From: Udo Lembke <ulembke@polarzone.de>
Subject: Re: [PATCH 10/12] kvm: enable smp > 1
Date: Wed, 19 May 2010 22:02:50 +0200
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To: Avi Kivity <avi@redhat.com>
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Avi Kivity schrieb:
> On 05/19/2010 12:57 PM, Udo Lembke wrote:
>> Jan Kiszka schrieb:
>>> ...
>>> --enable-io-thread?
>>>
>>> If you had it disabled, it would also answer my question if -smp works
>>> without problems without that feature.
>>>
>>> Jan
>>>
>> Hi,
>> i have a dumb question: what is the "--enable-io-thread"? Is this a 
>> kvm-switch?
>
> It's a ./configure switch for upstream qemu (don't use with qemu-kvm 
> yet).
>
>> My kvm 0.12.4 don't accept this switch. I'm know only "threads=n" as 
>> smp-parameter and "aio=threads" as drive-parameter.
>>
>> Because i look for a solution for a better io-performance of 
>> windows-guest with more than one cpu...
>
> Unrelated, what are your smp issues?
>
If i use one cpu i got a good io-performance:
e.g. over 500MB/s at the profile "install" of the io-benchmark h2benchw.exe.
( aio=threads | SAS-Raid-0 | ftp://ftp.heise.de/pub/ct/ctsi/h2benchw.zip 
| hwbenchw.exe -p -w iotest 0)
The same test but with two cpus gives results between 27 and 298 MB/s!

Also in real life it's noticeable not only with an benchmark. I use a 
win-vm with two cpu for postscript-ripping and have a performance drop 
due to the bad io.

Udo