We have a throughput problem with two UML instances on the same machine, both using preconfigured tuntap devices. The problem appears both with a bridged setup and a routed setup.
Our setup is very simple - two virtual machines on the same host, both have a preconfigured tap device, one virtual machine tries to talk to the other. What happens is that at first the transfer goes blazingly fast for a few seconds, and then the transfer rate starts dropping rapidly. Tcpdumping the ports show that TCP is bursty, only getting a few packets at a time through, and resend times grow longer. Throughput goes down to almost total stalls and less than 1Mbit/s. If there's a 10Mbit traffic limiter attached to the interfaces either inside the virtual machines or to the tap devices, traffic flows evenly between the machines. When the limit is increased, traffic starts to become more and more bursty, and at some point, total throughput starts decreasing. If the traffic goes over a physical cable, such as a crossover cable over two network cards on the same machine, the 100Mbit/s maximum throughput is easily attainable. There are two likely explanations for this behaviour: Something somewhere is buffering a few packets, and dropping the rest. When the transfer rate goes high enough and there's no physical interface to create delay, the buffer gets full and the majority of packets are discarded. However, no dropped counters rise on any of the interfaces, nor are any kernel messages printed from uml or the host. Or, there's a locking bug somewhere that stalls the transfer for a while every now and then. If anyone has any idea what is really going on, I'd appreciate it. -- Naked ------------------------------------------------------- This SF.Net email is sponsored by: IntelliVIEW -- Interactive Reporting Tool for open source databases. Create drag-&-drop reports. Save time by over 75%! Publish reports on the web. Export to DOC, XLS, RTF, etc. Download a FREE copy at http://www.intelliview.com/go/osdn_nl _______________________________________________ User-mode-linux-user mailing list User-mode-linux-user@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/user-mode-linux-user
