[dpdk-dev] [PATCH] ethdev: expose link status and speed using xstats

[Kyle Larose]

Hi Harry,

On Wed, Jan 20, 2016 at 9:45 AM, Van Haaren, Harry
<harry.van.haaren at intel.com> wrote:
> Hi Kyle,

>
> In theory we could create a new API for this, but I think the current xstats API is a good fit for exposing this info, so why create extra APIs? As a client of the DPDK API, I would prefer more statistics in a single API than have to research and implement two or more APIs to retrieve the information to monitor.
>

You create new APIs for many reasons: modularity, simplicitly within
the API, consistency, etc. My main concern with this proposed change
relates to consistency. Previously, each stat had similar semantics.
It was a number, representing the amount of times something had
occurred on a chip. This fact allows you to perform operations like
addition, subtraction/etc and expect that the result will be
meaningful for every value in the array.

For example, suppose I wrote a tool to give the "rate" for each of the
stats. We could sample these stats periodically, then output the
difference between the two samples divided by the time between samples
for each stat. A naive implementation, but quite simple.

However, if we start adding values like link speed and state, which
are not really numerical, or not monotonic, you can no longer apply
the same mathematical operations on them and expect them to be
meaningful. For example, suppose a link went down. The "rate" for that
stat would be -1. Does that really make sense? Anyone using this API
would need to explicitly filter out the non-stats, or risk nonsensical
output.

Let's also consider how to interpret the value. When I look at a stat,
there's usually one of two meanings: it's either a number of packets,
or it's a number of bytes. We're now adding exceptions to that rule.
Link state is a boolean. Link speed is a value in mbps. Duplex is
pretty much an enum.

We already have the rte_eth_link_get function. Why not let users
continue to use that? It's well defined, it is simple, and it is
consistent.

Thanks,

Kyle