On Kernel Construction

1
On ?Kernel Construction

• Jochen Liedtke
• GMD German National Research Center for
  Information Technology
• 15th ACM Symposium on Operating System Principles
  (SOSP)
• December 3-6, 1995

2
?-kernels advantages

• Early ?-kernels Brinch Hansens (1970) and
  Hydra (1974)
• ?-kernel idea
• Advantages
• More modular system structure
• Server fault isolation (independence)
• More flexible and tailorable

3
problems

• Most existing ?-kernels have bad performance
• Bad performance ? Restricted flexibility
• Believed-to-be causes
• Increased used-kernel and address-space switches
• But performance studies are kernel-specific

4
Liedtkes Thesis

• A ?-kernel does the job IF
• Properly designed and
• Carefully implemented
• Minimality
• Elegance Efficiency
• Architectural Flexibility integration

5
Roadmap

• Primitives that ?-kernels must implement
• Achievable flexibility
• Achievable performance
• Cost no portability!

6
Primitives in a ?-kernel

• Determining criterion functionality
• A concept is OK inside the ?-kernel if left
  outside would prevent implementation of the
  systems required functionality
• Assume
• System must support interactive and/or not
  completely trustworthy apps
• Hardware implements page-based VM
• System must support
• Independence
• Integrity

7
Primitives address spaces

• Address space mapping that associates each
  virtual page to a physical page
• Why is this needed inside?
• Basic operations
• Grant, map, flush

8
Primitives Threads and IPC

• Thread an activity executing inside an address
  space
• Why is this needed inside?
• so cross-address (aka IPC)
• Note that grant and map need IPC

9
Primitives Unique identifiers

• UIDs for threads or tasks or comm. channels
• Why are uid required?
• Cryptography as an alternative

10
Flexibility

• Memory manager a server managing the initial
  address space is a classical MM
• Device driver a process which directly access
  I/O ports mapped into its address space and
  receives messages from HW through IPC
  (interrupts)
• Remote comm. devices drivers access comm. HW,
  server translated local messages to external
  comm. protocols
• Unix server implemented by IPC

11
Performance

• Cost of kernel-user mode switches implementation
  problem
• Cost of address-space switches implementation
  problem
• Memory system degradation is not inherent to the
  approach

12
Non-Portability

• Portable ?-kernels
• Cannot take advantage,
• Nor circumvent performance problems of specific
  hardware
• Additional layer cost in performance
• Lack of portability goes beyond code
• Algorithms and internal concepts
• ?-kernels are the processor dependent basis for
  portable OSs