Benefit of running multiple cores on GUEST?

[Rootman]

Been using virtualization products for years, I used to use the VMW product but recently switched to VB because performance was dismal on the other. VB seems to run a lot better, even with the SAME guests imported into VB.

I have a Quad core I7 CPU on the host, I have setup at least two cores when setting up a Windows guest so that the multi processor HAL gets loaded and multi core is available.

So for an average GUEST being used as a simple Windows XP system for average web surfing, email, watching videos etc. is there ANY advantage of running multiple cores on the GUEST? Any DISadvantages? In adjusting the cores up and down there SEEMS to be a little better performance when using more than one core, am I just imagining it?

[Perryg]

For each core you add there is an overhead. That said I find that 2 is all that I need and adding more (as you have seen) rarely makes a difference, except for a very few times where the app could actually use the additional cores. Even then it was negligible.

[BillG]

I agree with Perry. Most apps are not multi-threaded, so they cannot take advantage of multiple CPUs. If the application you are running is single threaded, running multiple cores is counter productive.

[mpack]

A decent machine running a modern OS can nearly always benefit from 2 cores, usually it allows the host OS to get on with background tasks without impacting a user mainly running in the 2nd core. No matter how busy either core gets, the other core still runs smoothly.

To get any benefit from more than 2 cores you would have to be running multiple CPU intensive apps, each running in its own core, or else running one multithreaded app which has multiple CPU intensive threads. However, CPU intensive apps are rare, period, because customers don't like them. An app that needs to max out multiple cores to get decent performance? Even more rare. By the same token, running two CPU intensive apps side by side must also be rare: users generally don't.

Really, the trend towards more and more cores is largely a marketing gimmick (*). It allows them to imply that more cores (or more memory) == more performance, when it fact that is true only in rare circumstances. Maybe if you are doing continuous real time 3D graphics rendering then there's a benefit. For most everyone else there's no benefit.

(*) As is the push to 64bit IMHO.

All my VMs run on one core, however I rarely run CPU intensive apps, and never so in a VM.

[martyscholes]

We regularly run two cores as well so that Windows / Linux can do all of their background stuff without impacting foreground apps. We did notice that >2 cores on our server leads to huge performance losses. I suspect this is because our server has 4x dual-core CPUs, each with its own memory space.