Daniel Lezcano [off-list ref] writes:

3. General observations
-----------------------

The objective to have no performances degrations, when the network
namespace is off in the kernel, is reached in both solutions.

When the network is used outside the container and the network
namespace are compiled in, there is no performance degradations.

Eric's patchset allows to move network devices between namespaces and
this is clearly a good feature, missing in the Dmitry's patchset. This
feature helps us to see that the network namespace code does not add
overhead when using directly the physical network device into the
container.

Assuming these results are not contradicted this says that the extra
dereference where we need it does not add measurable to the overhead
in the Linus network stack.  Performance wise this should be good
enough to allow merging the code into the linux kernel, as it does
not measurably affect networking when we do not have multiple
containers in use.

Things are good enough that we can even consider not providing
an option to compile the support out.

The loss of performances is very noticeable inside the container and
seems to be directly related to the usage of the pair device and the
specific network configuration needed for the container. When the
packets are sent by the container, the mac address is for the pair
device but the IP address is not owned by the host. That directly
implies to have the host to act as a router and the packets to be
forwarded. That adds a lot of overhead.

Well it adds measurable overhead.

A hack has been made in the ip_forward function to avoid useless
skb_cow when using the pair device/tunnel device and the overhead
is reduced by the half.

To be fully satisfactory how we get the packets to the namespace
still appears to need work.

We have overhead in routing.  That may simply be the cost of
performing routing or there may be some optimizations opportunities
there.

We have about the same overhead when performing bridging which I
actually find more surprising, as the bridging code should involve
less packet handling.

Ideally we can optimize the bridge code or something equivalent to
it so that we can take one look at the destination mac address and
know which network namespace we should be in.  Potentially moving this
work to hardware when the hardware supports multiple queues.

If we can get the overhead out of the routing code that would be
tremendous.  However I think it may be more realistic to get the
overhead out of the ethernet bridging code where we know we don't need
to modify the packet.

Eric