I have read that linux uses free memory for caching, to make system faster. However, both Nagios and Paessler PRTG monitoring system show me that my memory usage is critical. I could change Nagios mem_usage script to sum free and cached memory, but would that be correct information? I doubt that they misunderstood Linux memory usage. Lets say I have 8 GB RAM. 5 GB are used, 2 GB is cached, and I have 1 GB of free memory. Real available memory should be free+cached (3 GB)? If some new application would need additional 3 GB RAM, could it take everything from cache and free without using swap, or is there a minimum that should be in cache? Real example: $ cat /proc/meminfo MemTotal: 5984256 kB MemFree: 137052 kB Buffers: 140484 kB Cached: 3439616 kB SwapCached: 244 kB Active: 3148824 kB Inactive: 2341768 kB ... My monitoring tools show that I have 137 MB free RAM, however I have ~3,5 GB in Cache. Thanks!