Comment by throwaway17_17 | Hacker News Reader

Comment by throwaway17_17 | Hacker News Reader

Parent Full thread

I am pretty okay with the code (I'm essentially talking about the usage syntax for the library and its type) shown in the examples. However, at this point any parallel computing implementations must address the baseline issues presented in "Scalability! But at what COST? (McSherry,Isard,Murray 2015)" a paper whose central question is can a parallel computation exhibit a Configuration that Outperforms a Single Thread (the COST in the title). [1] There is a good discussion of the paper and its applicability to parallel (and distributed) computation implementations in Richard Feldman's 2024 Distributed Systems talk "Distributed Pure Functions". [2]

At this point in the life-cycle of the concept of parallel computation, I think it has become somewhat imperative that devs in the area begin to honestly evaluate the practicality and benefits/drawbacks of using the techniques for a given application area and attempt to 'sell' their libraries, techniques, idioms, etc using a more transparent approach. Also, I generally think that people that argue for more prevalence of parallel code, especially those arguing for the default being parallel (or concurrent), have to wrestle with and address these same issues.

Again, I don't dislike the premise of the library, think the usage examples seem very sensible and well designed, and I really like parallel computation as an area of study in general. Further, I really think that setting out a task for one's self

'to try building a library using modern features from the C++ standard library. I’ve used coroutines for task encapsulation and C++23 expected for exception handling, while trying to maintain good performance.'

after taking inspiration from two well respected and frequently utilized libraries in the space is great and the internals of the library I saw look clean and well architected.

1 - https://www.usenix.org/system/files/conference/hotos15/hotos... 2 - https://youtu.be/ztY1YRiaSiE?si=npBREw9vdF5dHcJh&t=350

I think you are misapplying that paper? This as a library is the "batteries" to C++'s no-batteries-included standard library which does not implement asynchronous coroutines at all.

The paper is much more on the side of application and system performance. But you couldn't even write such a system without a library like this providing you the tools to do so. This is much more in the domain of "basic tool for ecosystem" than "library for specific tasks". It's on the user of the tool to address the paper's question, not the builder of the tools.

You are not incorrect in stating that the primary focus of the paper is more on the application side. However, I think providers of a parallel computation infrastructure would benefit from profiling a wide range of potential use cases across several work load sizes. This could then lead to a section in a README where the baseline overhead was broken down per workload/worksize measurements and a back of the envelope estimate by an application developer would be more particularly motivated when deciding which infrastructure tool may be the best fit for their application's specific requirements.

"Concurrency != parallelism" is an important distinction in this context. The base C++ coroutines feature is not about threads or parallel processing, but rather is a generalization of the concept of "subroutine" with respect to control flow and stack usage. An example using coroutines to service many tasks (if not otherwise involving threading features) is not much different (at the level of what the CPU sees) from a single threaded implementation using a loop or continuation passing to process concurrent tasks. Performance should be identical between both the language-supported coroutines code and the manually implemented single threaded loop if the work is batched the same.

I would tend to agree with your open assertion that Concurrency != Parallelism. However, I'm not sure that it is really germane in this situation. I am aware that library in question here uses C++ coroutines for 'task encapsulation' according to the developer; however, this library is being compared directly to TBB and OpenMP which are two of the 'go-to' implementations for parallel computation. So I don't think the focus on parallelism in this comment chain is in any way inaccurate or misapplied.

Thank you for your thoughtful feedback.

I've just skimmed through the paper, and it raises interesting and valid point about scalability in parallel computing. I'll definitely look into it more thoroughly, as well as the talk you mentioned.

I'm glad you find the usage examples well-designed and appreciate your positive remarks about the library's architecture. Thank you again for your insights.

Reply on news.ycombinator.com