Thread sleep VS wait condition

Question

EDIT :

The point here is to know the more efficient solution for implement a wait loop which poll a condition on every iteration. By efficient, I mean "efficient for CPU scheduling".

I know that the wait condition used in my code is not a "real wait condition" used witch a "wakeOne"/"wakeAll" instruction but I want to know if using a fake wait condition is more effective for the CPU than a sleep.

Original post :

Here is 2 code snippets, which does the same thing : wait until something happen. This code is used in a pool of worker threads. So when a thread wait, the others (or some of the others) should process their instructions.

The first use a "sleep", the second a "wait condition". They are written with Qt, but could be easily translated to C++11, Boost or any thread library.

The both works well, but is there any performance difference ? I remember that I've read somewhere :

The "sleep" cause an active wait, so the CPU spend time to wait.

The "wait condition" make the CPU wait for an event, so the CPU switch to another thread execution during the wait

Do I remember well ? Is that true ? Using a wait condition is more effective to execute multiple threads in parrallel ?

The "sleep" version :

while (someCondition == false)
{
    sleep(100);
}

// Do some work

The "WaitCondition" version :

QMutex mutex(QMutex::NonRecursive);
QWaitCondition waitCondition;

while (someCondition == false)
{
    QMutexLocker locker(&mutex);
    waitCondition.wait(&mutex, 100);
}

// Do some work

Answer 1

EDITED:

Both versions are identical, as both "block" or "suspend" (that is - remove it from the "runnable" list in scheduler) the calling thread.

http://pubs.opengroup.org/onlinepubs/9699919799/functions/sleep.html http://pubs.opengroup.org/onlinepubs/9699919799/functions/pthread_cond_timedwait.html

During sleep and wait for CV the thread does not use any clock cycles of the CPU.

ORIGINAL:

The difference is significant. The first example (with sleep) will react to an event within 0-100ms. The other - with condition variable - will react "instantly". That is because the sleep will do just that - sleep for the exact amount of time you requested (note - let's assume that your system does not use POSIX-style signals). On the other hand, condition variable will wait for as long as you requested unless it receives a notification that the condition might have changed.