Re: [Qemu-devel] Re: qemu-kvm faster than qemu?

[Anthony Liguori]

On 08/25/2010 06:09 PM, walt wrote:
> On 08/25/2010 12:48 PM, Anthony Liguori wrote:
> > On 08/25/2010 02:37 PM, walt wrote:
> > > Hi qemu team,
> > >
> > > I just discovered that qemu now offers kvm support, so I decided to
> > > compare it to qemu-kvm.
> > >
> > > I'm running the latest git versions of both programs on an AMD64
> > > host running the latest kernel from Linus.git.
> > >
> > > The guest is Windows 7 on a qcow2 disk image and one kvm64 cpu.
> > >
> > > I find that qemu-kvm boots to the Win7 login prompt in 25 seconds,
> >> while qemu with kvm enabled takes about 45 seconds.
> > make sure you build qemu with --enable-io-thread.
>
> I built qemu both with and without io-thread, and it makes no difference
> at least in this particular experiment.
>
> (BTW, I found that qemu-kvm will not even compile with --enable-io-thread
> because qemu_mutex_lock_iothread becomes defined in two places.)
>
> > Also, what's your full command line?
>
> For qemu:
> /usr/local/bin/qemu-system-x86_64 -m 1024 -enable-kvm -cpu kvm64 w7.diff
>
> For qemu-kvm:
> /home/wa1ter/bin/qemu-system-x86_64 -m 1024 -enable-kvm -cpu kvm64 
> w7.diff

You'll get more accurate and repeatable results by using:

qemu-system-x86_64 -m 1024 -enable-kvm -cpu kvm64 -drive 
file=w7.diff,cache=none

Also try qemu-kvm with -no-kvm-irqchip.  I can't believe an in-kernel 
lapic would make this much of a difference with windows 7 but it's worth 
trying.

Regards,

Anthony Liguori

> Note that w7.diff is the same file in both cases: a qcow2 differencing 
> disk
> made from the original qcow2 backing file w7.img.
>
> Thanks for your reply.  I'm curious if anyone can reproduce this 
> difference
> in speed.