Re: [AD] rest and yield_timeslice

[Chris <kcat@xxxxxxxxxx>]

Elias Pschernig wrote:
> Well, not completely separate: If you do rest(x) with x other than 0,
> you always yield.

Though there's an important difference. When you do rest(x) where x > 0, 
on platforms capable of course, you technically don't yield. Yielding 
implies stopping _only if_ there's something else on your priority level 
or higher that wants the timeslice. When you rest, you're actually 
lowering the process's priority level so that it's on par with the 
system idle process, if not lower (eg. completely inactive), until 
another process restores it. However, when you yield you keep your 
priority level the same.. you're merely put at the end of the queue so 
other processes of equal or higher priority can use the timeslice.

Now.. Windows seems to do something like this for Sleep:

set_process(lowest_priority);
do {
} while (wait_time > 0);
set_process(restore_priority);

of course, it just so happens that restoring priority automatically puts 
you at the end of the scheduler's queue.. so you get the effect, however 
ineffient, of a plain yield. However, I think this is completely 
inefficient for a game programming library as most games only want to 
give up the CPU if they really have to, so you would first check that 
you actually need to wait:

if(wait_time > 0)
{
   set_process(lowest_priority);
   do {
   } while (wait_time > 0);
   set_process(restore_priority);
}

Now, which do you think would be better for games, where you have 
situations like this:
while(playing) {
  do_frame();
  rest(expected_frame_time/*10ms for 100fps*/ - accum_frame_time);
}
If something like that hits <=0, you don't want to rest at all.

To bring a real-world analogy.. If you want to drive for 2 miles, you 
get in your car, start it up, drive 2 miles, shut it off. But if you 
want to drive for 0 miles, would you get in your car, start it up, then 
just shut it off? No, one would think you don't even bother going to the 
car at all. It's the same here, IMO.