In the article, swap is mentioned. Apparently either processing each request requires some amount of memory, or new processes are fired up for concurrent reques

In the article, swap is mentioned. Apparently either processing each request requires some amount of memory, or new processes are fired up for concurrent requests (the Apache model). So, what I've seen happen in similar circumstances is that once swapping starts, some of the processes run 10x slower, while new load keeps coming in and piling up, running the server even deeper into the swap. Soon you have a thousand processes, load-average >100, and you can't run anything in the terminal. As mentioned in other comments, a proper defense against this would be to have a front-line system that anticipates such a situation and actively manages it: puts outstanding requests in a queue (throttling the load) and then errors out on new requests if there's still too many of them coming in.