Subject: Re: timecounters using broken TSC - PC-engines WRAP time problem
To: None <tech-kern@NetBSD.org>
From: theo borm <email@example.com>
Date: 11/29/2007 17:00:21
Joerg Sonnenberger wrote:
>On Thu, Nov 29, 2007 at 03:24:19PM +0100, theo borm wrote:
>>It mentions VMware, and my /guess/ was that timecounters can get confused
>>when the TSC is modified outside the netbsd scope (and that would suggest
>>potential problems with XEN too).
>Most modern systems just use the ACPI timer, which has a higher priority
>and is typically a lot more stable.
>>>Does your system have an AMD Geode SC1100 or similiar? I found a
>>>discussion for Linux that mentioned that the TSC is broken as it is not
>>>incremented during halt.
>>Yes. The fact that the TSC is broken is also mentioned in the dmsg, but
>>despite this it /is/ used.
>Can you try boot -d and "w tsc_is_broken=1"?
booting hd0a:netbsd (howto 0x40)
Stopped at netbsd:cpu_Debugger+0x4: popl %ebp
db> w tsc_is_broken=1
Hmm.... Continue booting....
brouter -> gdb --write /netbsd
(gdb) set tsc_is_broken=1
brouter -> reboot
brouter -> dmesg |grep timecounter
timecounter: Timecounters tick every 10.000 msec
timecounter: Timecounter "i8254" frequency 1189200 Hz quality 100
timecounter: Timecounter "clockinterrupt" frequency 100 Hz quality 0
It seems to work. Thanks.
Is this tsc_is_broken documented? is there a kernel compilation option too?
>>I'm wondering if using the TSC rather than the i8254 by default is such a
>>good idea. It may be more accurate than the i8254 /when/ it is working, but
>>if the i8254 clock is more reliable, I would prefer that.
>It is not so much a question of accuracy, but rather that the i8254
>timer is extremely slow on most systems.
At > 1Mhz I wouldn't call it /extremely/ slow ;-)
It is probably a very naive question, but what use would most (ordinary)
users have for a clock with better resolution than a microsecond? AFAIK
this clock is not used to directly time time-critical events (such as
outputting the next word in an audio stream), only passively, to time
and order events that occur at their own pace, and I guess the most
visible type of events (from a user's perspective) are timestamps of files.