ProxMox VM memory usage not matching VM reports

Summary

There are cases where in ProxMox the Summary for the VM in the ProxMox interface shows that the VM is using all (or almost all) of it's memory.  In many cases the ProxMox interface is reporting any memory the OS is using as buffer/cache as consumed memory. To get better integration/reporting of the VM to the ProxMox hypervisor you need to manually install the guest agent. 

Install the Agent

https://pve.proxmox.com/wiki/Qemu-guest-agent

Linux

Depending on the distribution you are using, just use the package manager (yum or apt) to add the package qemu-quest-agent

After the install is complete you need to POWER OFF the VM.  The daemon/service will not start with an enable or start nor will a reboot work.  You need to power off / power on the VM.  You also need to make an edit to the VM's options before you power off for the full effect. 

Windows

Windows is slightly more complicated.  You need the VirtIO drivers and that process is explained here: 

https://pve.proxmox.com/wiki/Windows_VirtIO_Drivers

You need an ISO that has the driver and the agent to install it on the system.

This has not been tested internally but it should likely be presumed that a power off / power on should be done here as well. 

Make a change to the VM before powering back on after installing the agent.

In the ProxMox interface select your VM and go to Options.  Select the QEMU Guest Agent option and click EDIT:

(By default it is disabled).  The edit screen will show:

Ensure that "Use QEMU Guest Agent" is checked and the "Run guest-trim after a disk move or VM Migration" is optional.