Rob Gubler wrote: > ... > I have a network of nodes all sending traffic to a central machine that is > running RTnet. All of the nodes involved are marching along on the same > TDMA schedule. The goal is to scale to ~ 100 nodes, on a 5 millisecond TDMA > slot schedule. In a (very) worst case scenario all nodes would transmit 2 > messages per 5 millisecond slot, where each message is 1500 bytes, directed > to the central machine. > > So that being said, > > packets/s = (2 messages) * (100 nodes) * (1000 milliseconds / 5 > milliseconds) = 40,000 > size of packets = 1500 > > That's approximately 487 Mbps > > Well, as far as latency requirements go.. The central machine needs to read > the packets, do some processing, and send a much smaller number of packets > back out, all in that TDMA slot.
Quite a heavy job for the central box. However, as you will use GBit Ethernet, you will also have a switched network. In that case you can and should coalesce the nodes timeslots so that packets arrive in bursts at the central machine, reducing its IRQ load. > > On a related note I was looking into the Intel 82575 driver and there is a > variable I can configure via modprobe that will rate limit the number of > interrupts per second that are triggered. That might be something > interesting for me to look at once I get it ported. That's also an option. Normally, you want reactive NICs and set this tunable to 1 IRQ/packet, but here is might be better to increase it on the central box. It should also become the master to avoid that it has to wait on TDMA Sync frame that may get delayed due to IRQ coalescing. Yet another option to handle these load scenarios would be a NAPI-like mode for RTnet, ie. polling for incoming packets. But that would require some smart API extension or it would collide with RTmac/TDMA and other reactive use cases. Jan
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