I plan to build a server mainly for running virtual machines. I will use mainly Qemu(with KVM support) and Virtualbox as hypervisors. There will be dozens of virtual machines and they do not execute computationally expensive tasks. I am aware that I need to pick a CPU with VT-x/AMD-v support and I would benefit from Hyper-threading, but how do less high-frequency cores compare to more lower-frequency cores in hypervisor environment? Let's say for example quad-core Haswell microarchitecture Core i7-4790K where four cores run at 4GHz versus octa-core Xeon E5-1428L v3 where all eight cores run at 2GHz. If any additional information is required, then please ask.

  • Hard to say unless you can run benchmarks. – Matt Sep 7 '15 at 10:09
  • I see. So in general there is not a huge difference between same microarchitecture 4x 4GHz cores and 8x 2GHz cores if physical CPU is used for running virtual machines? – Martin Sep 7 '15 at 21:44

This entirely depends on your needs, most would go for more cores as it means you may well be able to run more VMs, or VMs with more vCPUs, on a single host. But if you have a known number of VMs that would benefit from higher frequency CPUs then by all means go that way. That said I'd always prefer a Xeon over an iX CPU due to their additional reliability features.

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  • Does vCPU map to one core of physical CPU? And by reliability features you mean ECC memory support? – Martin Sep 7 '15 at 10:09
  • No, not usually - a vCPU is just one of the resources you assign to any given VM and Xeon's have far more RAS features (intel.com/content/www/us/en/processors/xeon/…) than the iX range - so things that would cause an i7 to crash may well not do so on a Xeon. – Chopper3 Sep 7 '15 at 10:14
  • I understand the RAS part now, but I'm still bit confused about the statement that more cores means that one is able to run more VMs. Why is that so? I mean for example in case of Core i7-4790K and Xeon E5-1428L v3 both are 16GHz in total if you sum the frequencies of the cores. Why should Xeon E5-1428L v3 support more VMs? – Martin Sep 7 '15 at 22:03
  • This is going to sound rude Martin but it's not meant to be, but this site is for (as we state right up front when you sign up) professional sysadmins, not beginners, and this is really basic stuff you're asking. The main issue is the ability and overhead of scheduling multiple VMs to have multiple vCPUs on a finite number of actual cores, the more there are the better - unless you have very specific requirements - the 'adding up' thing you did is an invalid assumption. – Chopper3 Sep 8 '15 at 7:13

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