When the host is set up, a percentage of system memory is reserved for the hypervisor, and all remaining memory is automatically allocated to Domain0.
A better solution is to set a minimum and maximum amount of memory for domain 0, so the memory can be allocated appropriately to the hypervisor. An appropriate minimum amount would at least be 512 MB.
On 64bit machine with more than 480 GB RAM Setting a maximum amount needs to be set, otherwise the machine may refuse to boot. Set the value at or below the boundary of 480 GB—the difference is reserved for the hypervisor (to be used for guest VMs).
Determine the amount of memory to set for domain 0.
At Domain0, type xm list to view the currently allocated memory.
Select the Xen section.
In dom0_mem= mem_amount where mem_amount is the maximum amount of memory to allocate to Domain0. Add K, M, or G, to specify the size, for example, dom0_mem=768M., add
Restart the computer to apply the changes.
To set a minimum amount of memory for Domain0, edit the dom0-min-mem parameter in the /etc/xen/xend-config.sxp file and restart xend. For more information, see Section 4.2, Controlling the Host by Modifying xend Settings.