Re: [Xen-devel] huge tcp performance-regression on pvops - kernel (+Solution)

[Keir Fraser <keir.fraser@xxxxxxxxxxxxx>]

Cc¹ing Jeremy. I think the patch of interest in linux-2.6.18-xen is 776,
which is the one that removes those lines from the front/back drivers in
favour of a solution integrated into the protocol stack.

 -- Keir

On 26/10/2009 23:37, "Ronny.Hegewald@xxxxxxxxx" <Ronny.Hegewald@xxxxxxxxx>
wrote:

> Setup: pvops dom0 kernel 2.6.31.4 from Jeremys git-repository from 2009-10-18
> 
> 
> Problem: Very slow network-performance (3-10 kbs) when tcp-packets are used
> (noticed when using scp, samba, nfs over tcp)
> 
> 
> 
> But this occurs only in the following situations:
> 
>     
> 
> 1.) domU to domU on same PC (domUs are paravirtualized linux-kernels)
> 
> 2.)  domU to another PC that doesn't use pvops-kernel (packets sent from
> another PC to domU works fine)
> 
> domU to dom0 and the opposite way works without performance-regression.
> 
> 
> Reason: bigger tcp-packets get dropped from the domU the tcp-packets are sent
> from (netstat -s in domU shows many retransmitted tcp-segments)
> 
> 
> 
> tcpdump shows that the bigger packets leave the vif from the domU they were
> sent from, but never arrive the vif from the domU they are sent to.
> 
> 
> This is caused by this lines in drivers/xen/netback.c at line 1325 :
> 
> 
> 
>                 if (skb->data_len < skb_shinfo(skb)->gso_size) {
> 
>                          skb_shinfo(skb)->gso_size = 0;
>                          skb_shinfo(skb)->gso_type = 0;
>                  }
> 
> 
> These lines were reverted from the linux-2.6.18-xen mercurial repository
> 
> 
> 
> on 2009-01-13 in changeset 774: 107e10e0e07c: netfront/back: do not mark
> packets of length < MSS as GSO
> <http://xenbits.xensource.com/linux-2.6.18-xen.hg?rev/107e10e0e07c>
> 
> 
> 
> I used the patch on the above mentioned pvops tree and the problem was gone.
> 
> 
> 
> I never noticed such problems on the 2.6.18-xen kernel or the forward-ported
> xen-kernel 2.6.31.4 (from Andrew Lyon)
> 



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