<DD>virtualization always imposes two kinds of overhead: context-switching and sandboxing (see below). by contrast, a multi-OS bootloader does not impose an overhead because it only provides a hardware compatibility layer, without imposing context-switching or sandboxing.

context-switching imposes an overhead because glue code is needed to allow the guest and host OS to run simultaneously. there is overhead each time the CPU switches between the guest and host OS, which is in proportion to the switching frequency. this means you want the timeslices for each OS to be as long as possible to minimize the overhead of saving/restoring the CPU context. you also need a dedicated timer on the CPU (untouchable by the guest OS) to preempt the guest thread at the end of the timeslice.

sandboxing imposes an overhead as follows:

- the VM container almost always makes system calls available as *platform-independent* BIOS calls rather than forwarding the system calls themselves (as chroot does).

- for security reasons, BIOS calls are almost always provided as wrappers around *system* calls, rather than native BIOS calls (which cannot be trapped by the host OS).