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Re: [Xen-users] Bad TCP accept performance

To: Russ Purinton <rpurinton@xxxxxxxxxxxxxxxxxxxxxxx>
Subject: Re: [Xen-users] Bad TCP accept performance
From: Pasi Kärkkäinen <pasik@xxxxxx>
Date: Wed, 25 May 2011 21:07:44 +0300
Cc: Carl Byström <cgbystrom@xxxxxxxxx>, xen-users@xxxxxxxxxxxxxxxxxxx
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On Wed, May 25, 2011 at 11:49:21AM -0400, Russ Purinton wrote:
> I thought that even traffic still went thru the NIC to loop.  I 
> can't seem to find anything on the net supporting this one way or the other, 
> though I've seen multiple posts about pinging to test and verify 
> the NIC card and drivers working properly.  Not sure what this means.
> Also, if it doesn't pass to the NIC, the NIC then the question arises how the 
> IP and TCP checksums are being applied if offloading is enabled and it 
> doesn't pass thru the NIC.  
> I had thought that on a physical host, the loopback ping would hit the 
> network card.  I'm guessing with a virtual host, the loopback ping would only 
> hit the Virtual NIC but likely not the physical NIC.  If it goes to the 
> virtual NIC, then it would be passing thru the PV drivers to the QEMU layer 
> supporting the virtual networking right?
> The packets shouldn't be visible even from Dom0 because they should stay 
> within the vNIC.  Again, not finding any supporting documentation on the 
> 'net, one way or the other, so feel free to prove me wrong.

Just check "ifconfig -a" output. IP is on the 'lo' interface, not on 'ethX' interface.
lo-interface is provided by the loopback-driver.

Even if you don't have any nic drivers loaded, you still have 'lo' and pinging 
localhost works.
Try it. rmmod your nic driver and ping localhost.

-- Pasi

> Thanks
> -----Original Message-----
> From: xen-users-bounces@xxxxxxxxxxxxxxxxxxx 
> [mailto:xen-users-bounces@xxxxxxxxxxxxxxxxxxx] On Behalf Of Pasi Kärkkäinen
> Sent: Wednesday, May 25, 2011 10:08 AM
> To: Russ Purinton
> Cc: Carl Byström; xen-users@xxxxxxxxxxxxxxxxxxx
> Subject: Re: [Xen-users] Bad TCP accept performance
> Using localhost ( goes thru loopback device only, ie. interface 
> 'lo',
> so it does NOT go through NIC card or xen/PV drivers..

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