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Subject: Re: Timing issue with Dummynet on high kernel timer interrupt
To: Luigi Rizzo <rizzo@iet.unipi.it>
References: <6545444AE21C2749939E637E56594CEA3C0DCCC4@gsp-ex02.ds.swin.edu.au>
 <5638B7B5.3030802@selasky.org>
 <6545444AE21C2749939E637E56594CEA3C0DE7FF@gsp-ex02.ds.swin.edu.au>
 <563B2703.5080402@selasky.org>
 <6545444AE21C2749939E637E56594CEA3C0E0BD9@gsp-ex02.ds.swin.edu.au>
 <563C6864.2090907@selasky.org>
 <CA+hQ2+hm2z0MkB-8w5xJM7+iz13r_ZjwmpZBnb30_D_48gaf-w@mail.gmail.com>
 <563C786C.1050305@selasky.org>
 <CA+hQ2+j0WiGgzV119M1ZQiXP5Z7fq7deVxDPkOhvTc7hpTETKw@mail.gmail.com>
 <563CC186.9000807@selasky.org>
Cc: Rasool Al-Saadi <ralsaadi@swin.edu.au>,
 "freebsd-net@freebsd.org" <freebsd-net@freebsd.org>,
 FreeBSD Current <freebsd-current@freebsd.org>,
 Alexander Motin <mav@FreeBSD.org>
From: Hans Petter Selasky <hps@selasky.org>
Message-ID: <563CD533.2000909@selasky.org>
Date: Fri, 6 Nov 2015 17:28:35 +0100
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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