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Date: Thu, 30 Sep 2010 12:19:38 -0700
From: "Venkateswararao Jujjuri (JV)" <jvrao@linux.vnet.ibm.com>
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Subject: Re: [Qemu-devel] QEMU throughput is down with SMP
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To: Stefan Hajnoczi <stefanha@gmail.com>
Cc: Qemu-development List <qemu-devel@nongnu.org>

On 9/30/2010 2:13 AM, Stefan Hajnoczi wrote:
> On Thu, Sep 30, 2010 at 1:50 AM, Venkateswararao Jujjuri (JV)
> <jvrao@linux.vnet.ibm.com>  wrote:
>> Code: Mainline QEMU (git://git.qemu.org/qemu.git)
>> Machine: LS21 blade.
>> Disk: Local disk through VirtIO.
>> Did not select any cache option. Defaulting to writethrough.
>>
>> Command tested:
>> 3 parallel instances of : dd if=/dev/zero of=/pmnt/my_pw bs=4k count=100000
>>
>> QEMU with  smp=1
>> 19.3 MB/s + 19.2 MB/s + 18.6 MB/s = 57.1 MB/s
>>
>> QEMU with smp=4
>> 15.3 MB/s + 14.1 MB/s + 13.6 MB/s = 43.0 MB/s
>>
>> Is this expected?
>
> Did you configure with --enable-io-thread?

Yes I did.
>
> Also, try using dd oflag=direct to eliminate effects introduced by the
> guest page cache and really hit the disk.

With oflag=direct , I see no difference and the throughput is so slow and I 
would not
expect to see any difference.
It is 225 kb/s  for each thread either with smp=1 or with smp=4.

- JV

>
> Stefan
>