This sounded promising but then I saw a performance benchmark [1] that was done on a pentium(!) with a very suboptimal hash table design compared to [2] and [3].
The Judy site itself [4] is full of phrases like "may be out of date", when the entire site was last updated in 2004. If it was already out of date in 2004...
It seems that Judy arrays are just kind of a forgotten datastructure, and it doesn't look promising enough for me to put in effort to revive it. Maybe someone else will?
Judy arrays are (under the covers) a kind of radix tree, so they are comparable to things like ART (adaptive radix tree), HAMT (hashed array-mapped trie), or my qp-trie. Judy arrays had some weird issues with patents and lack of source availability in the early days. I think they are somewhat overcomplicated for what they do.
Regarding cache-friendiness, a neat thing I discovered when implementing my qp-trie is that it worked amazingly well with explicit prefetching. The received wisdom is that prefetching is worthless in linked data structures, but the structure of a qp-trie makes it possible to overlap a memory fetch and calculating the next child, with great speed benefits. Newer CPUs are clever enough to work this out for themselves so the explicit prefetch is less necessary than it was. https://dotat.at/prog/qp/blog-2015-10-11.html
I guess I partly got the idea for prefetching from some of the claims about how Judy arrays work, but I read about them way back in the 1990s at least 15 years before I came up with my qp-trie, and I don't know if Judy arrays actually use or benefit from prefetch.
The Judy site itself [4] is full of phrases like "may be out of date", when the entire site was last updated in 2004. If it was already out of date in 2004...
It seems that Judy arrays are just kind of a forgotten datastructure, and it doesn't look promising enough for me to put in effort to revive it. Maybe someone else will?
[1] http://www.nothings.org/computer/judy/
[2] https://probablydance.com/2017/02/26/i-wrote-the-fastest-has...
[3] https://probablydance.com/2018/05/28/a-new-fast-hash-table-i...
[4] http://judy.sourceforge.net/