Looks really interesting. I assume this suggests that ClickHouse is going to gradually switch to using this library for network and I/O, thus addressing the main weakness (in my mind) of C++ thread-per-connection servers, which is, they (surprisingly!) create too many threads and can't really handle more than, say, a thousand active connections at the same time. It mostly matters for async INSERTs in this case of course, not for SELECTs, although generally it applies to both.
I’ve been working on one for C: https://github.com/xtellect/cactus
It’s narrower than Silk/SeaStar: continuation stealing for CPU-bound recursive code, not a general async I/O fiber runtime.
[0] https://github.com/scylladb/seastar