One of the biggest impediments to widespread use of virtual machines is the performance overhead incurred by running a virtual machine. Listed below are various performance parameters and application behavior.

1. Calculate the CPI for the system listed above assuming that there are no accesses to I/O. What is the CPI if the VMM performance impact doubles? If it is cut in half? If a virtual machine soft ware company wishes to obtain a 10% performance degradation, what is the longest possible penalty to trap to the VMM?

2. I/O accesses oft en have a large impact on overall system performance. Calculate the CPI of a machine using the performance characteristics above, assuming a non-virtualized system. Calculate the CPI again, this time using a virtualized system. How do these CPIs change if the system has half the I/O accesses? Explain why I/O bound applications have a smaller impact from virtualization.

3. Compare and contrast the ideas of virtual memory and virtual machines. How do the goals of each compare? What are the pros and cons of each? List a few cases where virtual memory is desired, and a few cases where virtual machines are desired.

4. Section 5.6 discusses virtualization under the assumption that the virtualized system is running the same ISA as the underlying hardware. However, one possible use of virtualization is to emulate non-native ISAs. An example of this is QEMU, which emulates a variety of ISAs such as MIPS, SPARC, and PowerPC. What are some of the difficulties involved in this kind of virtualization? Is it possible for an emulated system to run faster than on its native ISA?

Transcribed Image Text:

Priviliged 0/S Accesses I/0 Access Time (Includes Time to Trap to Guest Performance Impact to Performance 1/0 Access Trap to the Impact to Trap per 10,000 per 10,000 Base CPI Instructions Guest 0/S to VMM Instructions 0/S) 15 cycles 175 cycles 1100 cycles 1.5 120 30