The Memory pane of Parallels Desktop Preferences allows you to adjust the maximum amount of physical memory (RAM) that the system will reserve for all virtual machines running on your Mac.
By default, the Automatically option is selected. In this case, the total amount of RAM to be allocated to all running virtual machines is automatically calculated by the system based on the following main factors:
The allocated amount of memory is shared among all running virtual machines. You can configure the amount of physical memory for a particular virtual machine on the General tab of Virtual Machine Configuration.
You can redefine the default behaviour and manually set the amount of memory to be reserved for all active virtual machines. To this effect, select the Manually option and specify the needed value by:
Locking Parallels Desktop Preferences
If you want to prevent Parallels Desktop Preferences from further unauthorized changes, click the Lock icon at the bottom of the window. The next time anybody wants to change the settings in any pane of Parallels Desktop Preferences, an administrator's password will be required.