Jumbo packets and hardware checksumming

Jumbo packets are packets that carry more payload than the normal 1500 bytes. Even the definition of a jumbo packet is unclear; different people use different lengths.

For jumbo packets to work, the protocol stack, the drivers, and the network switches must all support jumbo packets:

If you can use jumbo packets with io-pkt, you can see substantial performance gains because more data can be moved per packet header processing overhead.

To configure a driver to operate with jumbo packets, do this (for example):

# ifconfig wm0 ip4csum tcp4csum udp4csum
# ifconfig wm0 mtu 8100
# ifconfig wm0

For maximum performance, we also turned on hardware packet checksumming (for both transmit and receive) and we've arbitrarily chosen a jumbo packet MTU of 8100 bytes. A little detail: io-pkt by default allocates 2 KB clusters for packet buffers. This works well for 1500 byte packets, but for example when an 8 KB jumbo packet is received, we end up with 4 linked clusters. We can improve performance by telling io-pkt (when we start it) that we're going to use jumbo packets, like this:

# io-pkt-v6-hc -d i82544 -p tcpip pagesize=8192,mclbytes=8192

If we pass the pagesize and mclbytes command-line options to the stack, we tell it to allocate contiguous 8 KB buffers (which may end up being two adjacent 4 KB pages, which works fine) for each 8 KB cluster to use for packet buffers. This reduces packet processing overhead, which improves throughput and reduces CPU utilization.