdown. It turns out that the more Network Interface Cards (NICs) you have in your system, the worse the problem gets. I have three NICs, including WiFi, which slows my performance to a theoretical maximum of 9MB/second.