Utilization Monitor, Calculator, and Simulator

HP Capacity Advisor performs numerous functions. At a fundamental level, it collects data from utilization monitoring daemons on systems and workloads. Using this data, Capacity Advisor can report on utilization of memory, core processing units, I/O bandwidth, and power. It can calculate what resource utilization would look like if the load were increased or decreased. Further, it can simulate what resource utilization might look like if loads are combined. This goes beyond simple addition of maximum loads to provide a dynamic addition of components based on the time sequence of utilization measurements taken from the actual loads running on real systems (see “Peaks and Sums”).

Data Handling for Virtual Machines

Data collected by Capacity Advisor for use in the Profile Viewer and historical utilization reports can vary from data reported by tools that run inside a virtual machine. These tools include HP-UX and Linux commands such as top, ps, and sar, and on Microsoft Windows, the task manager or perfmon.

This data can vary in a couple of ways. For example:

Capacity Advisor usually reports CPU utilization that is greater than what is reported by the VM guest. The operating system inside a virtual machine only knows about the CPU time used by threads that simulate the virtual processors. However, there are also threads that simulate the IO cards and disks in the virtual system. Capacity Advisor collects data on all of these threads, which can cause the Capacity Advisor data for a specific virtual machine’s CPU utilization to be greater than the number of virtual CPU cores (vCPUs) associated with the virtual machine. Under certain load conditions, this can result in CPU utilization of more than 100% being reported for a virtual machine.
Capacity Advisor data for VM host CPU utilization can be lower than what the guest operating system reports, especially during periods of high utilization. Capacity Advisor records physical CPU utilization, which is the utilization of the actual cores on the VM host; that is, the total utilization reported by Capacity Advisor is based on the CPU time that the VM host allotted to each virtual machine. In contrast, the guest OS records virtual CPU utilization, which includes time when the VM wants to run, but is suspended while another VM is using the resources on the VM host.

Because Capacity Advisor corrects for these effects, the collected data has much less “noise” in it, and better reflects the CPU time that was actually used by any one VM.

Utilization Monitor, Calculator, and Simulator

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Data Handling for Virtual Machines