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SubjectRe: Jiffy question and sound.
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
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