Now available to the general public is Windows Server 2008 (WS08) Hyper-V 24 Logical Processor (LP) Support. This revises our RTM support limit of 16 Logical Processors. We also increased the number of supported running virtual machines from 128 to 192. This update also has the side benefit of improving workloads that start large numbers of processes by increasing the process address space cache limit from 192 to 384. All in all this update will allow you to get even more mileage from WS08 Hyper-V.
You can find the needed update here http://www.microsoft.com/downloads/details.aspx?FamilyID=fe36823a-7e5a-4262-9bf5-d6b3ae3ad375&DisplayLang=en
Here is some Q & A to help you figure out how and when to install this update;
Q: Should everyone install this update?
A: No. Install this update only on machine needing 24LP support or if you are running more than 192 processes in the root (no recommeded) / guest. Keep in mind there are very few workloads that create this many processes and there are only two that come to mind. Terminal Services and Hyper-V running > 192 virtual machines (VM).
Q: Does this mean I can now run 192 VM's on my 16 logical processor system?
A: No. We only support an 8 virtual processor (VP) to 1 logical processor (LP) limit.
Q: What does 8 VP to 1 LP mean?
A: This mean is you have 8 logical processors you can run up to 64 virtual processors in any combinaiton. For example 16 Windows Server 2008 (WS08) VM's with 4 VP each, or 64 Windows Server 2003 VM's with 1 VP each, or 8 WS08 4VP VM's with 32 W2k3 1VP VM's. You get the picture.
Q: What is a logical processor aka LP?
A: A logical processor is what we call anything that shows up in Windows as a processor in task manager. More precisely it is the "number processors * number of cores * 2 if you are running Hyper Threading (HT) or now called Symmetric Multi Threading (SMT)." For example a dual proc quad core system with SMT enabled has 2 * 4 * 2 = 16 LP. A quad proc dual core system has 4 * 2 = 8 LP.
Leading the 24LP effort was a blast and it is also why my blog has been pretty quiet recently.