Check manual page of mem.used


Usage of physical and virtual RAM

Distribution: official part of Check_MK
License: GPL
Supported Agents: Linux

This check measures the current usage of physical RAM and virtual memory used by processes. You can define a warning and critical level for the usage of virtual memory, not for the usage of RAM.

This is not a bug, it's a feature. In fact it is the only way to do it right (at least for Linux): What parts of a process currently reside in physical RAM and what parts are swapped out is not related in a direct way with the current memory usage.

Linux tends to swap out parts of processes even if RAM is available. It does this in situations where disk buffers (are assumed to) speed up the overall performance more than keeping rarely used parts of processes in RAM.

For example after a complete backup of your system you might experiance that your swap usage has increased while you have more RAM free then before. That is because Linux has taken RAM from processes in order to increase disk buffers.

So when defining a level to check against, the only value that is not affected by such internals of memory management is the total amount of virtual memory used up by processes (not by disk buffers).

Check_MK lets you define levels in percentage of the physically installed RAM or as absolute values in MB. The default levels are at 150% and 200%. That means that this check gets critical if the memory used by processes is twice the size of your RAM.

Hint: If you want to monitor swapping, you probably better measure major pagefaults. Please look at the check kernel.

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Discovery

mem.used creates one check on each host that provides the section <<>>. If that section also outputs information about the Windows page file, then the check <<>> is being used instead.