thread::yield() vs Sleep()ing

From: simonwlb (conic_at_[hidden])
Date: 2002-03-07 20:01:02

I'm using thread::yield() in my multi-threaded cross-platform 
application to avoid "CPU burn".

This is because I have threads that are in an infinte loop in order 
to do some polling.  There are no mutexes to wait on in the loop.

void X::threadFunction()
{
  for(;;)
  {
    if( m_x->isReady() )
      doSomething();
    .
    .
    .
    boost::thread::yield();
  }
}

If the answer is "you should almost never poll like that - try to use 
mutexes etc. as much as possible" please say so (I'm a newbie to 
threads)!

In any case, if I choose to have a thread in an infinite loop anyway, 
I have three options (assuming I have no mutexes available to me):
1. Don't yield or Sleep at all.
2. Sleep for a while - say for example, 10 ms.
3. Yield.

Option 1 is clearly wrong.  The CPU usage will go to 100% and the 
whole machine may slow down to an unusable speed.

Option 2 works well in my emperical tests.  The thread loop executes 
plenty fast enough and my Windows CPU meter shows virtually 0% CPU 
usage.  10ms is a bit of an arbitrary figure though - My code needs 
to work on a variety of software and hardware platforms, so the ideal 
number is hard or impossible to know.

Option 3 also works well in my emperical tests and seems like the 
neat solution (no arbitrary waiting period).  My application and the 
system as a whole both run smoothly and fast.  The only irksome thing 
is that the CPU meter shows 100% usage.  That's a bit alarming for me 
and the users of my software.

My questions:
1. Should thread loops without logical wait conditions be avoided as 
much as possible?  Is that the problem?

2. Should I just yield() or should I pause for a time period?

3. How do you select a time period to wait?

Simon Bailey