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Computer Science CS677: Distributed OS Lecture 6, page
Computing Parable
- The Lion and the Fox
- Courtesy: S. Keshav 1
Types of Hypervisors
- Type 1: hypervisor runs on “bare metal”
- Type 2: hypervisor runs on a host OS
- Guest OS runs inside hypervisor
- Both VM types act like real hardware 2
Computer Science CS677: Distributed OS Lecture 4, 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 kernel 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 3
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 4
Computer Science CS677: Distributed OS Lecture 4, page
Virtual machine Interface
- Standardize the VM interface so kernel can run on bare
hardware or any hypervisor
7
Memory virtualization
- OS manages page tables
- Create new pagetable is sensitive -> traps to hypervisor
- hypervisor manages multiple OS
- Need a second shadow page table
- OS: VM virtual pages to VM’s physical pages
- Hypervisor maps to actual page in shadow page table
- Two level mapping
- Need to catch changes to page table (not privileged)
- Change PT to read-only - page fault
- Paravirtualized - use hypercalls to inform 8
Computer Science Lecture 4, page
I/O Virtualization
- Each guest OS thinks it “owns” the disk
- Hypervisor creates “virtual disks”
- Large empty files on the physical disk that appear as “disks” to the guest OS - Hypervisor converts block # to file offset for I/O
- DMA need physical addresses
- Hypervisor needs to translate CS677: Distributed OS 9
Examples
- Application-level virtualization: “process virtual
machine”
Computer Science Lecture 4, 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 13
Code and Process Migration
- Motivation
- How does migration occur?
- Resource migration
- Agent-based system
- Details of process migration 14
Computer Science CS677: Distributed OS Lecture 6, page
Motivation
- Key reasons: performance and flexibility
- Process migration (aka strong mobility )
- Improved system-wide performance – better utilization of system-wide resources
- Examples: Condor, DQS
- Code migration (aka weak mobility)
- Shipment of server code to client – filling forms (reduce communication, no need to pre-link stubs with client)
- Ship parts of client application to server instead of data from server to client (e.g., databases)
- Improve parallelism – agent-based web searches 15
Motivation
- Flexibility
- Dynamic configuration of distributed system
- Clients don’t need preinstalled software – download on demand 16
Computer Science CS677: Distributed OS Lecture 6, page
Models for Code Migration
- Alternatives for code migration. 19
Do Resources Migrate?
- Depends on resource to process binding
- By identifier: specific web site, ftp server
- By value: Java libraries
- By type: printers, local devices
- Depends on type of “attachments”
- Unattached to any node: data files
- Fastened resources (can be moved only at high cost)
- Fixed resources
- Local devices, communication end points 20
Computer Science CS677: Distributed OS Lecture 6, page
Resource Migration Actions
- Actions to be taken with respect to the references to local resources when migrating code to another machine.
- GR: establish global system-wide reference
- MV: move the resources
- CP: copy the resource
- RB: rebind process to locally available resource Unattached Fastened Fixed By identifier By value By type MV (or GR) CP ( or MV, GR) RB (or GR, CP) GR (or MV) GR (or CP) RB (or GR, CP) GR GR RB (or GR) Resource-to machine binding Process-to- resource binding 21
Migration in Heterogeneous Systems
- Systems can be heterogeneous (different architecture, OS)
- Support only weak mobility: recompile code, no run time information
- Strong mobility: recompile code segment, transfer execution segment [migration stack]
- Virtual machines - interpret source (scripts) or intermediate code [Java] 22
