On Fri, Nov 26, 2004 at 04:31:21PM +0100, Marco Mellia wrote:

The point is:
while a PCI-X linux or (or click) box can receive (receive just up to
the netif_receive_skb() level and then discard the skb) up to more than
wire speed using off-the-shelf gigabit ethernet hardware, there is no
way to transmit more than about half that speed. This is true
considering minimum sized ethernet frames.

That's more-or-less what I'm seeing.

Theoretically, the maximum #pps you can send on gigabit is p=125000000/(s+24)
where s is the packet size, and the constant 24 consists of the 8B preamble,
4B FCS and and 12B inter-frame gap.

On an e1000 in a 32b 66MHz PCI slot (Intel server mainboard, e1000 'desktop'
NIC) I'm seeing that exact curve for packet sizes > ~350 bytes, but for
smaller packets than that, the curve goes like p=264000000/(s+335)  (which
is accurate to +/- 100pps.)  The 2.64e8 component is exactly the theoretical
max. bandwidth of the PCI slot the card is in, the 335 a random constant
that accounts for latency.  On a different mobo I get a curve following
the same formula but different value for 335.

The same card in a 32b 33MHz PCI slot in a cheap Asus desktop board gives
something a bit stranger:
- p=132000000/(s+260) for s<128
- p=132000000/(s+390) for 128<=s<256
- p=132000000/(s+520) for 256<=s<384
- ...

Again, the 132000000 corresponds with the theoretical max. bandwidth of
the 32/33 bus.  I'm not all that sure yet why things show this behavior.


--L