device polling and weird timer interrupt count from vmstat

[Oliver Fromme]

Artem Kuchin wrote:
 > I enabled device polling in the kernel, in nics and
 > set HZ=1000.

HZ=1000 is the default anyway.  You only need to set it
if you want a value other than 1000.

For example, on my notebook I have set HZ=600 because that
machine often used for various media playback (video).
600 is a multiple of 24, 25 and 30, which are common frame
rates of video files.  Aligning the kernel's scheduling
quantum with the frame rate improves video playback.
On a very fast processor I would probably use HZ=1200,
but mine is only an older single-core.

 > How, when i do
 > 
 > omni2# vmstat -i
 > 
 > i see
 > 
 > interrupt                          total       rate
 > irq14: ata0                           47          0
 > irq15: ata1                           41          0
 > irq28: em0                          2268          4
 > irq72: twe0                        58380        120
 > cpu0: timer                       965994       1995
 > cpu3: timer                            1          0
 > cpu1: timer                            1          0
 > cpu2: timer                       965857       1995
 > Total                            1992589       4116

Looks perfectly normal.

 > What i don't understand is why timer rate on each cpu is 1995? I have
 > set it to 1000, not 1995 or 2000. I have seen it showing 2000 on
 > another box.

The timer frequencies are not necessarily the same as the
HZ setting.  They are related, but don't have to be the
same.  It depends on several things, in particular the type
of your processor and sysctl kern.timecounter settings.

There are several formulas, dependencies and requirements
for time counters.  For example, the frequency must be
set so the counter does not roll over in less than about
max(2 msec, 2/HZ sec), which depends on the bit width of
the counter (see src/sys/sys/timetc.h).  On the other
hand, you want a counter which runs as fast as possible,
so you get better precision for time keeping.  For those
reasons the cpu timer can be set to a multiple of HZ.

I've made a quick survery on a few machines around me
(unless otherwise noted, all of these run FreeBSD/i386
6-stable and have the default of HZ=1000):

Pentium III, 800 MHz:
cpu0: timer                   1893955760       1999

Xeon 5160, 3000 MHz:
cpu0: timer                   2466663483       1996

Athlon64, 2200 MHz:
cpu0: timer                   3964659397        960

Athlon64 X2, 2x 2000 MHz, 7-current:
cpu0: timer                    847083650       2000
cpu1: timer                    847081624       2000

VIA C3 Nehemiah, 1000 MHz, HZ=250:
irq0: clk                      167288669        249

 > 3) Is timer int really generated on each cpu? Am i really wasting cpu
 > time on ~4000 ints per second?

4000 ints per second is rather nothing on any modern CPU.
Have a look at the top(1) display of an otherwise idle
system.  The "%interrupt" column should be zero, even if
it's processing 4000 timer interrupts per second.  As far
as I know, the cpu timer interrupt handler is very light-
weight.

 > 4) does twe driver use polling? whay about twa?

No, they don't.  They aren't even NIC drivers.  Currently
polling is only implemented for (some of) the NIC drivers.

 > how to check it in the sources? 

The polling(4) manual page lists all supported devices.

Best regards
   Oliver

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