Re: [Qemu-devel] About qemu emulation speed (a question) and supported OS

[Jim C. Brown]

On Tue, Sep 13, 2005 at 08:36:29AM -0400, Alexandre Leclerc wrote:
> Hi all,
> 
> I'm new to qemu and my question is simple and is probably due to my
> ignorance. If I compare qemu and vmware, there is a great deal of
> emulation speed differences.
> 
> - Is it because of what qemu is? (i.e. it is a full emulator of many
> platforms, etc. Meaning that vmware is probably only specialised for
> x86...)

Yes. qemu does dynamic translation, which allows for arbituary combinations
(PPC host and x86 guest, x86 host and PPC guest, SPARC host and PPC guest,
etc.) but is also slower than running code via virtualization, which is what
VMware does (virtualization only works when host and guest are same, e.g.
x86 host and x86 guest or PPC host and PPC guest; it's also not possible (or
at least very hard) to generalize this so virtualizers tend to be tied to a
single computer arch).

> - If no, is it possible that one day qemu reaches the speed of vmware?
> 

qemu itself? Nope.

kqemu/qvm86 don't have this limitation though. Fabrice had said that he wants
kqemu to be able to do total virtualization (both kernel and userland bits);
basically all the translation code of qemu would be left unused but the hardware
emulation would still be shared.

(I'm not sure why Fabrice wants kqemu to do total virtualization: possibly he
wants to turn it into a commercial product.)

> I'm just asking because I'll probably need an emulator for a host
> linux guest winxp in few time. This is for a production environment
> and to develop in windows (so programming in the vm all day long, so
> speed is very important since it impacts productivity.) So I must know
> if it is better for me to buy a vmware licence.
> 

VMware has good speed gains for Windows guests because of the VMware tools
package - it uses custom guest drivers which allow it to get a huge speedup
by avoiding the burden of emulated I/O. qemu doesn't do this, and while it isn't
too difficult to add, I doubt anyone will bother in the near future.

I use qemu (sans kqemu/qvm86) to program in Windows. I use 2000, I found
XP unbearably slow (but the speed would be improved when using kqemu). I find
that the speed of using cygwin gcc in win2000 to be acceptable, but slower than
linux/gnu gcc. It would be much faster with kqemu but I don't know how much.

The nice thing about FOSS: you can give it a test run to see if its fast enough
for your purpose or not.

-- 
Infinite complexity begets infinite beauty.
Infinite precision begets infinite perfection.