From the website (https://s3fifo.com/), it claims that it needs no locking (backing scalability claims). This seems like an important part of their work too, unless I've missed some obvious trick that everyone uses. Naively, I would think that you can't update a hash table (to find the cache items efficiently?) and the queues at the same time without a lock. They surely aren't doing a linear search through the queue looking for a match