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We have serveral servers with Intel i7 2600 quad core processors all running Ubuntu 12.04 LTS and they are used as hosts of virtual machines on libvirt/kvm.
My question is, should we turn hyper-threading on? From experience, if it is on, we see 8 CPUs on the host, if it is off, we see 4. We cannot see any noticable differences in performance so I was wandering if any of you have made some benchmarks or have some experience or other usefull information w.r.t. HT and KVM?
Hyperthreading was a mess when it was first introduced. Many if not most of the tests would show that there was a performance hit when enabled. My opinion is that if you are using modern distro and up to date apps and you need many cpu's either virtual or real then use hyperthreading. Most of the early problems that were common have been resolved. Most of the apps are smp aware and HT aware. You won't really have an 8 core but for most smp aware apps it should help. Many apps are still only good for a couple of cores. I never turn it off anymore for my uses.
TobiSGD brings up a good point about use as the basis for answer.
it depends on the workload. generally speaking, HT helps when you have IO bound loads and/or heavily congested CPU. If you are not overcomitting your CPU and have no disk/network bottlenecks, HT will probably not provide any advantages, and might, under certain conditions, even hinder performance