Write code to make CPU usage display a sine wave

Question

Write code in your favorite language and let Windows Task Manager represent a sine wave in CPU Usage History.

This is a technical interview quiz from Microsoft China. I think it's a good question. Especially it's worth knowing how candidate understand and figure out the solution.

Edit: It's a good point if may involve multi-core(cpu) cases.

Answer 1

A thread time slice in Windows is 40ms, iirc, so that might be a good number to use as the 100% mark.

unsigned const TIME_SLICE = 40;
float const PI = 3.14159265358979323846f;
while(true)
{
    for(unsigned x=0; x!=360; ++x)
    {
        float t = sin(static_cast<float>(x)/180*PI)*0.5f + 0.5f;
        DWORD busy_time = static_cast<DWORD>(t*TIME_SLICE);
        DWORD wait_start = GetTickCount();
        while(GetTickCount() - wait_start < busy_time)
        {
        }
        Sleep(TIME_SLICE - busy_time);    
    }
}

This would give a period of about 14 seconds. Obviously this assumes there is no other significant cpu usage in the system, and that you are only running it on a single CPU. Neither of these is really that common in reality.

Answer 2

Here's a slightly modified @flodin's solution in Python:

#!/usr/bin/env python
import itertools, math, time, sys

time_period = float(sys.argv[1]) if len(sys.argv) > 1 else 30   # seconds
time_slice  = float(sys.argv[2]) if len(sys.argv) > 2 else 0.04 # seconds

N = int(time_period / time_slice)
for i in itertools.cycle(range(N)):
    busy_time = time_slice / 2 * (math.sin(2*math.pi*i/N) + 1)
    t = time.perf_counter() + busy_time
    while t > time.perf_counter():
        pass
    time.sleep(time_slice - busy_time);    

A CPU-curve can be fine-tuned using time_period and time_slice parameters.

Answer 3

With the literally hundreds (thousands?) of threads a PC runs today, the only way I can think to even come close would be to poll CPU usage as fast as possible, and if the usage% was below where it should be on the curve, to fire off a short method that just churns numbers. That will at least bring the typical low usage UP where needed, but I can't think of a good way to LOWER it without somehow taking control of other threads, and doing something such as forcing thier priority lower.