VM Memory usage doesn't match OS Memory Usage

[TheVMinator]

I'm getting alerts from the Windows OS that memory is 90% utilized.  However the "usage %" in vROps and vCenter performance charts never goes above 50%.  There is no memory swapping happening either.   Why is the OS reporting it is out of RAM when vCenter thinks the VM is only using 55%?

[jasnyder]

Are you actually getting errors allocating memory (i.e. 'Out of Memory' errors) or is it an alert threshold getting triggered?  Try running this tool in the VM - https://docs.microsoft.com/en-us/sysinternals/downloads/rammap

RAMMap - Windows Sysinternals | Microsoft Docs

And see what results you get.

Also, can you report total GB usage for guest and host according to vROPS/vSphere?  It could be an active memory thing or something in the way vSphere is handling memory for the VM.