P1L2: Basics

Question 1

Name the design principle of a microkernel. Also, state the services that are part of the Kernel.
What functionality must be efficient?

Answer 1

Extreme separation of mechanism and policy.

• Only bare minimum of OS services
  • <ins>Inter process-Communication</ins>
  • <ins>Memory Management</ins>
  • <ins>CPU-Scheduling</ins>
  • <ins>Enough to represent an executing application, its address space & context (thread)</ins>
• Everything else runs in user space (e.g apps, Filesystem, device driver)
• Communication between services in user space and clients in user space ALL VIA message passing IPC => must work efficiently

Question 2

Name the design principles of a Modular OS.

Answer 2

• Design:Similar to microkernel. Core only has basic functionality + knowledge how to load additional kernel modules
• <ins>Only Hardware services + Basic Higher-level services are part of the OS</ins>
• Extensibility & Customisable through kernel modules
• Kernel exposes Interface that every module has to implement!!

Question 3

Name the design principles of a Monolithic OS.

Answer 3

• Design: No separation of policy and Mechanism
• All functionality already part of the OS’s Kernel
• No extensibility

Question 4

Name advantages and disadvantages of Microkernels

Answer 4

Advantage:

• Flexibility (seperation of mech. and policy) -> adding service is just done in user space without affecting kernel
• Verifiable (smaller code size)
• Security (most services run in user-space, failure or insecurity does not affect kernel) - OS does not easily crash
• Easy to modify kernel (less code)

Disadvantage:

• Message passing IPC for all communication is inefficient (always user-system crossing) - Windows NT -> Windows XP
• Not very portable

Question 5

Name advantages and disadvantages of Monolithic Kernels

Answer 5

Advantage:

• Compile time benefits because all functionality already part of Kernel
• Testable as a whole (no external code that can crash the system like for Kernel modules)
• Performance: Few user-kernel crossings vs. Microkernel

Disadvantage:

• Failure in OS service can bring down whole system (bluescreen of death)
• Too much code (maintenance bad)
• Not very extensible

Question 6

Name advantages and disadvantages of Modular OS’s

Answer 6

Advantage:
- Flexibility - can add kerrnel module
- vs. Monolithic: smaller memory footprint -> better performance
- vs. Microkernel: less IPC
Disadvantage:
- Performance: Indirection via standart kernel module interface can add overhead
- Bug in Kernel module can crash system