Re: [Qemu-devel] QEMU throughput is down with SMP

[Stefan Hajnoczi]

On Thu, Sep 30, 2010 at 1:50 AM, Venkateswararao Jujjuri (JV)
<address@hidden> 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?

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

Stefan