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Re: [Qemu-devel] time synchronization with qemu

From: Piotr Krysik
Subject: Re: [Qemu-devel] time synchronization with qemu
Date: Wed, 15 Sep 2004 07:37:11 -0700 (PDT)


When qemu is waiting to read data from your host 
disk, the clock may slow down. For start try to 
work on small images that fit in ram.

My guess is that your guest uses timer interrupt 
as time source and when there should be two or more 
interrupts while qemu is waiting for disk read 
to complete, only first interrupt will be delivered. 
The solution is to modify qemu source code to use 
asynchronous disk IO. A patch was send on this list 
a few months ago (search the list archive for AIO), 
but I didn't try (or even analyze it), so I cannot 
tell if it can help.



--- giovanni bracco <address@hidden> wrote:

> I have tried but it does not works well and  I 
> think that the reason is that inside a qemu system 
> the clock is not stable and depends on the load 
> of the ost machine (which depends, in my case of 
> the 3 qemu running together, also on the load on 
> the other qemu systems). I suppose that ntp does 
> not work well with very unstable clocks!
> Up  to now the best solution I have found is to 
> run a cron job on the host which sets every minute 
> the clock of the 3 qemu systems using "rsh rdate 
> -s host.ip" but still the time difference between 
> qemu systems is til between 3 and 10 seconds (even 
> if there is a reset about every minute!)
> Any suggestion is welll accepted!
> Giovanni

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