Re: [Qemu-devel] [Bug 1066055] [NEW] Network performance regression with vde_switch

[Stefan Hajnoczi]

On Fri, Oct 12, 2012 at 05:34:23PM -0000, Edivaldo de Araujo Pereira wrote:
> I've noticed a significant network performance regression when using
> vde_switch, starting about one week ago (10/05/2012); before that date,
> I used to get about 1.5 Gbits host to guest, but now I can only get
> about 320 Mbits; I didn't find any modification in net/vde.*, just in
> hw/virtio*.
> 
> My command line: 
>  qemu-system-i386 -cdrom /bpd/bpd.iso -m 512 -boot d -enable-kvm \
>   -localtime -ctrl-grab -usbdevice tablet \
>   -device 
> virtio-net-pci,mac=52:54:00:18:01:01,netdev=vde0,tx=bh,ioeventfd=on,x-txburst=32
>  \
>   -netdev vde,id=vde0 -vga std -tb-size 2M -cpu host -clock unix
> 
> My host runs a kernel 3.6.1 and my guest runs a kernel 3.5.4; the same
> problem happens with other host and guest versions, too.
> 
> I know there are better ways of running a guest, but using vde I get a
> cleaner environment in the host (just one tun/tap interface to
> manage...), which is quite good when running some accademic experiments.
> 
> Interestingly, at the same time I've noticed a performance enhancement
> of about 25~30 % when using a tun/tap interface, bridged or not.

Hi Edivaldo,
It would be great if you can help find the commit that caused this
regression.

The basic process is:

1. Identify a QEMU release or git tree that gives you 1.5 Gbit/s.
2. Double-check that qemu.git/master suffers reduced performance.
3. git bisect start <bad> <good>
   where <bad> and <good> are the git commits that show differing
   performance (for example, bad=HEAD good=v1.1.0)

Then git will step through the commit history and ask you to test at
each step.  (This is a binary search so even finding regressions that
happened many commits ago requires few steps.)

You can read more about git-bisect(1) here:
http://git-scm.com/book/en/Git-Tools-Debugging-with-Git#Binary-Search
http://www.kernel.org/pub/software/scm/git/docs/git-bisect.html

The end result is the commit introduced the regression.  Please post
what you find!

Stefan