My IT Blog

I’m often asked the question when looking at vSphere statistics – “What is the difference between PCPU Used and PCPU Utilized and why don’t they match?” Let’s take a look as it can be somewhat complex. First lets start with some definitions: Time Stamp Counter (or TSC) – is a 64 bit register available on all modern processors that counts clock cycles at a consistent rate and is not affected by changes in clock frequency. Unhalted Cycles – another count of ‘all’ clock cycles, but this one is dependent on the clock frequency and therefore the rate can change if the clock frequency changes (due to things like power management or Hyper-Threading). Wall Clock Time – refers to elapsed real world time. Okay now lets define our two counters using the above definitions: PCPU utilized (which is TSC based) = (non-idle state TSC cycles)/wall clock time PCPU used (which is unhalted cycle based) = (unhalted cycles)/wall clock time So assuming a non Hyper-Threaded system, and no pow...