Re: Jiffy question and sound.

[watermodem]

watermodem wrote:
> 
> Matti Aarnio wrote:
> >
> > 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.)
> >
> 
> My sound card uses ALSA and ALSA wasn't available yet for
> the new kernel.  So.. Noting that LOOPS_PER_SEC was what
> failed in the kernel I modified it and compiled.  I am
> not associated in anyway with the ALSA folks just wanted
> too listen to music while working away.  I have no idea
> why it needs it or if it is busy-looping... (I hope not).

I noticed that if I kill the "artsd" daemon and then
let it naturally restart when starting another music player
the problem goes away for awhile.   When artsd starts using
more than 1.7% of the CPU then the problem occurs.  So I think
I was looking at the wrong code.  Perhaps the problem is with
the daemon.

> 
> >   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
> -
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