How to optimize server ?

I have 2 point relate to server.
1. task run very slow, especially Find neighbors of items. my server have 8cpu 4 core, 16g RAM.
2. how to optimize cache of redis ? i just run testing with about 50k items, but 16G ram redis it not enought. while in reality i have over 5 milion items. and over 1000 category.
3. Simply calculate, 50k items => 16G RAM, 500k item => 160G RAM, 5000k => 1600G RAM ? OMG, terrible. i was change database type for cache, but it very very slow.

Thanks.
Would be very grateful if there is any solution to help me improve