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Date: Thu, 30 Sep 2010 10:13:56 +0100
Message-ID: <AANLkTimDeFQcnC+0OAb0XgkYrSKNNmkUASRQJRifY_SL@mail.gmail.com>
Subject: Re: [Qemu-devel] QEMU throughput is down with SMP
From: Stefan Hajnoczi <stefanha@gmail.com>
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To: "Venkateswararao Jujjuri (JV)" <jvrao@linux.vnet.ibm.com>
Cc: Qemu-development List <qemu-devel@nongnu.org>

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=3D/dev/zero of=3D/pmnt/my_pw bs=3D4k coun=
t=3D100000
>
> QEMU with =A0smp=3D1
> 19.3 MB/s + 19.2 MB/s + 18.6 MB/s =3D 57.1 MB/s
>
> QEMU with smp=3D4
> 15.3 MB/s + 14.1 MB/s + 13.6 MB/s =3D 43.0 MB/s
>
> Is this expected?

Did you configure with --enable-io-thread?

Also, try using dd oflag=3Ddirect to eliminate effects introduced by the
guest page cache and really hit the disk.

Stefan