How may cores do I need for X amount of vCPUs?

Posted on 2014-02-10
Medium Priority
Last Modified: 2014-02-11

I want to know how many vCPUs can I run on a physical host with 4 core Xeon processor?

I am thinking of buying one of those new Mac Pros but the entry level one.  This configuration comes with a 4 Core 3.7ghz Xeon processor and 12 GB of RAM, though I think I will bump this up to 16 GB, which can be done cheaply enough.

Do you think this processor will be powerful enough to run upwards of 12 windows virtual machines, IE a few windows 7 and 8 hosts and server 2003/2008/2012 servers as well as a fairly complex GNS3 lab on the actual host operating system.  

Do you think I will need to get the 6 core Xeon configuration instead of the 4 core.

I am finding VMWare's (for example) thesis statement description of vCPUs to physical cores a bit obfuscating and ADHD inducing (though entirely relevant I am sure).  But I gather, there isn't really a limit logically, and it depends on what you are running on your hosts.

But I just want some opinions based on your actual experience.  Do you think a 4 core Xeon is adequate for 12 windows VMs of a mix between server and consumer OS'?

Thanks for everyone's feedback.
Question by:CnicNV
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Accepted Solution

sda100 earned 336 total points
ID: 39847591
Hello there,

You don't say which Xeon you're referring to but I'll take a stab at the E3-12xx series (quad core with hyperthreading).

There's no hard and fast rule really to how many physical cores you need, just some pointers to bear in mind, mainly typical usage scenario - if you're not using the VMs then you can have a lot more than 12!

From my own experience for a cloud-based office environment, 8 cores (or 4 cores with hyper-threading) is ample to be able to run 20 Windows VMs (10 of which are in concurrent use, the others just sitting idle), 2 Windows servers (1 x DC and 1 x SQL), and a couple of low-resource linux VMs.  I also have 64GB RAM on that ESX box.

The above works for me because I know the typical usage and monitor it occasionally to make sure it's acceptable.  If you have heavy use then you'll need a greater spec or less VMs.

For each Windows VM I allocate 2 vCPUs and 2.5GB RAM, so I'm on the limit of the envelope there, but it works fine.

One thing I remember from reading around a bit is that if you allocate too many vCPUs to a VM, it has to wait until x physical cores are free before it can process the instruction, so you might end up with some physical cores doing nothing waiting for x to become free at the same time.  I wouldn't really allocate any more than 2 vCPUs per VM anyway in general usage.

If in doubt, over-spec your box - you're never going to downgrade, but you're very likely to upgrade or add VMs over time.

Best wishes,

Expert Comment

ID: 39847598
I've just realised you're talking about Apple hardware, so just substitute ESX for VMware in my previous answer.

Assisted Solution

computication earned 332 total points
ID: 39847604
A vCore is a reservation on a physical core. In other words, if a virtual machine with 2 vCores assigned wants to calculate something, 2 physical cores need to be available. So if you have 4 cores and 12 virtual machines with each just one vCore assigned, you will run into CPU ready wait time, issues. Now if this is for a lab setting, it might be acceptable but it will not be fast. A more pressing problem will be the memory. Do not grant more memory to virtual machines than there is available, physically. This will cause swapping and even on a SSD, that will result in significant slow down. I'm not sure for how long you'll need the test setup, but it might be cheaper to rent some cloud resources. It will definitely be cheaper to buy a customized/self build Windows machine or even a Hackingtosh.

Hope it helps, kind regards,

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LVL 123

Assisted Solution

by:Andrew Hancock (VMware vExpert / EE MVE^2)
Andrew Hancock (VMware vExpert / EE MVE^2) earned 332 total points
ID: 39847621
Memory is often the bottleneck not Cores on the Physical Processor.

But I would opt for 64GB RAM, and 6 Cores.

Also look at the storage options, to ensure you have fast storage for these VMs.

You may also want to find out from Apple, when the new Mac Pro 6, is certified for use with VMware ESXi 5.5.

Check the VMware Hardware Compatability Lists HCL here

The VMware Hardware Compatibility List is the detailed lists showing actual vendor devices that are either physically tested or are similar to the devices tested by VMware or VMware partners. Items on the list are tested with VMware products and are known to operate correctly.Devices which are not on the list may function, but will not be supported by VMware.


Author Comment

ID: 39847772
Thanks for the responses everyone, I appreciate the feed back.

Response to computication

It will definitely be cheaper to buy a customized/self build Windows machine or even a Hackingtosh.

Yeah, you're probably right for a consumer level custom build.  But I was looking at workstation level products from Lenovo and HP, and they are fairly expensive as well, IE 4k$.  Though I am sure I could get one cheap used from some company that ditched the lease or whatever.  But, the Mac isnt actually all that bad price wise compared to others in the workstation segment.  Plus its really small for what it is, and well its also cool looking.  So it would be nice for a home lab with such a small footprint and that doesn't feel like I have a server in my room or something.  Finally as there are now a lot of people using Macs, it would be good for me to learn the ins and outs of its OS while I lab for actual relevant things, even if it is all "vertical marketed" up.

Response to Andrew Hancock

You may also want to find out from Apple, when the new Mac Pro 6, is certified for use with VMware ESXi 5.5.

I checked this and currently its not supported, however, a VMware moderator said that there will likely be support for it with the next release of ESXi.  I can just use fusion, Virtual Box or some other non bare metal hypervisor while I wait.
LVL 123
ID: 39847789
You may find ESXi 5.5 works fine on the Mac Pro 6, it's just not been tested as yet!

Expert Comment

ID: 39847800
ESXi is free to download and use.  Why not give it a whirl?

Author Comment

ID: 39847903
Response to Andrew Hancock

You may find ESXi 5.5 works fine on the Mac Pro 6, it's just not been tested as yet!

I believe, though I cant remember 100% was that the forum post was initiated by someone who was trying to install it on the latest Mac Pro but wasn't enable to because of some error.  I wish I had bookmarked the post.

Response to sda100

ESXi is free to download and use.  Why not give it a whirl?

For sure, I have no problem with trying to get it working.

Thanks everyone.

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