[Xen-users] poor outbound network performance on DNATed domU

[Christoph Fritsche <chr.fr@xxxxxxx>]

Hi everyone,

I experience a weird network behavior. There is a poor network 
performance for a domU behind DNAT when delivering data from the domU to 
the outside world. The other way around is surprisingly not affected. 
I'm using bridged networking on xen 3.2.1 with almost stock Debian etch.
The domU behind DNAT delivers a http download with about 57.67 KB/s. For 
example another domU with ordinary bridged networking delivers the same 
file to the same client with 10.73 MB/s.
The strange thing is, that uploading to the nated domU is not affected. 
Sending the previously downloaded file back to the domU via ftp was done 
 with 10743.2 kB/s.
Dom0 and DomU log files do not contain any hints what might be the problem.

Does anyone have similar experiences or even an advice where to look at?

A detailed description of my setup follows.

network layout:
bridge name     bridge id               STP enabled     interfaces
eth0            8000.001a4df5dbf6       no              peth0
                                                        vif3.0
xenint          8000.2e53c90b0fb5       no              pdummy0
                                                        vif3.1
                                                        vif6.0

Eth0 is configured with a public IP, xenint a private one (172.31.0.1). 
Internal domUs are not directly attached to eth0 and use dom0 as gateway.
In iptables OUTPUT and FORWARD policy is ACCEPT and NAT rules are:
$IPTABLES -t nat -A PREROUTING -i eth0 -d $INET_IP -p TCP --dport 21 -j 
DNAT --to-destination 172.31.0.4
$IPTABLES -t nat -A PREROUTING -i eth0 -d $INET_IP -p TCP --dport 81 -j 
DNAT --to-destination 172.31.0.4:80
$IPTABLES -t nat -A POSTROUTING -o eth0 -s 172.31.0.0/24 -j MASQUERADE

system:
Debian etch amd64 with self compiled 2.6.18 debian + xen patched kernel
linux-tree-2.6.18               2.6.18.dfsg.1-18etch1
libxenstore3.0                  3.2.0-3~bpo4+2
xen-hypervisor-3.2-1-amd64      3.2.0-3~bpo4+2
xen-shell                       1.8-3~bpo40+1
xen-tools                       3.9-2~bpo40+1
xen-utils-3.2-1                 3.2.0-3~bpo4+2
xen-utils-common                3.2.0-1~bpo4+1
xenstore-utils                  3.2.0-3~bpo4+2

Whenever it looks like an kernel issue, I will send my .configs in.

Thanks in advance,
Christoph

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