RE: [Xen-users] Slow TCP performance between Windows Vista andXenPV-on-HVM guest

["Robert Dunkley" <Robert@xxxxxxxxx>]

I have always had to disable LSO in my setups (Although the other
acceleration features work fine). Could it possibly be a NIC driver
issue in Dom0?


Rob

-----Original Message-----
From: xen-users-bounces@xxxxxxxxxxxxxxxxxxx
[mailto:xen-users-bounces@xxxxxxxxxxxxxxxxxxx] On Behalf Of Fischer,
Anna
Sent: 12 June 2010 04:40
To: James Harper; xen-users@xxxxxxxxxxxxxxxxxxx
Subject: RE: [Xen-users] Slow TCP performance between Windows Vista
andXenPV-on-HVM guest

> Subject: RE: [Xen-users] Slow TCP performance between Windows Vista
and
> XenPV-on-HVM guest
> 
> > > Are you capturing packets on the windows machine or on the Dom0?
> >
> > Dom0. Note that the Windows machine doesn't even run Xen or
anything,
> it is
> > just some random machine on the network. Only the Linux guest runs
on
> Xen.
> 
> Yes, I'd figured that.
> 
> >
> >
> > > If you are using tcpdump on dom0, make sure you use '-s0' so that
> you
> > > capture the entire packup, and possibly '-v' as well. Without
> capturing
> > > the entire packet, tcpdump can't tell you if the checksum is
correct
> or
> > > not. Even if the checksum is incorrect on Dom0 it doesn't
> necessarily
> > > tell you that there is a problem though. A bad checksum on
received
> > > packets on the windows machine would definitely suggest a problem
> > > though.
> >
> > I capture with Ethereal. I definitely catch all packet. If this was
a
> checksum
> > problem, then communication wouldn't work at all. However, SSH and
> other
> > (slower) connections work just fine. The problem is only on bulk
data
> transfer
> > using TCP. If the Linux guest was sending a packet with an invalid
> checksum,
> > then the Windows guest would *never* send out the ACK. However, it
is
> actually
> > sending out the ACK, but only after the retransmit, to ACK the
> *retransmitted*
> > packet. If this was a checksum problem, then the retransmitted
packet
> would
> > also have an invalid checksum and so it would basically never be
> ACKed.
> >
> > I have read about Vista's TCP "auto-tuning" feature, and I wonder if
> something
> > like this might be the problem here that the Xen guest cannot cope
> with?
> >
> 
> It might then be a 'large send' problem. 

Yes, my guess was that it must be something like this.

> That would manifest itself as
> low volume traffic being mostly okay, but as the throughput increased,
> >MTU sized packets would be sent from DomU via Dom0, with the intent
> that the hardware will split them up into <=MTU sized. If those were
> dropped somewhere then the retransmit would happen, and the retransmit
> would typically not use the 'large' packet, so it would probably work.

Is that so? I don't know much about the TCP implementation, but would it
disable offloading for a retransmit?


> tcpdump should show >1500 byte packets in Dom0 on the vif interface
> belonging to the DomU, and in the DomU if this is happening.

No, I only see < 1500. I capture on the VIF and on the physical device
in Dom0.


> Use ethtool in DomU to disable as many offload features as possible
and
> see if things improve.

Hardware offload is disabled on the NIC inside the Linux guest, on the
VIF in Dom0 and also on the NIC in Dom0. All offload features, including
checksum offload. My guess was also that this must be the problem, as I
said before it actually works with exactly the same guest running on
VMWare. But obviously on VMWare it doesn't run the Xen netfront/netback
drivers, so my guess was that some configuration on there might be the
issue. But as I said, switching off hardware offload does not make any
difference at all. At the moment it does not run any HW offloading.

Thanks,
Anna

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