I found https://kb.paessler.com/en/topic/9973-how-to-correctly-monitor-linux-real-memory-usage
which looks like it would solve my problem if I was running PRTG on a linux box but my PRTG is monitoring from Windows. In Linux, all available "free" memory is usually taken to buffer hard disk access. The result is, that the "free real memory" SNMP OID (22.214.171.124.4.1.2021.4.6.0) tells PRTG, that there is only some 10 or 20 MB available. Is there a way to implement this solution within PRTG that is monitoring from a windows to all my Linux servers?