Description
The NAT mode guest interface is truely a great feature, but there are some optimization/scheduling problems on outbound traffic to remote hosts. I have tried this on several hosts (linux and windows) each with different hardware configurations, the result is the same - a throughput less than 500 kbyte/s (on windows hosts less than less than 100 kbyte/s). There is no problem when sending data from the guest to it's host.
I have spent days debugging and profiling, but the problem is not a part of my expertees so at last I had to give up.
I have communicated quite a lot with the vbox developers via IRC passing on debug and profiling information - they say the problem is due to low prioritized scheduling on the host side.
Reproduce
1. Install a guest
2. Set up NAT on NIC1
3. Boot up on the guest and send something rather large to a remote host (for instance using scp on a linux guest).
Odd observations
1. Outbound traffic to the host is runs fast (8-10 Mbyte/s on my system)
2. Traffic to remote host on LAN very slow (less than 100 kbyte/s)
3. When I profile VBoxDD.dll outbound traffic speed jumps to 30X speed (more than 3 mbyte/s) (only on LAN)
4. While attempting to do some manual profiling in pcnetAsyncSendThread() (src/VBox/Devices/Network/DevPCNet.cpp) I opened a file for output and closed it again each time. This appearenty up prioritized the thread for transmitting network packets and the speed suddenly went drastically up.
My opinion
I think there is a great potential in NAT with port forwarding, because it enables you (in conjunction with VBoxHeadless.exe) to distrubute a virtual machine and run it as a normal service (ie. Windows Service) the only thing missing is outgoing traffic speed :-)
Best regards Jakob Simon-Gaarde
btw: vmware's NAT has the same defect. So this could be a place where VirtualBox can offer a little more :-)
