Time keeping Issues with the low-resolution TSC timecounter

[Jung-uk Kim]

On Wednesday 15 June 2011 04:39 pm, Jung-uk Kim wrote:
> On Wednesday 15 June 2011 03:27 pm, Ian FREISLICH wrote:
> > > > The problem I noticed first is that it takes unusually long
> > > > until a key press is repeated. With the default eventtimer
> > > > (HPET) it seems to take about 4s, which can be slightly
> > > > improved by switching to i8254.
> > > >
> > > > The "error beep" seems to take longer than usual, too,
> > > > and the system "feels sluggish" in general.
> > > >
> > > > An effect that is easier to measure is that the system is
> > > > unable to properly keep the time. Again the problem is less
> > > > severe when using i8254 instead of HPET:
> > >
> > > [SNIP]
> > >
> > > First of all, please do not mix timecounter issues with
> > > eventtimer. They are not directly related.
> > >
> > > Can you please show me verbose boot messages *without* your
> > > patch? Does "sysctl kern.timecounter.hardware=HPET" help
> > > *without* touching eventtimers?
> >
> > I have the same issue with my system (Atom N270).  The effect
> > that I see is about 29 wall clock seconds are recorded as 1
> > system second.
>
> Can please you send me output from the following?
>
> sh -c 'count=10; while [ $count -gt 0 ]; do count=$((count - 1));\
> sysctl kern.timecounter; sleep 1; done'

The OP sent me some data.  The following is cooked data from what I 
got:

TSC		Delta		Elapsed Time
---------------+---------------+------------
2245026970		
2249708604	4681634		0.3003693189
2255874998	6166394		0.3956301509
2260656402	4781404		0.3067704701
2261993048	1336646		0.0857579744
2264175164	2182116		0.1400025497
2266445706	2270542		0.1456758803
2266987003	541297		0.0347291162
2267645095	658092		0.0422225757
2268274965	629870		0.0404118782
2268979787	704822		0.0452207294
2270482069	1502282		0.0963850274
2271140877	658808		0.0422685136
		
HPET		Delta		Elapsed Time
---------------+---------------+------------
1322734365		
1337156221	14421856	1.0072408644
1351548130	14391909	1.0051493276
1365949265	14401135	1.0057936833
1380376349	14427084	1.0076059946
1394823985	14447636	1.0090413726
1409273964	14449979	1.0092050107
1423719753	14445789	1.0089123757
1438167064	14447311	1.0090186742
1452613630	14446566	1.0089666424
1467062977	14449347	1.0091608710
1481522037	14459060	1.0098392393
1495969404	14447367	1.0090225853

As you can see, HPET increases normally (within errors from sleep(3) 
accuracy, syscall overhead, etc.) but TSC-low is totally erratic (and 
too low).  I don't know how this can happen, though. :-(

I need some time to figure it out.

Jung-uk Kim