Chorus and other Microkernels

1
Chorus and other Microkernels

• Articles By
• Jon Udell
• Peter D. Varhol
• Dick Pountain

Presented by Jonathan Tanner and Brian Doyle
2
What is a Microkernel?

• Provides IPC, Scheduling, Real-Time Events and
  Memory Management
• All else is a service which can be plugged in as
  an external module at run-time.

3
Monolithic vs. Micro

• Microkernel

Monolithic
Scheduler
Device Drivers
File System
Memory Manager
IPC
Scheduler
Device Drivers
Memory Manager
IPC Real-Time Events
File System
4
Windows NT Micro or no?

• Has Core Kernel Size
• All Modules ARE NOT simply user level
• Security, I/O and others run in Executive Mode.

5
Win NT Kernel
User Level Space
File System
Netscape
Executive / Privileged Space
Security
I/O
Kernel Space
Scheduler
Memory Manager
IPC
6
The Chorus Microkernel

• Aspirations of a New Operating System
• Multitasking
• Networking
• Fault Tolerance
• Symmetric Multiprocessing
• Massive Parallelism
• Binary Compatibility with Industry Standard
  Software
• Object Oriented Design

7
The Chorus Microkernel

• Chorus Systems are built on a tiny nucleus
  (typically only 50-60 Kbytes in size vs.
  Monolithic which can be 400 Kbytes or more) which
  includes.
• Scheduling
• Memory Management
• Real-Time Events
• Communications

8
Chorus Lexicon

• Actor - The Equivalent of a Unix process it
  provides an execution context for one or more
  threads.
• Ports - Queues attached to actors by which
  threads of one actor send messages to threads of
  another.
• Site - The basic unit of computing hardware,
  consisting of one or more processors, memory, and
  I/O devices.
• Thread - The unit of execution in Chorus. It
  has the same meaning as it does in Windows NT. A
  Thread does not need a private address space but
  does need its own stack. Under Chorus the Actor
  owns the address space.

9
Chorus Microkernel
ports
Actors
Site
Site
Communication Medium
10
The Chorus Microkernel

• Multitasking Real-Time Executive
• Allocates Local Processors
• Schedules Threads Using Priority-Based Preemptive
  Scheme
• Optional Time-Slicing
• API for Thread Creation/Destruction
• Synchronization via Semaphores, Spin Locks, and
  Mutexes.
• Chorus Philosophy - Provide a variety of
  efficient low-level mechanisms, leaving the
  choice of performance trade-offs to the
  sub-system builder.

11
The Chorus Microkernel

• The Memory Manager
• Offers Segments and Regions
• The virtual address space of an actor is divided
  into contiguous regions that map a portion of a
  segment into physical memory.
• System actors called Mappers manage segments,
  allocating regions as needed.

12
The Chorus Microkernel

• The Supervisor
• Dispatches Interrupts, Exceptions and Traps.
• These events are dispatched to dynamically
  defined device drivers and other real-time event
  handlers at run time.
• Response time is fast enough for Chorus to be
  applied to real-time control systems.

13
The Chorus Microkernel

• The Interprocess Communications (IPC)
• Delivers messages between ports
• Two Communication Modes
• Simple, non-blocking, asynchronous send/receive
  protocol in which messages are not acknowledged.
• RPC (Remote Procedure Call) with full
  client/server semantics.

14
The Chorus Microkernel

• Above the Kernel - Everything else in the OS is
  a server. (Operating in User Mode or Kernel
  Mode)
• File Managers
• Stream and Socket Managers
• Device Drivers
• Unix System V

15
The Chorus Microkernel

• The ability to support conventional Operating
  Systems as Sub-Systems means you could develop
  Multiple Personalities
• OS/2
• Unix
• Windows
• IBM is basing its future OS strategy on a similar
  idea, implementing it on the Mach-3.0 microkernel.

16
Chorus Systems Unix

• A Trend in OS Development - Restructuring of
  traditional monolithic operating systems into
  independent servers.
• Primary Concerns
• Efficiency Can a microkernel-based modular
  operating system provide performance comparable
  to that of the monolithic kernel.
• Compatibility Portability, Standardization and
  Compatible Interfaces are needed for applications
  as well as device drivers and streams modules.

17
Chorus Systems Unix

• Supervisor Actors
• Share the supervisor address space.
• Can execute in kernel mode.
• Are truly separate entities from the nucleus
  compiled, linked and loaded independently.
• Utilize privileged instructions and connected
  handlers.
• Connected Handlers
• Created dynamically by Supervisor Actors to catch
  hardware interrupts, system call traps, and
  program exceptions.
• Allows for a common interface for the nucleus.
• Interrupt processing time is greatly reduced,
  allowing real-time applications to be implemented
  outside the nucleus.

18
Chorus Systems Unix

• Chorus Unix - Split into four servers
  (Supervisor Actors)
• Process Manager A Unix process was implemented
  by the Chorus Actor.
• File Manager
• Device Manager
• Socket Manager
• Unix System Calls - Implemented by a
  process-level library.

19
Chorus Systems Unix

• Some Early Problems
• Unix Signals Mono-threaded actors used priority
  messages.
• Device Drivers Needed to reside within the
  kernel.

20
Chorus Systems Unix

• Experience Gained Implementing a rich operating
  system environment such as Unix provides insight
  into the basic operating system services that a
  microkernel must provide.
• Supervisor Actors
• Threads
• Connected Handlers