Re: [RFC PATCH v1 0/5] Adjust wording for NUMA vs. socket ID in DPDK

[Bruce Richardson <bruce.richardson@intel.com>]

On Fri, Sep 06, 2024 at 03:02:53PM +0200, Morten Brørup wrote:
> > From: Burakov, Anatoly [mailto:anatoly.burakov@intel.com]
> > Sent: Friday, 6 September 2024 14.46
> > 
> > On 9/6/2024 2:37 PM, Morten Brørup wrote:
> > >> From: Anatoly Burakov [mailto:anatoly.burakov@intel.com]
> > >> Sent: Friday, 6 September 2024 13.47
> > >> To: dev@dpdk.org
> > >> Subject: [RFC PATCH v1 0/5] Adjust wording for NUMA vs. socket ID in DPDK
> > >>
> > >> While initially, DPDK has used the term "socket ID" to refer to physical
> > >> package
> > >> ID, the last time DPDK read "physical_package_id" for socket ID was ~9
> > years
> > >> ago, so it's been a while since we've actually switched over to using the
> > term
> > >> "socket" to mean "NUMA node".
> > >>
> > >> This wasn't a problem before, as most systems had one NUMA node per
> > physical
> > >> socket. However, in the last few years, more and more systems have multiple
> > >> NUMA
> > >> nodes per physical CPU socket. Since DPDK used NUMA nodes already, the
> > >> transition was pretty seamless, however now we're faced with a situation
> > when
> > >> most of our documentation still uses outdated terms, and our API is ripe
> > with
> > >> references to "sockets" when in actuality we mean "NUMA nodes". This could
> > be
> > >> a
> > >> source of confusion.
> > >>
> > >> While completely renaming all of our API's would be a huge effort, will
> > take a
> > >> long time and arguably wouldn't even be worth the API breakages (given that
> > >> this
> > >> mismatch between terminology and reality is implicitly understood by most
> > >> people
> > >> working on DPDK, and so this isn't so much of a problem in practice), we
> > can
> > >> do
> > >> some tweaks around the edges and at least document this unfortunate
> > reality.
> > >>
> > >> This patchset suggests the following changes:
> > >>
> > >> - Update rte_socket/rte_lcore documentation to refer to NUMA nodes rather
> > than
> > >> sockets - Rename internal structures' fields to better reflect this
> > intention
> > >> -
> > >> Rename --socket-mem/--socket-limit flags to refer to NUMA rather than
> > sockets
> > >> -
> > >> Add internal API to get physical package ID [1]
> > >>
> > >> The documentation is updated to refer to new EAL flags, but is otherwise
> > left
> > >> untouched, and instead the entry in "glossary" is amended to indicate that
> > >> when
> > >> DPDK documentation refers to "sockets", it actually means "NUMA ID's". As
> > next
> > >> steps, we could rename all API parameters to refer to NUMA ID rather than
> > >> socket
> > >> ID - this would not break neither API nor ABI, and instead would be a
> > >> documentation change in practice.
> > >>
> > >> [1] This could be used to group lcores by physical package, see e.g.
> > >> discussion
> > >>      under this patch:
> > >> https://patches.dpdk.org/project/dpdk/cover/20240827151014.201-1-
> > >> vipin.varghese@amd.com/
> > >
> > > Thank you for cleaning this up, Anatoly.
> > >
> > > I would prefer to take one more step and also rename functions and
> > parameters, e.g. rte_socket_id() -> rte_numa_id().
> > >
> > > For backwards compatibility, macros/functions with the old names can be
> > added.
> > >
> > 
> > I don't think we can do such changes without deprecation notices, but
> > it's a good candidate for next release.
> 
> Perhaps we can keep ABI compatibility by adding wrapper functions with the old names/parameters, which simply call the same functions with the new names/parameters.
> 
> The Devil is in the details, and I haven't looked deeply into this. So take with a grain of salt.
> 
> > 
> > I have thought about including parameter renames in this patchset, but
> > for now I decided against doing so. I can certainly include this in the
> > next revision if that's something community is willing to accept.
> 
> I agree with your decision on this. Renaming the parameters without renaming the functions could be confusing.
> 

I actually wonder if that is true. If we are simply renaming the parameters
without:
a) changing their types
b) changing the function behaviour
then it is neither an API nor an ABI break. If we were to do so, it would
be like changing a comment, since the actual parameter name is purely a
convenience to hint to the user what the value being passed actually does.

That only applies for function parameters though. For any defines or macros
that need renaming, then we are into API break territory and we would want
backward compatible versions of same.

/Bruce