If you install Oracle VM from the .iso image or by using Oracle System Assistant, dom0_mem should be set correctly. If you install the Oracle VM software from PXE boot or some other customized environment and you have a large-memory system, you might need to set dom0_mem.
Use this formula:
dom0_mem = 502 + int(physical_mem_MB * 0.0205)
For example, if your system has 128 GB (131072 MB) of memory, you need to increase dom0_mem to 3188 MB.
To increase the memory, add the text dom0_mem=3188m to the kernel line in grub.conf. For a first-time boot, you can interrupt the boot process at the grub menu and edit the grub.conf file.