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of the memory management functions can be carried out by the guest without
requiring host intervention. Nested page tables can be enabled in the Acceleration
tab of the system settings in the VirtualBox GUI.
Unlike Logical Domains, memory used by guests is under the control of the host
platform. Although the guests can take advantage of hardware virtualization fea-
tures such as nested page tables to reduce the overhead of memory management,
it is still possible to request more memory for a guest than the host system has
available. If a shortfall of memory occurs due to this type of oversubscription, the
host operating system will start demand paging, which may drag down the per-
formance of the guest machines and other applications and services running on
the host. VirtualBox does not yet provide a memory ballooning feature that would
enable the host to take pages back from a guest in the event of a memory shortfall.
Figure 5.4 shows an example of a guest whose memory allocation exceeds the
recommended maximum.
Figure 5.4
Setting Memory for an Oracle VM VirtualBox Guest
To help prevent overallocation of the host memory, the VirtualBox GUI will
display a warning if it thinks that too much memory is being configured for a
guest. In doing so, it considers only the total amount of memory on the system,
not how much of that is actually free. It will display a warning if more than 50%
of memory is allocated to a guest and a stronger warning if more than 80% is
reserved for a guest.
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