CPU Power Management

AMD PowerNow! for the Opteron series of server CPUs dynamically manages the CPU clock speed based on Utilization. The speed takes a few milliseconds to change, and it is not clear exactly what speeds are supported, but one report stated that the normal speed of 2.6GHz would reduce to as low as 1.2GHz under a light load. This report also shows CPU detailed configuration and power savings. http://www.gamepc.com/labs/view_content.asp?id=opteron285&page=3

The problem with this for capacity management is that there is no indication of the average clock rate in the standard system metrics collected by capacity planning tools. PowerNow! is described by AMD at http://www.amd.com/us-en/0,,3715_12353,00.html and drivers for Linux and Windows are available from http://www.amd.com/us-en/Processors/TechnicalResources/0,,30_182_871_9033,00.html. In the future, operating systems may be able to take the current speed into account, and estimate the capability utilization, but the service time is higher at low clock rates, so we will still see some confusing metrics.

The current PowerNow! implementation works on a per-chip basis, and Opteron’s have two complete CPU cores per chip that share a common clock rate. In a multiprocessor system made up of several chips, each pair of processing cores could be running at a different speed, and their speed can change several times a second.

Our basic assumption for well behaved workloads, that mean service time is a constant quantity, is invalidated in a very non-linear manner, and utilization measurements will also move in mysterious ways....