5.5 Performance and security isolation

Performance isolation is a critical condition for quality-of-service (QoS) guarantees in shared computing environments. Indeed, if the run-time behavior of an application is affected by other applications running concurrently and, thus, is competing for CPU cycles, cache, main memory, and disk and network access, it is rather difficult to predict the completion time. Moreover, it is equally difficult to optimize the application. Several operating systems, including Linux/RK [270], QLinux [343], and SILK [44], support some performance isolation, but problems still exist because one has to account for all resources used and to distribute the overhead for different system activities, including context switching and paging, to individual users a problem often described as QoS crosstalk [348].

5.1 Virtualization

Virtualization simulates the interface to a physical object by any one of four means:

1. Multiplexing. Create multiple virtual objects from one instance of a physical object. For example, a processor is multiplexed among a number of processes or threads.

2. Aggregation. Create one virtual object from multiple physical objects. For example, a number of physical disks are aggregated into a RAID disk.

3.Emulation.Constructavirtualobjectfromadifferenttypeofphysicalobject.Forexample,aphysical disk emulates a random access memory.

4. Multiplexing and emulation. Examples: Virtual memory with paging multiplexes real memory and disk, and a Virtual address emulates a real address; TCP emulates a reliable bit pipe and multiplexes a physical communication channel and a processor.

((problem quastion))

In Section 5.5 we stated that a VMM is a much simpler and better-specified system than a traditional operating system. The security vulnerability of VMMs is considerably reduced

because the systems expose a much smaller number of privileged functions. Research the literature to gather arguments in support of these affirmations. Compare the number of lines of code and system calls for several operating systems, including Linux, Solaris, FreeBSD, Ubuntu, AIX, and Windows, with the corresponding figures for several system virtual machines in Table 5.1.