I tested with upstream Debian in my virtual machines: Squeeze has a server load of 7-10% (which seems high, but might be related to using a VM). When upgraded to Debian Wheezy the load goes up to 40% as in Ubuntu 12.04. When I boot the old 2.6 kernel from Squeeze, the load goes back to the original values.
On Ubuntu 12.04 I tried several share and mount options. The only change that showed an effect was mounting with -o proto=udp which reduced the load to around 15% which is still more than the old kernel, but much better than the 40% with tcp.
(end cite)
I'm rather surprised that not more people are running into this issues because it seems to be a show-stopper for Ubuntu NFS servers.
I have added logs of a test setup in Bug #1077612 and steps how to reproduce in Bug #1006446 (before noticing that was marked duplicate)
Citing from the summary in Bug #1077612:
I tested with upstream Debian in my virtual machines: Squeeze has a server load of 7-10% (which seems high, but might be related to using a VM). When upgraded to Debian Wheezy the load goes up to 40% as in Ubuntu 12.04. When I boot the old 2.6 kernel from Squeeze, the load goes back to the original values.
On Ubuntu 12.04 I tried several share and mount options. The only change that showed an effect was mounting with -o proto=udp which reduced the load to around 15% which is still more than the old kernel, but much better than the 40% with tcp.
(end cite)
I'm rather surprised that not more people are running into this issues because it seems to be a show-stopper for Ubuntu NFS servers.