Flow ID, LACP, and igb

[Alan Somers]

On Mon, Aug 26, 2013 at 11:18 AM, Justin T. Gibbs <gibbs at freebsd.org> wrote:

> 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.
>
It's actually worse than this.  If two inbound TCP packets get sent to the
same queue on different igb ports, then they will have the same flowid.
That could happen even if the switch is distributing packets just fine.

>
> 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.
>
 It also affects outbound TCP packets for streams that originated on the
host.  For example, it affects tcp-mounted NFS clients.

>
> Are there other checksums we should be looking at in addition to FNV?
>
s/checksums/hashes/

>
> Thanks,
> Justin
>
>