CPU Utilization Is Wrong on PCs, and Getting Worse Every Year

CPU utilization is wrong. That’s the argument Brandon Gregg, Netflix’s senior performance architect, has leveled against one of the most fundamental performance measurement tools we use when evaluating a system. 

According to Gregg, CPU utilization as reported by Windows isn’t just wrong it’s actively getting worse over time.

If you’ve ever dug into this topic, you’re aware of some of the ways that CPU utilization isn’t reported accurately. Ever since Intel (and now AMD) added Hyper-Threading / SMT support, there’s been a discrepancy between how cores are presented in Task Manager and what resources are actually available.

Windows, Linux, and other operating systems report the total number of cores and measure CPU utilization as if each logical core was actually a physical core. But that’s not the problem Gregg is discussing.

First, there’s the problem of thread stalling. If you see your CPU running at 90 percent load, you might think it looks like this:

In reality, Gregg points out, what might be going on is something akin to this, in which the CPU is stalled and waiting for data but isn’t actually doing any work.

 If you think about it, you’ve probably seen this in action. If you’ve ever performed a rendering or Photoshop manipulation that really tasked your CPU, performance even UI performance may slow to a crawl while the workload is executing. 


There are ways to avoid this problem by setting the total number of active threads or the priority of the workload itself, but if you’ve worked with computers for any length of time you’ve probably seen instances where 100 percent CPU utilization didn’t actually mean 100 percent CPU utilization. 

The problem, according to Gregg, is that memory accesses often slow the system. This is known as the CPU-DRAM gap, and it’s a topic we’ve discussed before at ET.