Question

I have a multi-threaded web application with about 1000~2000 threads at production environment.

I expect CPU usage on w3wp.exe but System Idle Process eats CPU. Why?

Answer 1

The Idle process isn t actually a real process, it doesn t "eat" your CPU time. the %cpu you see next to it is actually unused %cpu (more or less).

The reason for the poor performance of your application is most likely due to your 2000 threads. Windows (or indeed any operating system) was never meant to run so many threads at a time. You re wasting most of the time just context switching between them, each getting a couple of milliseconds of processing time every ~30 seconds (15ms*2000=30sec!!!!).

Rethink your application.

Answer 2

the idle process is simply holding process time until a program needs it, its not actually eating any cycles at all. you can think the system idle time as available cpu

Answer 3

System Idle Process is not a real process, it represents unused processor time.

This means that your app doesn t utilize the processor completely - it may be memory-bound or CPU-bound; possibly the threads are waiting for each other, or for external resources? Context switching overhead could also be a culprit - unless you have 2000 cores, the threads are not actually running all at the same time, but assigned time slices by the task scheduler, this also takes some time.

Answer 4

You have not provided a lot of details so I can only speculate at this point. I would say it is likely that most of those threads are doing nothing. The ones that are doing something are probably IO bound meaning that they are spending most of their waiting for the external resource to respond.

Now lets talk about the "1000~2000 threads". There are very few cases (maybe none) where having that many threads is a good idea. I think your current issue is a perfect example of why. Most of those threads are (apparently anyway) doing nothing but wasting resources. If you want to process multiple tasks in parallel, espcially if they are IO bound, then it is better to take advantage of pooled resources like the ThreadPool or by using the Task Parallel Library.

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