I used to run 4 VMs on my dual core machine but actually was running nothing seriousely. A while back I read an article from Ubuntu about its cloud setup, mentioning you should not run VMs more than number of cores you have, I was laughing at the article at that time, since it sounds ridiclous – as long as CPU can handle the load, we should run as many VMs as possible and – the bottleneck seems coming freom memory to me.
Obviously I was wrong. Once I started testing Casasandra, I got lots of timeouts from 4 VMs, I checked memory, there was no problem at all, i.e. every VM’s memory is in physical memory. I was also doubt Ubuntu is not doing as good as Fedore, obviously Iwas wrong either, then I recalled the article, shut down 2 VMs and … you know, things work like a chime.
There could be saomew thesis telling why CPU is the bottleneck, but I don’t have time to dig it out, the most important lesson I took from this case is, don’t take anythingn in granted, respect preofessional advices