If for some reason the memcached service is not running on the server, and you active it on the prestashop backend, it will not work, you will need to reboot the server for it to take effect. (maybe restarting the memcached service and apache will also do the trick).
What if you want to disable memcached in the server?
you can go to:
In our particular case, memcached offered some mysql cpu optimization, but at a high php-fpm cost, so at the end of the day, doing a couple of benchmarks using screaming frog, we noticed that the site was crawled twice as fast with memcached disabled.
Maybe its a case of bad configuration, we'll have to take a look at that later, but for the moment we'll leave it off.