[Python-Dev] General concerns about C API changes

Raymond Hettinger raymond.hettinger at gmail.com
Tue Nov 13 22:06:35 EST 2018

Overall, I support the efforts to improve the C API, but over the last few weeks have become worried.  I don't want to hold up progress with fear, uncertainty, and doubt.  Yet, I would like to be more comfortable that we're all aware of what is occurring and what are the potential benefits and risks.

* Inline functions are great.  They provide true local variables, better separation of concerns, are far less kludgy than text based macro substitution, and will typically generate the same code as the equivalent macro.  This is good tech when used with in a single source file where it has predictable results.   

However, I'm not at all confident about moving these into header files which are included in multiple target .c files which need be compiled into separate .o files and linked to other existing libraries.

With a macro, I know for sure that the substitution is taking place.  This happens at all levels of optimization and in a debug mode.  The effects are 100% predictable and have a well-established track record in our mature battle-tested code base.  With cross module function calls, I'm less confident about what is happening, partly because compilers are free to ignore inline directives and partly because the semantics of inlining are less clear when the crossing module boundaries.

* Other categories of changes that we make tend to have only a shallow reach.  However, these C API changes will likely touch every C extension that has ever been written, some of which is highly tuned but not actively re-examined.  If any mistakes are make, they will likely be pervasive.  Accordingly, caution is warranted.

My expectation was that the changes would be conducted in experimental branches. But extensive changes are already being made (or about to be made) on the 3.8 master. If a year from now, we decide that the changes were destabilizing or that the promised benefits didn't materialize, they will be difficult to undo because there are so many of them and because they will be interleaved with other changes.

The original motivation was to achieve a 2x speedup in return for significantly churning the C API. However, the current rearranging of the include files and macro-to-inline-function changes only give us churn.  At the very best, they will be performance neutral.  At worst, formerly cheap macro calls will become expensive in places that we haven't thought to run timings on.  Given that compilers don't have to honor an inline directive, we can't really know for sure -- perhaps today it works out fine, and perhaps tomorrow the compilers opt for a different behavior.

Maybe everything that is going on is fine.  Maybe it's not. I am not expert enough to know for sure, but we should be careful before green-lighting such an extensive series of changes directly to master.  Reasonable questions to ask are: 1) What are the risks to third party modules, 2) Do we really know that the macro-to-inline-function transformations are semantically neutral. 3) If there is no performance benefit (none has been seen so far, nor is any promised in the pending PRs), is it worth it?  

We do know that PyPy folks have had their share of issues with the C API, but I'm not sure that we can make any of this go away without changing the foundations of the whole ecosystem.  It is inconvenient for a full GC environment to interact with the API for a reference counted environment -- I don't think we can make this challenge go away without giving up reference counting.  It is inconvenient for a system that manifests objects on demand to interact with an API that assumes that objects have identity and never more once they are created -- I don't think we can make this go away either.  It is inconvenient to a system that uses unboxed data to interact with our API where everything is an object that includes a type pointer and reference count -- We have provided an API for boxing and boxing, but the trip back-and-forth is inconveniently expensive -- I don't think we can make that go away either because too much of the ecosystem depends on that API.  There are some things that can be mitigated such as challenges with borrowed references but that doesn't seem to have been the focus on any of the PRs.

In short, I'm somewhat concerned about the extensive changes that are occurring.  I do know they will touch substantially every C module in the entire ecosystem.  I don't know whether they are safe or whether they will give any real benefit.

FWIW, none of this is a criticism of the work being done.  Someone needs to think deeply about the C API or else progress will never be made.  That said, it is a high risk project with many PRs going directly into master, so it does warrant having buy in that the churn isn't destabilizing and will actually produce a benefit that is worth it.


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