[j-nsp] LAG/ECMP hash performance

[Saku Ytti]

On Thu, 29 Aug 2019 at 23:10, <adamv0025 at netconsultings.com> wrote:

> All this time watching the thread I'm thinking if it's just coincidence that
> in some cases no matter how good the hash value is the available buckets
> skew the balancing. (and I guess that's why three are knobs to shift the
> hash around.

Obviously there is no perfect solution, but alarm bells should ring if
you notice pattern when there is imbalance, such as closely grouped
flows. With ideal diffusion the pattern would be random any 5tuple is
equally likely to collide with any other 5tuple. With CRC it seems
some 5tuples are far more likely to collide with some other 5tuples,
and closely group, high volume traffic naturally occurs in very
typical network scenarios. Like CDN network internal traffic or SP VOD
distribution etc.
Bear in mind solution NOK owner found for his problem, adding
completely static set of input bits! Which does support (or add
confirmation bias at least:) to the explanation that the hash has poor
diffusion quality.

-- 
  ++ytti