At this present moment, it is not advisable to run memcached other than for Distributed Polling.

LibreNMS can store SQL results in memcached to achieve performance advantages of in-memory value storage and removing work load for frequent queries off the MySQL backend.

To enable memcached in your install you need to have memcached installed and the PHP extension php5-memcached or php-memcached and add the following lines to your config.php:

$config['memcached']['enable']  = true;
$config['memcached']['host']    = "localhost";
$config['memcached']['port']    = 11211;

By default values are kept for 4 Minutes inside the memcached, you can adjust this retention time by modifying the $config['memcached']['ttl'] value to any desired amount of seconds.

This means that you can see what appears to be stale data for up to 4 minutes. If you edit an alert rule for example then those changes may not show immediately.

It's strongly discouraged to set this above 300 (5 Minutes) to avoid interference with the polling, discovery and alerting processes.

If you use the Distributed Poller, you can point this to the same memcached instance. However a local memcached will perform better in any case.

By default memcached on many distributions starts itself with 64 MB of memory for it to store data in. If you have lots of devices or look at graphs frequently, it might be worth it to expand memcached's footprint a bit. Generally this can be done in /etc/memcached.conf, replacing -m 64 with -m 512, or however many megs of memory you want to allocate for memcached. Then restart the memcached service.