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[Qemu-devel] High CPU Usage with Linux 2.6.24 and Windows XP Guest (but

Subject: [Qemu-devel] High CPU Usage with Linux 2.6.24 and Windows XP Guest but not with 2.6.23
From: Steve Fosdick
Date: Thu, 6 Mar 2008 10:58:38 +0000
Guys,

If I run Linux kernel 2.6.24.3 and then start a qemu virtual machine running 
Windows XP as the guest operating system the CPU usage is high, always close to 
100%, and the virtual machine slower than normal.

Once I am able to log in to windows, task manager shows the CPU usage 
permanently at 100%, even when Windows should be idle, and higher than normal 
usage from the csrss process.  I attach a screen shot of task manager showing 
this.  Looking at the CPU usage from Linux when windows is idle it is notable 
that most of the CPU usage is user-mode.

By comparison, with exactly the same virtual hard disk image and the same 
version of qemu (and kqemu) and kernel 2.6.23.12 the CPU usage is much lower 
and windows runs faster.  When windows is idle the CPU usage is low and when 
Windows is active the CPU usage is approximately 2/3 user and 1/3 system.

The qemu versions concerned are as follows:

qemu  0.9.0
kqmeu 1.3.0pre11

The hardware is an AMD64 processor and 1GB RAM.

The problem with 2.6.24 seems only to occur with kqemu and also appears in 
dependant of whether dynamic ticks is enabled.  A couple of timings should 
illustrate the difference.

>From VM start to login prompt: no kqemu=2m0s, 2.6.23=1m2s, 2.6.24=1m27s.
>From login to last systray icon: no kqemu=6m25s, 2.6.23=1m47s, 2.6.24=4m46s

Does anyone have any insight as to what may be happening or what tools I could 
use to gather enough information to help you guys diagnose the problem?

Regards,
Steve.
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