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CPU resources and reservations

A virtual CPU reservation or a binding is a good way to give an important virtual machine guest guaranteed CPU time when other guests on the machine are busy.A virtual machine reservation is still not as good as a real CPU; however, if the host is using more CPU than is available, even a guest with a reservation or high shares is not entirely isolated.

For PureConnect products, we recommend that you use a minimum of two virtual CPUs.For example, virus applications can update virus definitions on one thread at real-time priority that dominates the CPU.On single-core machines, this kind of update causes the server to become unresponsive to the job it is trying to perform. It’s easy to create a VM with only one virtual CPU, but it’s rare to see any physical server hardware with one core and no hyper-threading.

PureConnect software is designed to take advantage of symmetric multiprocessing. Subsystems can dynamically adapt and use the processors for executing threads simultaneously so making more virtual processors available to the guest helps performance.

Question	Answer
Does having more virtual processors increase scheduling latency in VMWare?	The scheduling latency is low on VMware with virtual processors.To determine this low latency, Genesys used the Time Stamp Counter (TSC) and the Multimedia Timer (MMT). We did a comparison on the samples at a 1-ms frequency with the reported clock speed of the virtual processors from the perspective of the guest. We measured to see the jitter in MMT compared to real time. Under VMware, the TSC times almost match the exact MM timer intervals (measured from edge to edge, but not necessarily consecutive edges). We did extensive testing with such measurements, since we have also investigated the platform for handling RTP streaming. At the application level, the extra virtual CPUs give processes more threads to work with and determine how the thread pools are dynamically created.
Is a reservation handled the same way on Hyper-V as in VMware?	No, the vendors have different approaches to reservations.In some cases, we ask you to reserve the core CPU MHz you are using.If the CIC server CPU is not available when it is required, you will miss calls.
Do Hyper-V and VMware use cores the same way?	VMware does not count hyper-threaded (HT) cores in the reservation pool.Hyper-V does count hyper-threaded cores.When setting reservations, you cannot run as many virtual machine guests on a VMware host as you can on the same hardware with Hyper-V. For example: Let’s say our host has this specification: E5620 @ 2.40 GHz with two physical sockets with four physical cores each (eight physical cores), with hyper-threading resulting in 16 logical CPUs. In this case, VMware only allows reservation of the physical cores, after subtracting the system overhead of running the hypervisor itself. So, in VMware, there would be approximately 17,420 MHz available for guests after it allocates CPU resources to running the hypervisor. (2.40 GHzX8 physical cores) about 1780-MHz hypervisor overhead = 17,420 MHz Hyper-V would make 38,400 Mhz available to guests with 100% reservations of all the virtual CPUs. 2.40 GHzX16 logical CPUs = 38,400 MHz