RE: [PATCH v2 0/2] ethdev: add random item support

[Michael Baum <michaelba@nvidia.com>]

On  Monday, 11 September 2023 20:53 Stephen Hemminger wrote: 
> 
> On Mon, 11 Sep 2023 18:55:45 +0200
> Morten Brørup <mb@smartsharesystems.com> wrote:
> 
> > > From: Michael Baum [mailto:michaelba@nvidia.com]
> > > Sent: Monday, 11 September 2023 08.42
> > >
> > > Add support for matching random value using new
> > > "rte_flow_item_random" structure.
> >
> > I get it. It can be used for things like stochastic sampling.
> >
> > However, it doesn't provide support for e.g. 1/100 or 1/500.
> >
> > So here's a suggestion:
> >
> > Instead of "value" (which is irrelevant) & "mask" (which is what
> > really controls the probability), wouldn't it be better if "value"
> > was an inverse probability (and "mask" was irrelevant)? E.g.
> > value=500 means that there is a 1 of 500 probability of a match.
> >
> > Would this kind of random item better serve the purpose?
> >
> > Or is the random item, in its current form, also required for other
> > purposes?
> >
> 
> For netem (Linux kernel) needed to support expressing a percent in fixed point
> number. The solution was to express it as:
>     Percent = X / UINT32_MAX

I concur with the notion that numbers can be utilized for probability calculations. However, it's important to acknowledge that when employing numbers, we are essentially working with a superset of possibilities due to their inherent versatility.

To illustrate this, consider the scenario where we transmit a numerical value to an application and use that same value across different stages of the pipeline. Take, for instance, the following use case:

In the initial stage, there is a 20% probability of selecting a packet for sampling before proceeding with the pipeline.

Towards the end of the pipeline, if the packet was initially sampled, we sample it once more.

The challenge arises when attempting to replicate this process solely using a numerical value, as there is no guarantee that the same packet will be chosen both in the initial and final stages. Conversely, by duplicating the random number and using it as a reference for matching, we can effectively implement the aforementioned usage scenario.

I hope this clarifies the distinction and benefits of utilizing random numbers in our probability calculations.