I have a Ubuntu server with 8GB memory and it's using less than 2GB.
I have a PHP app that takes 16 seconds to load a page.
I have a large database.
I noticed that if I did a database dump the disk IO is very high.
I changed query_cache_size = 916777216
That should be 1GB of memory for MySQL.
I did a database dump again and it read it all from the disk.
There is still only less than 2GB of memory used.
I think the key to optimizing this app is to cache most queries in memory. What's the next step.