Re: Linux 2.2.18: /proc/apm slows system time (was: Linux 2.2.19pre3)

From: Erik Mouw (J.A.K.Mouw@ITS.TUDelft.NL)
Date: Fri Dec 29 2000 - 07:42:52 EST

On Thu, Dec 28, 2000 at 02:53:37PM +0100, Matthias Andree wrote:
> Forget this all.
> I found the problem trigger, it's reading from /proc/apm, for a reason I
> cannot currently see.
> Current config, as far as it's APM-related:
> # CONFIG_APM_DO_ENABLE is not set
> # CONFIG_APM_CPU_IDLE is not set
> # CONFIG_TOSHIBA is not set
> I had found out that my clock was slow while dnetc was running. I had a
> dummy loader that just did while(1) {} which did not slow my clock. Now, I
> straced that dnetc beast and found out that it reads /proc/apm quite
> often.

I got the same problem while running the GNOME battery_applet, which
checks /proc/apm every 2 seconds.

> I can have my clock almost halt with this one:
> while cat /proc/apm ; do : ; done
> If I leave this running for 15 s, my system time drifts back 11 s.

Same over here on an Asus M8300 notebook (Celeron 500, 128MB, Intel
440MX chipset). Output from /proc/apm:

  1.14 1.2 0x03 0x01 0x03 0x09 100% -1 ?

I also enabled CONFIG_APM_ALLOW_INTS to see if it makes any difference,
but apparently it doesn't.

However, reading from /proc/apm also triggers other weird problems:

- Sound hickups: mp3 output starts to sound "scratchy". problem
  disappears after restarting mp3 player or after a couple of reads
  from /proc/apm. This is with mpg123, xmms, and madplay.
- USB drop outs, especially for bulk devices like scanners or USB audio
  devices. For sound it usually takes a second or so to restart.
- Received characters dropped on serial line. I thought my serial port
  was broken, because a 16550 is supposed to have a receive buffer.

All these problems went away when I removed the GNOME battery_applet.

I got the same problems with my previous notebook (Asus P6300, PII 266,
112MB, Intel BX/ZX chipset). It might be a BIOS problem, because both
notebooks use a Phoenix BIOS. OTOH, I can create the same problems with
USB on my desktop (Asus P5A motherboard, K6 333, 160MB, Ali 1541
chipset, Award BIOS) when I run the GNOME battery_applet. So is this an
Asus problem, or a general APM problem?


J.A.K. (Erik) Mouw, Information and Communication Theory Group, Department
of Electrical Engineering, Faculty of Information Technology and Systems,
Delft University of Technology, PO BOX 5031,  2600 GA Delft, The Netherlands
Phone: +31-15-2783635  Fax: +31-15-2781843  Email:
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