"Btw, we rolled this out over 3 weeks ago and I think you're the first person to ask about it on HN. There was one earlier question by email. I think that qualifies as a splash-free dive."
It was running BC. I had high hopes for switching to CS because I'd heard the same thing you had, but when I tried it, HN slowed to a crawl. This stuff is so unpredictable.
Arc uses mutable cons, but Racket has immutable cons. So it's a problem.
In Racket BC mutable and immutable cons use the same struct at the C level, so both are quite fast and almost interchangeable, if you cross your fingers that the optimization passes don't notice the mess and get annoyed (somewhat like UB in C).
In Racket CS immutable cons are implemented as cons in Chez Scheme, but mutable cons are implemented as records in Chez Scheme, so they are not interchangeable at all.
Arc used a magic unsafe trick to mutate immutable(at the Racket level) cons that are actually mutable(at the Chez Scheme level) cons. The trick is slow because the Racket to Chez Scheme "transpiler" doesn't understand it and does not generate nice fast code.
One solution is to rewrite Arc to use mutable cons in Racket, but they are slow too because they are records in Chez Scheme that have less magic than cons in Chez Scheme. So my guess it that it will be a lot of work and little speed gain.
[Also, ¿kogir? asked a long time ago in the email list about how to use more memory in Racket BC, or how to use it better or something like that. I think he made a small patch for HN because it has some unusual requirements. Anyway, I'm not sure if it was still in use.]
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The takeaway is that mutable cons are slow in Racket.
I was the person who emailed him about it earlier.
2024-09-05, me:
On another topic, I just noticed that the 700+ comments on https://news.ycombinator.com/item?id=41445413 all render on a single page. Hurray! Is the pagination approach obsolete now? I know that you've commented several times about wanting to optimize the code so pagination wasn't necessary. I don't know if that's finished or if pagination will have to go on the next time there's a big breaking story.
Dan G:
Yes: the performance improvements I've been working on for years finally got deployed, so pagination is turned off for now.
(In case you're curious, the change is that Arc now runs over SBCL instead of Racket.)
...
Btw you're the only person I know of who's noticed this and pointed it out so far!
I have very mixed feelings about how much I know about this site.
"Btw, we rolled this out over 3 weeks ago and I think you're the first person to ask about it on HN. There was one earlier question by email. I think that qualifies as a splash-free dive."
I had no idea and I'm an HN addict!