10G and socket alloc failure

While testing out the ixgbe driver we've observed this failure in the stack
code, here is the info:

The test engineer is using iperf, typically with 16 threads. If the
driver is using
either legacy or MSI interrupts we will see broken pipes, in dmesg its
due to sonewconn() failing in syncache_socket().

Whats interesting is that when I have multiple RX queues configured and
using MSI/X this doesnt happen, at least not very often.

It does not seem to hurt performance horribly, iperf just spawns another
thread, but I was wondering if there is some underlying tuneable I dont
know about that would stop this from happening??

And any theory about why it doesnt happen with multiple queues?

Jack