Flow ID, LACP, and igb

[Justin T. Gibbs]

Hi Net,

I'm an infrequent traveler through the networking code and would
appreciate some feedback on some proposed solutions to issues Spectra
has seen with outbound LACP traffic.

lacp_select_tx_port() uses the flow ID if it is available in the outbound
mbuf to select the outbound port.  The igb driver uses the msix queue of
the inbound packet to set a packet's flow ID.  This doesn't provide enough
bits of information to yield a high quality flow ID.  If, for example, the
switch controlling inbound packet distribution does a poor job, the outbound
packet distribution will also be poorly distributed.

The majority of the adapters supported by this driver will compute
the Toeplitz RSS hash.  Using this data seems to work quite well
in our tests (3 member LAGG group).  Is there any reason we shouldn't
use the RSS hash for flow ID?

We also tried disabling the use of flow ID and doing the hash directly in
the driver.  Unfortunately, the current hash is pretty weak.  It multiplies
by 33, which yield very poor distributions if you need to mod the result
by 3 (e.g. LAGG group with 3 members).  Alan modified the driver to use
the FNV hash, which is already in the kernel, and this yielded much better
results.  He is still benchmarking the impact of this change.  Assuming we
can get decent flow ID data, this should only impact outbound UDP, since the
stack doesn't provide a flow ID in this case.

Are there other checksums we should be looking at in addition to FNV?

Thanks,
Justin