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Date: Wed, 25 Aug 2010 18:28:54 -0500
From: Anthony Liguori <anthony@codemonkey.ws>
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Subject: Re: [Qemu-devel] Re: qemu-kvm faster than qemu?
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To: walt <w41ter@gmail.com>
Cc: qemu-devel@nongnu.org

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.
>
>