I wrote an infinite loop in python to print No! in the output.

while True:

When I execute it, I see its process (pythonw.exe) in Windows-Task-Manager that uses 60% of the CPU. I did a right-click on its process and change the priority to Low. it use 60% of CPU again! Then I change the priority to Real-Time. It still uses 60% of CPU !

Update: My total CPU usage for all process is 80%!!! and 20% is free! Why this python loop doesn't use this 20%?!

What's wrong with priority? Why it doesn't change?

  • 1
    Because the x86 instructions required to do this don't take more than 60% of a single clock cycle – Ramhound Sep 26 '14 at 16:32
  • Would you please explain it more clear as an answer? Why those instructions doesn't take more than 60% of a single clock cycle in x86? is it different in x64? – TheGoodUser Sep 26 '14 at 16:40
  • X64 is an extension of the x86 architecture, what part, don't you understand exactly – Ramhound Sep 26 '14 at 16:57
  • @Ramhound 1- x86 instructions required to do this don't take more than 60% of a single clock cycle. Why?! And 2- Why when I set its priority to Low it still use 60%? Why doesn't it reduces? – TheGoodUser Sep 26 '14 at 17:06
  • Because the load delta caused by this script and the load without the script the processor doesn't have to prioritize the threads. As for the other part unless you know x86 assembly any additional details wouldn't be helpful – Ramhound Sep 26 '14 at 17:09

The priority determines which thread gets to run when multiple threads are competing for CPU time. If nothing else wants the CPU, your program's thread(s) will get up to 100% CPU time, as and when needed, regardless of priority.

In your case the Python process is the only one needing a lot of CPU time, so it will get all it asks for. If your system were otherwise busy, then you would see your 60% drop much more than if it had normal priority.

You should test it against another CPU-consuming program running at the same time, eg in a CMD shell run test.cmd containing:

dir /s c:\
goto Loop

Then you will see the effects of changing the priority of your competing process. Note that the CPU requirements of this example will vary, depending on the CPU speed, the number of files in c:\ and disc cache sizes; redirecting the dir output to >nul: will increase the CPU load.

  • this batch file program uses only 3 percent! and again it doesn't change even if I change its priority! for that Python program, its CPU usage is fixed on 60% , while my total CPU usage is 80%! I mean, My CPU has 20% free !!! – TheGoodUser Sep 26 '14 at 16:51
  • You can't force a program to consume more time within a clock cycle than it needs – Ramhound Sep 26 '14 at 17:00
  • @TheGoodUser-Sp - I didn't want to make it too intensive, but if you redirect the output, as in dir /s c:\ >nul:, that will increase the CPU demand. If you comment out the dir line, you will create a CPU loop. – AFH Sep 26 '14 at 17:10
  • 1
    @TheGoodUser-Sp - As an adjunct, it is worth noting that unless other processes require more than 40% CPU, then the 60% your python program demands will always be available. You could also try running two copies of your program and changing the priority of one of them: they can't both get 60%, which guarantees that the CPU will be overstretched, so that priorities will be taken into account in the allocation of the CPU. – AFH Sep 26 '14 at 18:00
  • The dir command is not particularly CPU-intensive - it's disk-bound. – Jamie Hanrahan Sep 4 '15 at 10:22

There's nothing wrong with priority.

It is a common assumption that a low or medium priority set on a process will cause the operating system to limit the CPU time available to the process's threads - so, setting its priority higher should let it use more CPU!

But that is not what happens.

You can have a process set to the lowest possible priority, and if its code never needs to wait for anything, and if nothing else on the system wants CPU time, that process will get 100% of the CPU.

If a process gets less than 100% CPU, and nothing of equal or higher priority is trying to run, then that simply means that the process's threads are spending part of their time not wanting the CPU.

What is a thread doing if it doesn't want the CPU? Most of the time it's "waiting". Threads wait when they need something to happen that doesn't involve running the thread's own code in the CPU. Usually they're waiting for an I/O operation to complete. This could be an I/O request explicitly requested by the thread, or it could be, for example, a hard page fault. (On the other hand, time spent "waiting" for RAM read or write latency is all part of the thread's CPU time. That's not considered I/O.)

Your loop in Python includes a "print" funciton. That's an I/O operation, even if it's only to the screen.

You could use Windows Performance Analyzer to get details about just what pythonw.exe is doing that's making it wait.

You may wish to review my answer here.

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