Re: Jiffy question and sound.

[Matti Aarnio <matti.aarnio@zmailer.org>]

On Mon, Mar 19, 2001 at 12:20:47AM -0600, watermodem wrote:
> With the 2.4.0 kernel the loops_per_sec field was replaced (for i386)
> with current_cpu_data.loops_per_jiffy.
...
> #define LOOPS_PER_SEC current_cpu_data.loops_per_jiffy * 100

  The intention was to accomodate systems with faster than 2 GHz clock
  at which the LOOPS_PER_SEC counter spins around a bit too fast..
  ('signed long' at i386 handles 0..2G just fine, then it thinks the sign
   got inverted..  'unsigned long' works fine until 4 GHz processors.)

  Why does the ALSA need  LOOPS_PER_SEC ?
  Is it doing timing by busy-looping ?

> Now compiling the same  ALSA modules with 2.4.2 this problem happens
> much quicker and you don't need any other activity.  In fact it is hard
> to play more than half a song.  (MP3)
> It doesn't matter if what set of music players or tools I use the
> problem is quite visible.
> 
> When I boot back to the original 2.2.x kernel everything is perfect.   
> 
> So I guess I have a few questions here.
>  1)   Is a jiffy 100th of a second or is it smaller  (so my loop count
> is starving things.) (10ms) ?

	"HZ" is the answer.  E.g. Alpha has HZ=1024, while i386 has HZ=100
	Nearly all architectures have different values based on what some
	other UNIX uses at given system.

>  2)   Why is it so much worse in 2.4.2 than 2.4.0?
>  3)   Any other "gotch's" that are important to watch for when moving
> 2.2.x drivers to 2.4.x?

	The FAQ may have some pointers to "porting drivers to 2.4" documents.

> Thanks....
> Watermodem
> -
> Please read the FAQ at  http://www.tux.org/lkml/

/Matti Aarnio