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RE: [Xen-users] Very bad network performance between domU and dom0

To: "Klaus Schmidinger" <Klaus.Schmidinger@xxxxxxxxxx>, xen-users@xxxxxxxxxxxxxxxxxxx
Subject: RE: [Xen-users] Very bad network performance between domU and dom0
From: "Petersson, Mats" <Mats.Petersson@xxxxxxx>
Date: Thu, 25 Jan 2007 14:30:28 +0100
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Thread-topic: [Xen-users] Very bad network performance between domU and dom0

> -----Original Message-----
> From: xen-users-bounces@xxxxxxxxxxxxxxxxxxx 
> [mailto:xen-users-bounces@xxxxxxxxxxxxxxxxxxx] On Behalf Of 
> Klaus Schmidinger
> Sent: 25 January 2007 13:19
> To: xen-users@xxxxxxxxxxxxxxxxxxx
> Subject: [Xen-users] Very bad network performance between 
> domU and dom0
> I have a PC with an Intel Core 2 Duo E6600 2,40GHz F1066 CPU
> that runs openSUSE 10.2. The VT feature in the BIOS is enabled.
> The XEN version is 3.0.3_11774.
> uname -a:
> Linux tiger #1 SMP Mon Nov 27 11:46:27 UTC 
> 2006 x86_64 x86_64 x86_64 GNU/Linux
> I have two virtual mahines, one running SUSE 8.2, the other 
> Windows XP.
> The Linux machine mounts the /home directory from an external server
> via NFS, and the Windows machine does the same via Samba. In 
> both cases
> access to the files on the external server is very slow.
> My measurements showed that the data transfer rate between the
> external server and my PC that runs the dom0 is about 90MB/s over
> a gigabit ethernet connection, which is perfectly ok.
> The same measurements between the dom0 and the Linux domU machine show
> dom0 -> domU   27.7 MB/s
> domU -> dom0    7.9 MB/s

I take it this is a HVM domain and not a para-virtual kernel?
> which is very bad considering that this is all within the same
> hardware and nothing has to go over any external network connection.
> ifconfig doesn't report any network errors on any of the interfaces.
> I searched the archive of this ML and came across several threads
> with similar problems, but couldn't find any real solution.
> This is a real showstopper for me, because if I can't have good
> access to the NFS/Samba mounted home directory, there's no point in
> using virtual machines at all. So any hints that could help to
> improve this are greatly appreciated.

How have you configured your DomU/Dom0 cpu partitioning? Having one
dedicated CPU running Dom0 and another running DomU will help a little
bit because the context switching between the two is a little bit better
- how much better is a question for debate. 

Bear in mind tho', that the main culprit here is the fact that you're
using a emulated network card. It's unlikely that you'll ever get HIGH
performance with this, because it's just too software intensive (several
thousand cycles for each intercept by Xen of any operation that goes to
the hardware). 

The best solution is to use the para-virtual drivers - these should (in
theory at least) give you about the same performance as the para-virtual
kernel does on Xen, because it's using the same method. 

For linux-2.6, those are available in <xen source
directory>/unmodified_drivers. The README file in that directoy tells
you how to build these drivers. It's easiest if you do this ON the

There are drivers for Windows as well, but those are not available in
source-form, but I believe XenExpress contains drivers for Windows if
you want to get those. 

> Klaus
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