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

[Venkateswararao Jujjuri (JV)]

On 9/30/2010 2:13 AM, Stefan Hajnoczi wrote:
> 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?

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