Not all workloads are the same. Riva can be configured in an Active/Active/n-Active cluster. When sizing horizontally is not an option and increasing the number of virtual CPU's is preferred, it is important to factor possible virtualization overhead. When configuring a virtual machine to have more than 8 vCPUs, the term "monster VM" is often used.
A "monster virtual machine" is a virtual machine (VM) that typically has more than eight virtual CPUs (vCPUs) and may be configured with more than 255 GB of virtual RAM. Monster VMs are used to virtualize applications with large resource needs, such as Microsoft Exchange, Microsoft SharePoint, Microsoft SQL Server, or Oracle databases. Another common usage of the term "monster VM" is to identify that the VM uses more cores than are available on a single physical socket. If a VM host has 4 sockets and each socket has 8 cores, if a virtual machine uses more than 8 vCPUs, then it is considered a "monster VM".
VMware vSphere Blog - When to overcommit VCPU:pCPU for Monster VMs - http://blogs.vmware.com/vsphere/2014/02/overcommit-vcpupcpu-monster-vms.html
The details below provide specific metrics that can be monitored on the VMware performance monitor to confirm whether the VM is being starved of processing time causing a negative impact to performance. If this does occur, consider reserving the processing time to reduce the contention with other VMs or moving other VM to other hosts.
VMWare - https://kb.vmware.com/selfservice/microsites/search.do?language=en_US&cmd=displayKC&externalId=1017926
The following text is summarized in case the above link becomes unavailable. For all details, refer to the original source.
Troubleshooting a Virtual Machine That Has Stopped Responding: VMM and Guest CPU Usage Comparison (1017926)
Virtual machines depend on available host resources (CPU, Memory), and the guest operating system consumes those resources. A problem with resource availability or scheduling inside or outside the virtual machine may cause it to become unresponsive.
This article provides steps for using CPU performance metrics to determine whether a guest operating system is actually running, whether the virtual machine monitor (VMM) is running, or whether there is scheduling contention.
Note: This article is part of a series. For more information, see the parent article Troubleshooting a virtual machine that has stopped responding (1007819).
Four virtual machine CPU performance metrics can be used together to gain insight into the responsiveness of a virtual machine or its Guest OS:
These performance metrics can be reviewed using the Performance tab in the vSphere Client or using the esxtop or resxtop command-line utilities. Choose the most appropriate method for your environment.
Reviewing performance metrics using the vSphere Client
For more information on using custom performance charts, see the Customizing Chart Views section of the Resource Management Guide or the View Advanced Performance Charts section of the vSphere Monitoring and Performance Guide.
Interpreting CPU performance metrics
Article ID: 1506
Last updated: 13 Jun, 2016