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

[Sergey Bychkov]

----- 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
> cost.

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
>
> Ehm:
> 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
> > ==
> > 4096
> > ==
>
> 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
> > GNU/Linux
> > ==
>
> 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