pthread_cond_timedwait timing out late when large load put on CPU - c++

Question

In writing unit tests for an object, I am noticing that a pthread_cond_timedwait does not timeout soon enough when large loads are put upon the CPU. If these loads are not put on the CPU, everything works fine. When loads are put on to the system, however, I find that no matter the amount of time I set the timeout to, the true delay is off by about 50-100ms.
For example, here is a printout from a single interval of the program, where the last and current times are found using the function GetTimeInMs.
// Printout, values are in ms
Last: 89799240
Current: 89799440
Period Length: 200
Expected Period: 100
From all I have read this issue is usually caused by using relative times instead of absolute times, but as far as I can tell we are using absolute times correctly. If you wonderful people could help me figure out what is being done wrong here I would be very grateful.
The function utilizing timedwait is shown here. Note that based off of timing debugging I have done, I know the extra time generated is done via the timedwait call, so I have not included other code that would not be necessary.
bool func(unsigned long long int time = 100) // ms
{
struct timespec ts;
pthread_mutex_lock(&m_Mutex);
if (0 == m_CurrentCount)
{
// Current time + delay in ns
unsigned long long int absnanotime = (GetTimeInMs()+time)*1000000;
struct timespec ts;
ts.tv_nsec = absnanotime % 1000000000ULL;
ts.tv_sec = absnanotime / 1000000000ULL;
do
{
if (0 != pthread_cond_timedwait(&m_Condition, &m_Mutex, &ts))
{
// In the case I am testing, I hope to get here via timeout in 100 ms
pthread_mutex_unlock(&m_Mutex);
return false;
}
}
while (!m_CurrentCount);
}
pthread_mutex_unlock(&m_Mutex);
return true;
}
unsigned long long int GetTimeInMs()
{
unsigned long long int time;
struct timespec ts;
clock_gettime(CLOCK_MONOTONIC, &ts);
time = ts.tv_nsec + ts.tv_sec * 1000000000ULL;
time = time / 1000000ULL; // Converts to ms
return time;
}
The code used to initialize the class variables used in func.
void init()
{
pthread_mutex_init(&m_Mutex, NULL);
pthread_condattr_init(&m_Attr);
pthread_condattr_setclock(&m_Attr, CLOCK_MONOTONIC);
pthread_cond_init(&m_Condition, &m_Attr);
}
The CPU eater thread which simulates CPU load is running the following while loop.
void cpuEatingThread()
{
while (false == m_ShutdownRequested);
{
// m_UselessFoo is of type float*
m_UselessFoo = new float(1.23423525);
delete m_UselessFoo;
}
}

Answer 1

It's likely that, when the wait times out, the thread becomes ready without any priority boost or any other such action/s. If the box is loaded up, then the ready thread may not become running immediately.
It's common to apply temporary priority boosts to thread that become ready on signals - this tends to improve overall performance in the 'usual' case where the signal arrives before the timeout. The timeout is often more of an 'unusual' event, often signaling some sort of failure that will not be repeated and so threads becoming ready on timeout can wait their turn:)

Answer 2

For timed waits in general, the requirement is that they will wait at least as long as their argument. If you want precise times, this is not the right tool; you'll need something that guarantees particular times, and that's generally only available in a real-time operating system (RTOS).