[Asterisk-Dev] busy-wait loops in pbx_dundi

[Luigi Rizzo]

On Fri, Jun 10, 2005 at 07:59:40AM -0400, SteveK wrote:
> 
> On Jun 10, 2005, at 4:53 AM, Luigi Rizzo wrote:
> 
> > while applying the timeval changes, i noticed that pbx_dundi.c
> > is full of busy-wait loops of the form
> >
> >     while (!some_condition) {
> >         usleep(1);
> >     }
> >
> > could someone comment how one avoids that this code eats 100% of the
> > CPU and how high the risk is ?
> 
> The answer to that seems pretty obvious, Luigi.
> 
> Assuming that usleep works correctly (it more or less does), and the  
> test for some condition takes less than a microsecond to execute (it  
> surely does), then this loop won't use more than 0.1% of your CPU.

you are assuming that the interval is rounded up to the timer tick
which i see mentioned as a possibility on manual pages but not as
a mandatory behaviour. Never mind that linux or freebsd does so, you
cannot tell what some different kernel does.
(you are also assuming the tick to be >= 1ms, but that's a minor point),

To me this code seems unsafe.
If we are happy with the code sleeping for 1ms then we should
write usleep(1000) not usleep(1).

In any case my point was to figure out how quickly are we interested
in responding to those event, and putting a sensible value in
the timeout.

	cheers
	luigi