How does VirtualBox CALCULATE how many virtual CPUs are available?

Question

I've tried to get an answer to this question in VB IRC channel, I've looked around stackexchange, stackoverflow, superuser, and elsewhere. Answers come close, but not what I am wanting to know.

This is a curiosity question only, not one of necessity. I just want to know how things work. It has nothing to do with any bug, enhancement request, or security issue. If you feel this forum is not the place to get an answer to this, please refer me to the proper venue. Thanks. (Although it is hard for anyone to imagine that VBox's own forum could be the wrong place, I did not see an answer to my specific question or a place to post to an appropriate category.) Whatever happens, please don't close my question without at least pointing to a better resource (I hate when that happens!). Thanks again.

Now, the question: How does virtualbox's host driver calculate the total number of virtual CPU's to provide?

(Please note I will not respond to answers from people who did not really read the question, or at least first ask for more clarification. I think this is a VERY straight-forward question.)

Let me break the question down so as to be as precise and concise as possible as to what I am really asking. I am curious to know how the VirtualBox HOST software (whatever portion that may be) determines how many VIRTUAL CPUs appear on the configuration interface where the user selects how many VCPUs they would like to apply to a specfiic VM.

What I am NOT asking: I am NOT asking about the miracle of virtualization hardware, etc., in general; I understand multiple cores and multiple threading, VTx, etc. I am NOT asking how many I should use for a specfic VM or application. I am NOT asking for help in configuring any specific VM in my question. I am NOT asking anyone to ask ME why I need to know -- I told you already; I am merely curious. If my specific question does not interest you, that's fine. Again, this is just a simple, straight-forward question: How does VBox arrive at the number?

What I already know: It is true that, at least generally, the answer is 2x as many as physical CPUs; OK, if so, why 2x and not 3x or some other multiplier? (I know fractional amounts won't work for odd-number of cores or threads; I am just being as general as I can be.) For instance, on my Phenom II X6, VirtualBox presents me with up to 12 VCPUs. If the answer is the threads, well, that can't be since my particular Thuban does not have threads (some Thubans do, some don't). What my Thuban DOES have, though, is hypertransport, but not hyperthreading. Likewise, my old Phenom II X2 will allow 4 VCPU's in Virtualbox.

I have already read the numerous responses on the sites mentioned above admonishing users NOT to use more than one VCPU per VM because it adds overhead (for one thing, you must run the IOAPIC, which introduces a performance hit). I've also read posts where the question sounds like mine, but they do not ultimately give an answer to this.

Is the answer some kind of sigma sum or logarithmic formula? Is it complex enough to exceed this forum's formatting capabilities? Hard to imagine why it is so difficult to get an answer to this, which I figure would have been asked and answered many times over. I really want to know why it seems to be 2x usually; why that is the "magic" number. If I read the source code (assuming this is available), will the comments explain why?

I will really appreciate and admire the soul(s) who read and answer this question, and not some other question not being asked. I also hope you will not redirect me to the dark and hostile channels of IRC; there are some very sociopathic entities on IRC whose remarks remind me of some of the unsubs on Criminal Minds. Note that I said "some" -- there are helpful people there also. Not meaning to antagonize; I just hate going to IRC anymore. If you know of a specfic helpful nick on IRC with this, I'd appreciate that also.

BTW, I have been googling for answers to this and other questions and reading SO, SE, and SU boards and I see where some people respond with answers that are totally irrelevant. That's the reason for what may sound like a harsh tone by me. This is my first post, and I hope the response will be more positive than a few of my experiences on IRC.

Answer 1

It seems you are asking "How does the VirtualBox GUI calculate the available range for the Processor(s) slider under the System settings?"

Because VirtualBox is open source (and very clean, well written source) it isn't too hard to dig into the code and research out the answer. Digging down into the /src/VBox/Frontends/VirtualBox/src/settings/machine folder you can see all the UI* files that comprise the settings UI elements. The specific file that will have your answer is UIMachineSettingsSystem.cpp. Starting on about line 45 (as of revision 43459), you can see the following code:

/* Setup constants */
CSystemProperties properties = vboxGlobal().virtualBox().GetSystemProperties();
uint hostCPUs = vboxGlobal().host().GetProcessorCount();
mMinGuestCPU = properties.GetMinGuestCPUCount();
mMaxGuestCPU = RT_MIN (2 * hostCPUs, properties.GetMaxGuestCPUCount());

The mMinGuestCPU variable and associated GetMinGuestCPUCount() method (and mMaxGuestCPU / GetMaxGuestCPUCount()) should be relatively straightforward - typically it will be 1 for the min and the max will be the number of physical/logical cores available on the host.

Thus, to answer your question - the scale for the slider is typically 1 through two times the number of cores available. I would strongly encourage you to download the source code and dig into the methods that calculate those numbers and see for yourself how they are calculated and the nuances that are involved. Specifically the vboxGlobal().host().GetProcessorCount() method.