lecture04-encapsulation.md

Lecture 4 Encapsulation

Encapsulation Options

Option #1 Bare Metal

• Bare Metal: Give users entire machines
  • Pro: Good isolation, Software freedom, Best performance
  • Con: Limit allocation granularity, Software management tricky

Option #2 Process

• Process: Users are allocated traditional OS processes
  • Pro: Well-understood, Good performance, Debugging easy
  • Con: Performance isolation poor, Security questionable, Software freedom poor

Option #3 Containers

• Containers: Traditional OS hosts process containers, where each container looks like an empty OS
  • Pro: Decent software freedom, Good performance
  • Con: Possible security problems

Option #4 Virtual Machines

• Virtual machines: Users get a software container that acts like a physical machine
  • Pro: Decent isolation properties, Good software freedom
  • Con: Performance overhead, Imperfect performance isolation

Containers

Linux Containers

• Need OS-based protection and namespaces to limit power of guest application
• Leverage layered file system to enable easy composition of images (e.g., OverlayFS)
• Still need platform to deploy and manage running instances (e.g., Kubernetes)

Linux Namespaces

• Namespace: restrict what can a container see
  • Provide process level isolation of global resources
  • Processes have illusion they are the only processes in the system

Resource Allocation: Linux CGroups

• What and how much can a container use?
  • See upper bounds on resources that can be used
  • Fair sharing of certain resources
• Examples:
  • cpu: weighted proportional share of CPU for a group
  • cpuset: cores that a group can access
  • block io: weighted proportional block IO access
  • memory: max memory limit for a group

Virtual Machines

• Key Idea: Add software layer which emulates hardware interface
  • Fidelity
  • Isolation
  • Performance
• VM encapsulation provides broad advantages
  • Compatibility
  • Consolidation
  • State capture
  • Observability

Privileged Operations

• For performance, most instructions may execute directly
  • Especially those that execute in user mode
• Typically, a VMM must handle guest OS attempts to execute
  • Privileged instructions
  • Instructions that write/read privileged state
• Handling privileged instructions
  • Trap-and-emulate
  • Static software re-writing
  • Dynamic software re-writing

VMM Issues

• Managing multiple VMs
  • Multiple VMs may be handled through either partitioning or time-slicing
  • When the VMs are multiprocessor, gang-sheduling the virtual cores may improve performance
• Type I vs Type II
  • Type I advantages: performance, smaller code base
  • Type II advantages: convenience
• Ideally, isolation means that a VM behaves exactly as a PM