On Wed, Nov 17, 2010 at 11:38 AM, Daniel Hagerty <[email protected]> wrote: > Bill Bogstad <[email protected]> writes: > >> variable latency in the general case. This can screwup video >> streaming or VOIP pretty badly. You are in a single datacenter and >> aren't using those protocols, so we can hope it won't matter to you. >> (Be sure to test this. :-) > > Out of order delivery is pretty easy to get with or without > variable speed of light delays. Queueing and packet sizing are each > happy to provide on their own. > > TCP doesn't deal well with it, at least if you're mentioning it in > the same breath as performance. This is probably so for anything that > has to restitch something that has been fragemented and scrambled.
Good point. I hadn't considered the fact that unsynchronized queues are likely to generate out of order reception. In a datacenter, direct host port to host port cabling will eliminate any switch queuing issues. Light speed issues on cables running next to each other are probably not an issue. :-) Unfortunately, even if all host ports involved are dedicated to a single bi-directional netflow between the machines, host OS queueing and the priority of interrupt processing will probably result in the network layer on the destination machine seeing packets out of order. Many of the things that have been done to improve the performance of OS networks stacks is going to make seeing packets out of order more likely: processing multiple packets per interrupt on reception, allowing multiple CPUs to process incoming packets, queuing multiple outgoing packets to an interface per operation. At this point, I wouldn't even try to guess what performance will be. One possibility could even be lower performance when using multiple links; you could end up with lock contention on the receiver as multiple CPUs try to insert packets into the correct stream. Lots of fun.... Bill Bogstad _______________________________________________ bblisa mailing list [email protected] http://www.bblisa.org/mailman/listinfo/bblisa
