Re: [Bugme-new] [Bug 11316] New: severe performance regression for iptables nat routing

David Miller wrote:

From: Alex Williamson <redacted>
Date: Wed, 13 Aug 2008 20:08:20 -0600

quoted

git bisect traced the problem back to this changeset:

        commit e5a4a72d4f88f4389e9340d383ca67031d1b8536
        Author: Lennert Buytenhek [off-list ref]
        Date:   Sun Aug 3 01:23:10 2008 -0700
        
            net: use software GSO for SG+CSUM capable netdevices

I've verified that I can toggle the slowness by reverting this patch on
top of 8d0968ab (current head).  The problem is readily reproducible
using Ubuntu Hardy in a KVM VM with upstream, defconfig kernel.

Patrick I wonder if there a case where iptables NAT will COW the packet
when it really doesn't need to.

I don't think so, its using skb_make_writable everywhere, which checks
for skb_clone_writable, which should usually avoid COWing local TCP
packets. It would also be unlikely to have that much of a performance
impact (1MB/s -> 34kb/s).

It seems, if anything, using GSO should make things go a little bit
faster not slower... Hmmm...

Alex, could you post a tcpdump from both loopback and the outgoing
device from the machine you're doing NAT on?