Thread by @ErrataRob

BTW, the reason nginx has displaced Apache as the world's most used web server is because it's O(n) and Apache is O(n²).

https://twitter.com/stshank/status/1121206894809366528

What that means is the amount of work the server does for each arriving packet on nginx is roughly the same, regardless of the load. However, as load increases, the amount of work per packet Apache does increases.

Said another way, if you double the load on nginx, it doubles the amount of CPU needed, which is of course what you'd expect. However, if you double the load on Apache, it more than doubles the amount of CPU required.

This is the difference between "scalable" vs. "performance". With mere performance measurements at light load, you might see that Apache responds faster than nginx. But what you care is that at heavy load, Apache response times skyrocket while nginx stays roughly the same.

The underlying problem was recognized 20 years ago, but Apache has endured this long because of legacy and inertia.