The Internet is abuzz on newly launched Intel Xeon processors, there are reviews showing manifold increase in server performance, which is for some type of applications the number is 150%. We have seen multiple records being shattered. Xeon 55XX series is doing the exact thing in the server world, what Core2duo did to the desktop space back in 2006. The beauty of new Xeon is that, its brings in something for everybody, Database applications, web servers, business logic servers, IT infrastructure applications, virtualization, HPC etc etc. While the IT administrators are busy reading reviews and calculating how much money they can save replacing thier aging infrastructure, I did like to give a small information about a less talked feature in the new Xeon called PCU.
While the new Xeon got a brand new architecture, much discussed features are Integrated Memory Controller, Quick Path Interconnect, Turbo Mode (Any body remember the Turbo Switch on your computer cases back in old days, Turbo Mode gets you the Turbo speed without the need of the switch). But there is onething our architects added to Xeon architecture which is quite interesting but not talked much about is the Power Control Unit or PCU, I am going to provide a simple understanding of this feature without delving into complicated terminology of gates, Phase Locked Loops etc.
While desktop users wont tend to bother much about power usage, things work differently in the server world. Data center architects and managers spend hundreds of hours crunching numbers on how to make their Data centers run cool without paying heft electricity bills. So having a power efficient processor under the hood of the server which can efficiently manage its power consumption means, saving money on power bills not only with actual power saving on the server but also the related cooling cost of the data center. Now that you know why it is a big deal to have a intelligent Microprocessor, lets see what is this thing PCU.
PCU is an on-die micro controller introduced and dedicated to manage power consumption of the processor, this unit comes with it's own firmware and
gathers data from temperature sensors, monitors current, voltage and takes inputs from operating systems and not to forget that it takes almost a Million transistors to put this this micro controller on-die, while a million sound like a drop in an ocean in a billion transistor processor, considering the older Intel 486 processor had the similar transistor count and ran windows 3.x quite well.
In simple words the PCU controls voltage applied to the individual cores by using sophisticated algorithms, and hence sending the idle core to almost shut off level and reducing the power consumption. But let me explain this in more elaborated manner. In an older generation CPUs it wasn't possible to run each core on different voltages since they shared the same source and the idle cores still leaked power. But with the new generation Xeon, even though the four cores gets voltage from a common core voltage source, but thanks to a manufacturing material Intel uses we can run each core at different voltage level and have the ability independently clock them at different speeds. PCU can make this decision and nearly shuting off the idle core by cutting voltage to it and can intelligently increase the voltage to the active one of more cores bumping up the clock speed of one or more cores making them run faster, this is what we call as Turbo. To make this more simpleter to understand, I can provide a simple water tap example on how this works, supposedly think we have a long water pipe with four taps connected to it, when only one tap is busy filling up a bucket with water, we can turn off other three taps and divert the the water pressure to the running tap and let that fill the bucket faster.
We can always say why there is a need for on-die power management when the same can be achieved by any operating system using ACPI power states, PCU accepts power state requests from operating systems but uses its own built in logic to doubly ensure that the OS request holds merit. There are instances where the operating system instructs the CPU to go to lower power state only to wake it up next moment, adding PCU get this process a fine grained efficiency and helps our customer data center run much cooler.