I want to make a virtual computer with two computers, possible?

[Mazinho93]

Hello people, how are you? I want to link the laptop I have with the desktop I have too, creating a virtual computer with 6 cores. In case the laptop = 2 cores and the desktop 4, it's possible?

[scottgus1]

Interesting idea! Regrettably it is not possible with Virtualbox.

Also more cores in a guest, even one one physical host PC, make the guest OS run slower, due to extra scheduling commands on the host. Modern OS's are built for two cores, but more cores only help if there is parallel-processing software installed in the guest that will fill those extra cores.

[JustinH]

Also more cores in a guest, even one one physical host PC, make the guest OS run slower, due to extra scheduling commands on the host. Modern OS's are built for two cores, but more cores only help if there is parallel-processing software installed in the guest that will fill those extra cores.

Are you saying something like: I have 8 actual cores (and 8 logical = 16 total), set my VM to use only 4 of them, and this would typically cause the VM to run slower than if it was using only 2 cores.

[scottgus1]

et my VM to use only 4 of them, and this would typically cause the VM to run slower than if it was using only 2 cores.

Correct. Though I have not personally tested this, the information comes from forum gurus who know a whole lot more about the inside of a CPU than I do. I have had forum reports of sluggish guests that became more peppy when the core count was reduced. Of course dropping to 1 core is not always the best either: Windows 10 likes two cores.

[JustinH]

Thanks for the info.

[mpack]

Are you saying something like: ... and this would typically cause the VM to run slower than if it was using only 2 cores.

Correct. Even on a physical PC, adding cores often has no benefit. But those selling PCs need a growth path to entice buyers, and clock speeds stalled at physical limits several years ago. So now they imply that putting two engines in a car makes cars go twice as fast.

One nice thing about having two cores in a modern OS is that one can be dedicated to background tasks, so hardware events can now be handled with almost no latency and without interrupting the foreground. (Two car engine analogy: one engine is powering your heaters, radio etc, the other is driving your wheels).

Beyond two cores: any benefit presupposes that you had already maxed out existing cores, otherwise what did you want the extra cores to do with their time? Normally just sit idle, which I guess is harmless at least on a physical PC.

Unlike a physical PC, a hypervisor can't just ignore unnecessary cores. It has to assume that the guest (or the host) might have messed with them, therefore it has to cache the core registers to RAM and then restore them every time it does what the devs call a "world switch". That overhead exists for every core. If the OS isn't actually using that core then overhead is all the core provides. And don't forget that VM performance relies on the smooth operation of the host, so make sure the host has enough resources too.

So like Scott said. Start with 2 cores, and don't add any more unless you have more than one app running in the guest OS, each of which wants to max out a core. Or, one app that is specifically built to crunch numbers on a many cores as are available, e.g. bitcoin mining, video encoding. Things that will not make good use of multiple CPU cores: general office apps, most games. In fact most apps.