Timing issue with Dummynet on high kernel timer interrupt

[Hans Petter Selasky]

Hi,

I spent some time to write a test application to investigate this issue 
and I found some irregularities, that when kern.eventtimer.periodic=0, 
the timer appears to run very irregular.

Test software:
==============

fetch http://home.selasky.org:8192/privat/callout_test_dummynet.tar.gz
tar -zxvf callout_test_dummynet.tar.gz
cd callout_test_dummynet
make -m /usr/src/share/mk SYSDIR=/usr/src/sys all install

Test I:
=======

The following settings are placed in /boot/loader.conf and computer 
rebooted.
kern.hz=10000
kern.eventtimer.periodic=1

kldload callout_test

./test.sh
debug.total: 32604 -> 0
debug.total: 10021 -> 0
debug.total: 10020 -> 0
debug.total: 10020 -> 0
debug.total: 10020 -> 0
debug.total: 10020 -> 0
debug.total: 10020 -> 0
debug.total: 10020 -> 0
debug.total: 10019 -> 0
debug.total: 10021 -> 0
debug.total: 10020 -> 0
debug.total: 10021 -> 0
debug.total: 10019 -> 0
debug.total: 10020 -> 0
debug.total: 10018 -> 0
debug.total: 10021 -> 0
debug.total: 10020 -> 0
debug.total: 10020 -> 0
debug.total: 10020 -> 0
debug.total: 10020 -> 0
debug.total: 10020 -> 0
debug.total: 10020 -> 0
debug.total: 10020 -> 0
debug.total: 10020 -> 0

Test II:
========

The following settings are placed in /boot/loader.conf and computer 
rebooted.
kern.hz=10000
kern.eventtimer.periodic=0

kldload callout_test

./test.sh
debug.total: 20337 -> 0
debug.total: 10091 -> 0
debug.total: 10018 -> 1
debug.total: 10041 -> 0
debug.total: 10019 -> 0
debug.total: 10644 -> 0
debug.total: 10014 -> 1
debug.total: 10020 -> 0
debug.total: 10019 -> 0
debug.total: 10644 -> 0
debug.total: 10014 -> 0
debug.total: 10644 -> 0
debug.total: 10640 -> 0
debug.total: 10210 -> 0
debug.total: 10015 -> 0
debug.total: 10020 -> 1
debug.total: 10020 -> 0
debug.total: 10453 -> 0
debug.total: 10642 -> 0

If you load the computer, like a multi-CPU buildkernel, the value 
flattens out again.

Test III:
=========
The following settings are placed in /boot/loader.conf and computer 
rebooted.
kern.hz=1000
kern.eventtimer.periodic=1

kldload callout_test

./test.sh
debug.total: 5238 -> 0
debug.total: 1005 -> 0
debug.total: 1003 -> 0
debug.total: 1006 -> 0
debug.total: 1003 -> 0
debug.total: 1003 -> 0
debug.total: 1005 -> 0
debug.total: 1006 -> 0
debug.total: 1003 -> 0
debug.total: 1013 -> 0
debug.total: 1005 -> 0
debug.total: 1006 -> 0
debug.total: 1004 -> 0
debug.total: 1005 -> 0
debug.total: 1003 -> 0
debug.total: 1004 -> 0
debug.total: 1006 -> 0
debug.total: 1004 -> 0
debug.total: 1004 -> 0
debug.total: 1007 -> 0
debug.total: 1003 -> 0
debug.total: 1004 -> 0


The difference between 100021 and 10642 interrupts in a second for test 
II is quite big, close to 6.5% deviation. Does anyone have any clue if 
this is the expected behaviour when kern.eventtimer.periodic=1 or not?

MAV ?

--HPS