Check manual page of mem.win
Usage of main memory and page file
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 when 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 + swap) is really required, ignoring overlap and caches.
Commit Charge minus Memory Usage is approximately the amount
of the pagefile used for "relevant" data, but further optimizations
may lead to offsets (to the point where Commit Charge - Memory Usage
can be negative).
mem.win creates one check 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
being used instead.