4061 Session 27 (4/23)

Today

• Virtual Machines and Emulation

Today’s Objectives• Understand some of the challenges and

applications of virtual machines

Admin

• My office hours on Wednesday cancelled

• HW 5: Due day of final

• Quiz 5: Last day of class

• What is a virtual machine?

• What are some examples of virtual machines?

Virtual Machines

• One machine == one OS?– What about

• One machine, many OS• Many machines, one OS

• Portability of code– Why are Java and Python more portable than

C?

• Virtual machines are a new layer of abstraction

Why not 2?

• Why not run two operating systems at the same time on one box?

VM History

• 1972, IBM System/360

• Mainframes– Isolation between users– Era before user/kernel mode bit

• Little activity in 80s and early 90s

• Exciting area today

Jails

• “chroot jail”– Change the root directory of the calling process– Once a running process executes chroot("/home/jail"),

/home/jail becomes "/“– Can no longer access files above the new root

directory

• Jail is enforced by the OS, not the application• But applications may expect the presence of

“/bin/ls”, a scratch dir, devices, and shared libs

FreeBSD Jail

• Where chroot jail was weak partitioning, FreeBSD jail is strong partitioning– Create virtual machines.

• Popularly used in ISPs.

Virtual OS on the Desktop

• VMWare, Parallels, KVM, Xen

• Different approaches– Hardware emulation– Native virtualization (with hardware)– Paravirtualization

Mame

Wine

Java

• Bytecode

• JVM: JIT compiling

• Advantages– Portability– Security

Grid Computing

• Enable the virtualization of distributed computing resources to create a single system image across heterogeneous, geographically dispersed IT environments.