windows - 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.