|Distribution:||official part of Check_MK|
This check measures the current usage of physical RAM and
of the commit charge.
The "commit charge" was previously confusingly named "pagefile".
Please note that Microsoft themselves use this naming in
Due to how Windows optimizes memory management, physical memory
is in practice almost always fully utilized. The Memory usage this
check displays says how much memory is actually required by processes,
in contrast to caches that could be freed if the need arises.
In the same vein Windows may hold data in RAM and pagefile at the same
time or it may hold data in the pagefile even if there is RAM available,
when it determines this may improve performance.
As a result, the real pagefile usage is not only hard to
determine, it's also utterly useless when we want to determine
resource usage on the system.
The commit charge instead tells us how much of the memory used in
total (RAM + page file) is really required, ignoring overlap and caches.
Commit Charge minus used RAM is approximately the amount
of the pagefile used for "relevant" data, but further optimizations
may lead to offsets (to the point where Commit Charge - RAM can be negative).
The page file metric is determined using the fields 'ullTotalPageFile'
(current committed memory limit for the system or the current
process, whichever is smaller) and 'ullAvailPageFile' (maximum
amount of memory the current process can commit) of the MEMORYSTATUSEX
structure as returned by the windows API function 'GlobalMemoryStatusEx'.
One service is created for each host that provides data about the page file.
If that information is missing, a Unix-like system is assumed and
the check mem.used is creating one service instead.