e1000 performance issue in 4 simultaneous links

[Breno Leitao <leitao@linux.vnet.ibm.com>]

Hello, 

I've perceived that there is a performance issue when running netperf
against 4 e1000 links connected end-to-end to another machine with 4
e1000 interfaces. 

I have 2 4-port interfaces on my machine, but the test is just
considering 2 port for each interfaces card.

When I run netperf in just one interface, I get 940.95 * 10^6 bits/sec
of transfer rate. If I run 4 netperf against 4 different interfaces, I
get around 720 * 10^6 bits/sec.  

If I run the same test against 2 interfaces I get a 940 * 10^6 bits/sec
transfer rate also, and if I run it against 3 interfaces I get around
850 * 10^6 bits/sec performance. 

I got this results using the upstream netdev-2.6 branch kernel plus
David Miller's 7 NAPI patches set[1]. In the kernel 2.6.23.12 the result
is a bit worse, and the the transfer rate was around 600 * 10^6
bits/sec.

[1] http://marc.info/?l=linux-netdev&m=119977075917488&w=2

PS: I am not using a switch in the middle of interfaces (they are
end-to-end) and the connections are independents.

-- 
Breno Leitao <leitao@linux.vnet.ibm.com>