Transcript Chorus and other Microkernels

Chorus and other Microkernels
Articles By:
•Jon Udell
•Peter D. Varhol
•Dick Pountain
Presented by:
Jonathan Tanner and Brian Doyle
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.
Monolithic vs. Micro
Monolithic
Scheduler
•Microkernel
Device Drivers
File System
Memory
Manager
IPC
Device
Drivers
File
System
Scheduler
Memory Manager
IPC
Real-Time Events
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.
Win NT Kernel
User Level
Space
Executive /
Privileged
Space
Netscape
Security
File System
I/O
Kernel Space
Scheduler
Memory Manager
IPC
The Chorus Microkernel
• Aspirations of a New Operating System
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Multitasking
Networking
Fault Tolerance
Symmetric Multiprocessing
Massive Parallelism
Binary Compatibility with Industry Standard Software
Object Oriented Design
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.
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Scheduling
Memory Management
Real-Time Events
Communications
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.
Chorus Microkernel
ports
Actors
threads
threads
Site
Site
Communication Medium
The Chorus Microkernel
• Multitasking Real-Time Executive
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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.
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.
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.
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.
The Chorus Microkernel
• Above the Kernel - Everything else in the OS is a
server. (Operating in User Mode or Kernel Mode)
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File Managers
Stream and Socket Managers
Device Drivers
Unix System V
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.
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.
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.
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 processlevel library.
Chorus Systems Unix
• Some Early Problems
– Unix Signals: Mono-threaded actors used
priority messages.
– Device Drivers: Needed to reside within the
kernel.
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