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Computer Science CS677: Distributed OS Lecture 5, page
Module 1: Virtualization
- Virtualization: extend or replace an existing interface to
mimic the behavior of another system.
- Introduced in 1970s: run legacy software on newer mainframe hardware
- Handle platform diversity by running apps in VMs
- Portability and flexibility ! 1
Types of Interfaces
- Different types of interfaces
- Assembly instructions
- System calls
- APIs
- Depending on what is replaced /mimiced, we obtain
different forms of virtualization
! 2
Computer Science CS677: Distributed OS Lecture 5, page
Types of Virtualization
- Emulation
- VM emulates/simulates complete hardware
- Unmodified guest OS for a different PC can be run
- Bochs, VirtualPC for Mac, QEMU
- Full/native Virtualization
- VM simulates “enough” hardware to allow an unmodified guest OS to be run in isolation - Same hardware CPU
- IBM VM family, VMWare Workstation, Parallels, VirtualBox ! 3
Types of virtualization
- Para-virtualization
- VM does not simulate hardware
- Use special API that a modified guest OS must use
- Hypercalls trapped by the Hypervisor and serviced
- Xen, VMWare ESX Server
- OS-level virtualization
- OS allows multiple secure virtual servers to be run
- Guest OS is the same as the host OS, but appears isolated
- Solaris Containers, BSD Jails, Linux Vserver, Linux containers, Docker
- Application level virtualization
- Application is gives its own copy of components that are not shared
- (E.g., own registry files, global objects) - VE prevents conflicts
- JVM, Rosetta on Mac (also emulation), WINE ! 4
Computer Science CS677: Distributed OS Lecture 5, page
How Virtualization works?
- CPU supports kernel and user mode (ring0, ring3)
- Set of instructions that can only be executed in kernel mode
- I/O, change MMU settings etc -- sensitive instructions
- Privileged instructions: cause a trap when executed in user mode
- Result: type 1 virtualization feasible if sensitive instruction subset of privileged instructions
- Intel 386: ignores sensitive instructions in user mode
- Can not support type 1 virtualization
- Recent Intel/AMD CPUs have hardware support
- Intel VT, AMD SVM
- Create containers where a VM and guest can run
- Hypervisor uses hardware bitmap to specify which inst should trap
- Sensitive inst in guest traps to hypervisor ! 7
Type 1 hypervisor
- Unmodified OS is running in user mode (or ring 1)
- But it thinks it is running in kernel mode ( virtual kernel mode)
- privileged instructions trap; sensitive inst-> use VT to trap
- Hypervisor is the “real kernel”
- Upon trap, executes privileged operations
- Or emulates what the hardware would do ! 8
Computer Science CS677: Distributed OS Lecture 5, page
Type 2 Hypervisor
- VMWare example
- Upon loading program: scans code for basic blocks
- If sensitive instructions, replace by Vmware procedure
- Cache modified basic block in VMWare cache
- Execute; load next basic block etc.
- Type 2 hypervisors work without VT support
- Sensitive instructions replaced by procedures that emulate them. ! 9
Paravirtualization
- Both type 1 and 2 hypervisors work on unmodified OS
- Paravirtualization: modify OS kernel to replace all
sensitive instructions with hypercalls
- OS behaves like a user program making system calls
- Hypervisor executes the privileged operation invoked by hypercall. ! 10
Computer Science Lecture 5, page
Virtual Appliances & Multi-Core
- Virtual appliance: pre-configured VM with OS/ apps
pre-installed
- Just download and run (no need to install/configure)
- Software distribution using appliances
- Multi-core CPUs
- Run multiple VMs on multi-core systems
- Each VM assigned one or more vCPU
- Mapping from vCPUs to physical CPUs
- Today: Virtual appliances have evolved into docker containers CS677: Distributed OS (^)! 13 Computer Science Lecture 5, page
Use of Virtualization Today
- Data centers:
- server consolidation: pack multiple virtual servers onto a smaller number of physical server - saves hardware costs, power and cooling costs
- Cloud computing: rent virtual servers
- cloud provider controls physical machines and mapping of virtual servers to physical hosts
- User gets root access on virtual server
- Desktop computing:
- Multi-platform software development
- Testing machines
- Run apps from another platform ! 14
Computer Science Lecture 5, page
Case Study: PlanetLab
- Distributed cluster across universities
- Used for experimental research by students and faculty in networking and distributed systems
- Uses a virtualized architecture
- Linux Vservers
- Node manager per machine
- Obtain a “slice” for an experiment: slice creation service ! 15
Virtual machine Interface
- Standardize the VM interface so kernel can run on bare
hardware or any hypervisor
! 16
