It's a single-threaded process running a single core.

Don't know how much it is still true, but HN was originally implemented in arc.

The language homepage[1] says "Arc is unfinished. It's missing things you'd need to solve some types of problems. [...] The first priority right now is the core language."

Perhaps parallelism is still pending. A Ctrl-F on the tutorial doesn't turn up any hits for "process", "thread", "parallel", or "concurrency".

[1]: http://www.arclanguage.org

It's pretty easy if you're keeping everything in RAM and don't have layers upon layers of frameworks. You've got about 3B cycles per second to play with, with a register reference taking ~ 1 cycle, L1 cache (48K) = 4 cycles, L2 cache (512K) = 10 cycles, L3 cache (8M) = 40 cycles, and main memory = 100 cycles. This entire comment page is 129K, gzipping down to 19K; it fits entirely in L2 cache. It's likely that all content ever posted to HN would fit in 128G RAM (for reference, that's about 64 million typewriter pages). With about 30M random memory accesses being possible per second (or about 7 billion consecutive memory accesses - you gain a lot from caching), it's pretty reasonable to serve several thousand requests per second out of memory.

For another data point, I've got a single box processing every trade and order coming off the major cryptocurrency exchanges, roughly 3000 messages/second. And the webserver, DB persistence, and a bunch of price analytics also run on it. And it only hits about 30% CPU usage. (Ironically, the browser's CPU often does worse, because I'm using a third-party charting library that's graphing about 7000 points every second and isn't terribly well optimized for that.)

https://www.cryptolazza.com

Software gets slow because it has a lot of wasteful layers in between. Cut the layers out and you can do pretty incredible things on small amounts of hardware.

> Serious question, how is that possible?

You see, it is possible. Take a step back and look again to see the evidence: The site is called Hacker News, the first goal of this site was to prove that something useful can be created in an entirely custom programming language, and the site handles a huge amount of traffic, so it is a challenging task to let this run on a single core.

So the answer from a hacker's mind to why they let it run on a single core might simply be: Because they can.

On the other hand, YCombinator is a successful company, so buying a larger server would certainly be in their latitude. But that would be less intellectually appealing, and part of their success come from the fact that they decide as hackers, and don't always take the easiest path.