FreeBSD 10-R, Xen 4.1 guest, pf/NAT performance question

seanrees at seanrees at
Mon Apr 7 14:50:45 UTC 2014

Thanks for the tip. Yes, I had already run into TSO4 causing issues with
this VM, so it was switched off (ifconfig xn0 -tso4).

I also set net.inet.tcp.tso=0 (was 1) and tried again - no change. :(


On Mon, Apr 7, 2014 at 3:10 PM, Mark Felder <feld at> wrote:

> On 2014-04-07 07:57, seanrees at wrote:
>> Hi there freebsd-questions,
>> I've been batting my head against this problem for a few days now and not
>> having much progress, so I'm hoping to get pointers at what to look at
>> next.
>> I've got a FreeBSD 10-R guest in Xen 4.1 (I am just a customer of the Xen
>> provider; I don't run the Xen hypervisor myself). I use this instance to
>> terminate a VPN, for which I also NAT VPN clients with PF. I am seeing
>> unusually slow packet forwarding performance: 0.5mbit internet -> vpn
>> client, 2.0 mbit vpn client -> internet. (the numbers should be closer to
>> 10mbit/5mbit).
>> This guest is a duplicate of another Xen instance I have in another data
>> centre. I manage the configurations and packages centrally and aside from
>> IP address differences, the machines are configured identically. The
>> differences: it's 30ms closer to me and runs in Xen 3.4. I see performance
>> from this machine in the 10mbps range.
>> I've eliminated the obvious:
>>   - The problem VPS is fine network wise; can download tarballs from the
>> Internet at 100mbps.
>>   - VPS -> Home is fine; can download at ~10mbps; the problem is isolated
>> to forwarding Home -> VPS -> Internet and back.
>>   - I excluded OpenVPN as the cause by replicating the setup with ssh -w;
>> same performance.
>>   - SSH port forwarding (ssh -L) is fast; indicating to me the issue is
>> somewhere in the PF/kernel.
>>   - I checked TCP options by capturing traffic at varying points; these
>> seem fine. I see a good deal of TCP retransmits but the window sizes stay
>> the same.
>> Any thoughts on what to check next?
> Have you turned off TSO?
> ifconfig xn0 -tso
> or
> sysctl net.inet.tcp.tso=0
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