Link: Boosting NGINX Performance 9x

Boosting NGINX Performance 9x with Thread Pools

[...] the thread pool is performing the functions of the delivery service. It consists of a task queue and a number of threads that handle the queue. When a worker process needs to do a potentially long operation, instead of processing the operation by itself it puts a task in the pool’s queue, from which it can be taken and processed by any free thread.

Source: nginx.com