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Re: [Qemu-devel] More about slow clock in guest OS

From: Sergey Bychkov
Subject: Re: [Qemu-devel] More about slow clock in guest OS
Date: Mon, 18 Feb 2008 20:56:34 +0200

----- Original Message ----- From: "Mulyadi Santosa" <address@hidden>
To: <address@hidden>
Cc: <address@hidden>
Sent: 1.02.2008 4:41
Subject: Re: [Qemu-devel] More about slow clock in guest OS

After some investigations I can say that with the latest (2008/01/30) qemu from cvs, compiled with gcc-3.4 on linux x86_64 host, guest OS win2k3 works
not too good.
So, in other words "it works but not so well". may I interpret it as a progress?

Yes, now there is a choice: stable but slow (dynticks) and unstable but faster (rtc)

Unfortunately, there is no option "stable and correct" yet ;)

With "-clock dynticks" clock in OS is very slow - and windows time server
can't adjust.

Probably due to cost of Qemu timer rearming. And maybe Windows 2003
does certain dyntick by its own...thus enlarging the timer rearming

Tests say that in dynticks mode clock in VM is 4-5 times slower than in real world.

With "-clock rtc" hung periodically - for up to 5 minutes, 300 seconds.

pffff... too much timers get expired? lock contention somewhere...anybody?

I even stop experiments with "rtc" because of instability. Maybe somebody else will find this problem in the future, I'm focusing in testing "dynticks" mode now.

Thus..maybe it's Qemu's fault...

sudo $QEMU -net nic,model=rtl8139,macaddr=52:54:00:80:80:01 -net
tap,script=$DIR/qemu-ifup-br0,downscript=$DIR/qemu-ifdown-br0 -localtime -cdrom
/distrib/knoppix/KNOPPIX_V5.1.0CD-2006-12-30-EN.iso -m 384 -pidfile
$DIR/virt1-knoppix.pid -smp 1 -no-kqemu -clock rtc -vnc :9

1. can you simply use SDL output instead of VNC?
2. what if you don't use -localtime? just courious...
1. Yes, but main purpose is to run VM on linux server, without X.
2. The only difference is that initial clock is two hours out. Windows always assumes that hardware clock is local.

$ cat /proc/sys/dev/rtc/max-user-freq

ehm...I think 1024 is enough for most cases..
There were tests, and this parameter doesn't affect dynticks mode, so now it is in default value, 64.

$ uname -a
Linux *** 2.6.18-5-amd64 #1 SMP Sat Dec 22 20:43:59 UTC 2007 x86_64

is that kernel version of the host? can you upgrade it ? let's say to
the latest 2.6.24?
I will upgrade it when Debian releases next stable release :)

PS: could you try KVM too? but ehm...well, sounds like you don't have
VT enabled Intel processor or SVM enabled AMD processor. So, if you
indeed have one...try KVM..

At this time I apply some weird combination of standard and self-made tools to keep clock in VM in sync with outer world.
May be I will try to compare behaviour of different OSes in qemu and/or KVM.

Sergey Bychkow
ICQ: 21014758
FTN: 2:450/118.55

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