Windows System Idle Processes interfering with performance measurement

Question

I am doing some performance measurement of my code on a Windows box and I am finding that I am getting dramatically different results between measurements. A quick bit of ad hoc exploration during a slow one shows in the task manager System Idle Processes taking up almost 100% CPU.

Does anyone know what System Idle Processes actually means and what Windows features it may be running?

NB: I am not measuring performance using the task manager, I just used it to take a look at what else was running during a particularly slow measurement.

Please think before saying this is not programming related and closing the question. I would not ask it unless I thought there were grounds to say it is. In this case I believe it clearly is because it is detrimentally affecting my development and test environments and in order to sort it out I need to know a bit more about it. Programming does not start and end with the writing of the code.

Answer 1

System Idle Process is a sort of a default process that Windows runs on the processor when it has nothing else to schedule for running. This process is like a housekeeper that does things like trying to save system power etc.

If you're measuring the performance of your program, don't use the Windows Task manager. Use Performance Monitor instead (which you can start by typing 'perfmon' at the commandline). Or better still, use a profiler.

Answer 2

The idle process typically doesn't do any useful work except execute the HLT instruction, which puts that CPU core into a lower power state (C1). However, the fact that your benchmark is not consuming 100% CPU time does open the door for speculation about what is going on.

If your application is single-threaded and your test system is multicore/hyperthreaded/multi-CPU, then you should expect to see around 50% idle CPU time for two cores, 75% for four, etc. This is because the CPU time percentages in Task Manager include all cores. (I believe that older versions of Windows had an option to change this, but I don't see it on Vista.)

If the idle process is consuming a lot of CPU, that may indicate that your application is spending a lot of time sleeping. It might be waiting for data from some external source (e.g. a disk or network). It might be spending a lot of time waiting for synchronization objects (e.g. mutexes or events). It also might be spending a lot of time calling the Sleep() function. Profiling your code should identify where it's spending the time.

Getting fully reproducible benchmark results may require you to disable processor/disk/network-intensive background applications and services (e.g. search indexing, SMS software inventory, virus scanning, Windows Update downloads, IncrediBuild/distcc) or to connect the machine to an isolated network (or to no network at all).

I'm assuming that you wrote a benchmark for your application, and that you're just trying to use Task Manager to diagnose why the benchmark results aren't what you expected. Task Manager isn't an accurate way to measure application performance.

Answer 3

If you "system idle process" is taking up 100% then essentially you machine is bored, nothing is going on. If you add up everything going on in task manager, subtract this number from 100%, then you will have the value of "system idle process." Notice it consumes almost no memory at all and cannot be affecting performance.