measuring CPU, RAM, disk, and network performance (ubuntu 18.04)

hi.

i've built a web-app (a CMS, http://seductiveapps.com) that runs nicely and which is based on a growing number of opensource, free-to-use-commercially, libraries and software stacks.

now i'd like to know when one of them is running into trouble, performance-wise

what i have is :
3 functional ubuntu computers behind 1 adsl line, 1 dev machine, 1 server with plenty of RAM, and 1 spare server with lubuntu and just 4G of RAM. All have SSDs of plentiful size.
plenty of PHP and…

measuring CPU, RAM, disk, and network performance (ubuntu 18.04)