Most people who've spent a lot of time with Hyper-V have had experience of accidentally taking too much memory away from parent partition. This happens when they start too many virtual machines – and all of a sudden the performance and responsiveness of parent partition goes down significantly. The response from people who hit this's usually to stop last VM that they started, to reduce its memory, and then start it up again.
To help mitigate this issue, Microsoft provided a new registry entry that lets you override parent memory reserve with your own static memory reserve. This registry entry doesn't exist by default – but if you go to HKLM:\SOFTWARE\Microsoft\Windows NT\CurrentVersion\Virtualization registry key and create a new DWORD entry with a name of memoryreserve – you can then set value to the static amount of memory that you want to reserve for parent.
Note: if you set this value too low; VMs will be able to use too much memory and cause performance issues for you. Equally – the higher you set this the fewer VMs you can run.