[Qemu-devel] High CPU Usage with Linux 2.6.24 and Windows XP Guest (but not with 2.6.23)

[Steve Fosdick <lists@pelvoux.nildram.co.uk>]

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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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