Show the proportion of memory on the Hyper-V server consumed by all the virtual machines on the server over time.
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Ideally the proportion should be consistently reported around 80, definitely below 100 and no spikes above 100. Some highly tuned systems that are very dense may run higher than 80.

So long as the proportion stays under 100 there is enough available memory to service all virtual machines on the Hyper-V server. If the proportion exceeds 100 then virtual machine(s) are doing paging operations with the virtual machine itself and there will be performance degradation.