The Windows 8 Task Manager is showing 49% CPU utilization while Process Explorer is showing 100%. What would be causing this? Is it a Windows 8 bug?
My guess would be that Task Manager takes into account the current CPU speed and thus shows utilisation relative to the top speed. Task Manager just takes the CPU utilisation as returned from the system. In your case they are off by a factor of two, just like the current and maximum CPU speed.
You could try changing your power plan not do clock down the CPU to see whether they then report the same.
I went into my BIOS settings and disabled all of the following:
Even with all of these settings enabled, there is still an almost exact 2x difference between the CPU usage reported by the Task Manager and by Process Explorer. So it does not appear to be an issue of variable frequencies on the processor or from adding cores.
My guess is that Process Explorer uses a different method to calculate the CPU usage, but I'm not sure what that is. I'll try to find some information about this, and update the answer when I do.
Edit: this doesn't explain the 2x difference, but does explain the difference between how Process Explorer and the Task Manager calculate CPU usage:
Older versions of Windows were able to track only an approximation of actual CPU usage. At a clock-generated interrupt that on most systems has a period of 15.6 milliseconds (ms), Windows identifies the thread currently executing on each CPU. If the thread is executing in kernel mode, its kernel-mode time is incremented by 15.6 ms; otherwise, its user-mode time is incremented by that amount. The thread might have been executing for only a few CPU cycles when the interrupt fired, but the thread is charged for the entire 15.6-ms interval. Meanwhile, hundreds of other threads might have executed during that interval, but only the thread currently running at the clock tick gets charged. Windows Task Manager uses these approximations to report CPU usage even on newer versions of Windows that have more accurate metrics available. Task Manager further reduces its accuracy by rounding to the nearest integer percentage, so processes with executing threads that consume less than 1 percent of CPU time are indistinguishable from processes that do not execute at all. Finally, Task Manager does not account for CPU time spent servicing interrupts or deferred procedure calls (DPCs), incorrectly including that time with the System Idle Process.
Procexp represents CPU usage more accurately than does Task Manager. First, Procexp shows per-process CPU utilization percentages rounded to a resolution of two decimal places by default instead of to an integer. Second, Procexp tracks the time spent servicing interrupts and DPCs and displays them separately from the Idle process. Finally, Procexp uses additional system metrics so that processes consuming small amounts of CPU can be identified and, when possible, provide a more accurate account of actual CPU consumption. Different metrics are available on Windows XP, Windows Vista, and Windows 7 and their corresponding server versions. Procexp takes advantage of whatever is available to report the most accurate measures possible.
I also found this on the Sysinternals Forums, from the maker of ProcExp himself:
Process Explorer shows actual CPU usage now based on cycle counts. If you have speedstep or other CPU frequency throttling enabled that will cause work consuming some number of cycles to be calculated as a higher percentage of available cycles per second.
Has to do something with your processor being downclocked, Task Manager probably counts in downclocks while Process Explorer doesn't, you should set your power option to Performance and check if it shows 100% when maxed out.
Windows Defender will run under the "Task Manager" label. I'm guessing your computer was doing a weekly virus scan when you saw these numbers. I see this happening on my Windows 8 laptop too with the fan going at maximum.