Virtualmachines canlose performance fromanumberof events, such as the execution of privileged instructions, TLB misses, traps, and I/O.

These events are usually handled in system code. Thus one way of estimating the slowdown when running under a VM is the percentage of application execution time in system versus user mode. For example, an application spending 10% of its execution in system mode might slow down by 60% when running on a VM.
Figure 2.35 lists the early performance of various system calls under native execution, pure virtualization, and paravirtualization for LMbench using Xen on an Itanium system with times measured in microseconds (courtesy of Matthew Chapman of the University of New South Wales).

a.  What types of programs would be expected to have smaller slowdowns when running under VMs?

b. If slowdowns were linear as a function of system time, given the preceding slowdown, how much slower would a program spending 20% of its execution in system time be expected to run?

c. What is the median slowdown of the system calls in the table above under pure virtualization and paravirtualization?

d. Which functions in the table above have the largest slowdowns?
What do you think the cause of this could be?

Transcribed Image Text:

Benchmark Null call Null I/O Stat Open/close Install signal handler Handle signal Fork Exec Fork + exec sh Native 0.04 0.27 1.10 1.99 0.33 1.69 56.00 316.00 1451.00 Pure 0.96 6.32 10.69 20.43 7.34 19.26 513.00 2084.00 7790.00 Para 0.50 2.91 4.14 7.71 2.89 2.36 164.00 578.00 2360.00 Figure 2.35 Early performance of various system calls under native execution, pure virtualization, and paravirtualization.