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Re: [Xen-users] slow network with gplpv drivers in vlan setup

To: James Harper <james.harper@xxxxxxxxxxxxxxxx>
Subject: Re: [Xen-users] slow network with gplpv drivers in vlan setup
From: Matthieu Patou <mat+Informatique.xen@xxxxxxxxx>
Date: Tue, 09 Feb 2010 11:37:43 +0300
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James,
Any chance for an explanation ? what scatter/gather is supposed to
bring ?

Large Send is probably what is causing you problems. Some network cards
support large send offload only for untagged packets.
Well that's a bit strange because on 2 identical dom0 servers configured the same way I had this behavior only on one dom0 (the really loaded server).
  Linux doesn't seem
to know this though so the result is just that it doesn't work. Try
turning off large send and see what happens.

Ok so you suggest to change the value of 61440 to something smaller (ie. 8192)

Turning off scatter gather will (almost completely) disable large send
also, because windows is then limited to a total packet length of 4096
bytes which can't be broken up unless the MSS is really small.
The question that stay is what is drawback of not having  large send ?

Matthieu.

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