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CS533 Concepts of Operating Systems, Spring 2011
THE MACH SYSTEM
"Operating Systems Concepts, Sixth Edition" by
Abraham Silberschatz, Peter Baer Galvin, and Greg
Gagne, published by J Wiley, 2002.
Presented by: Shweta Ojha
OUTLINE
Introduction
MACH Architecture
Motivation
System Components
Process Management
Interprocess Communication
Memory Management
Programmer Interface
Summary
Introduction

MACH: operating system kernel

Microkernel

Developed at Carnegie Mellon University

Logical successor to Accent kernel


Developed as a replacement for the kernel in the BSD
version of UNIX
Basis of modern operating system kernels
•Mac OS X
•GNU Hurd
(Source: Wikipedia)
What is a Microkernel ?
Near minimum amount of software that can provide the
following mechanisms needed to implement an OS:
low-level address space management
thread management
inter-process communication (IPC) (Source: Wikipedia)
(Source: http://en.wikipedia.org/wiki/File:OS-structure.svg )
MACH - Architecture
BSD code outside the kernel
 Basic Mach features in the kernel
 Unix specific code in user mode
 BSD can be replaced with other OS
 Concurrently run multiple OS on top of microkernel

Motivation

Runs on uniprocessors and multiprocessors

Capable of functioning on heterogeneous hardware

Supports varying degrees of shared memory access:
• Uniform Memory Access (UMA)
• Non-Uniform Memory Access (NUMA)
• No Remote Memory Access (NORMA)


Function with varying intercomputer network speeds
Supports simultaneous execution of multiple
operating systems
Motivation




Distributed operating providing network transparency to
clients
Integrated memory management and interprocess
communication to provide communication based memory
management and for communication of large numbers of
data
Heterogeneous system support
Simple programmer interface with a good set of
primitives

Easy portability to a wide class of uniprocessors

Extensive library of utilities and applications
System Components
System Components

Task:
•Consists of a virtual address space
•Contains one or more threads
•Protected access to system resources via ports

Thread:
•Basic unit of execution
•Runs in the context of a task
•Threads within a task share task's resource (ports, memory)

Port:
•Mechanism to reference an object
•Protected by kernel managed capabilities – port rights
•Communication by sending messages to ports

Port set:
•Group of ports sharing a common message queue

Message:
•Basic method of communication between threads

Memory Object:
•Source of memory accessed by mapping into task's address space
Process Management

Basic Structure:
•Tasks & Threads

Create task:
•Similar to Unix (FORK)

Parallelism:
•1 Task has multiple threads
•Threads on parallel processors
•Faulty thread delayed, others continue

Operations:
•Suspend Task => Suspend all threads
•Resume Thread ≠> Resume Task

Synchronization Primitives:
•Mach IPC → exchanging messages
•Thread synchronization calls (start , stop)
•Semaphores (wait, signal)
MACH- Threads

User level threads with kernel support

C Threads influenced POSIX P Threads standard

C Threads package
•Thread control routine: create
destroy
wait
yield
•Mutual exclusion through spinlocks: mutex_alloc
mutex_free
mutex_lock
mutex_unlock
•Synchronization through condition variables: condition_alloc
condition_free
condition_wait
condition_signal
CPU Scheduling

Only threads are scheduled (not tasks)

Thread priority = exponential average of CPU usage

Global run queues & per processor (local) run queues
•Local run queue absolute priority over global run queue

Maintains a list of idle processors

Constant time quantum over entire system
•Thread time quantum Ξ 1/ Number of threads

Yielding CPU while waiting for resource
1st Call: Thread ------------------> Scheduler
Alert: Thread Block
2nd Call: Thread moved off the run queue till event
Exception Handling



Exception Handler = Thread in the task(exception occurred)
RPC messages: synchronize & communicate between victim &
handler
Two granularities of exception handling
•Error handlers: per-thread handling
•Debuggers: per-task handling
•Error handlers have higher precedence over Debuggers

Process:
Wait
routine
Victim
RPC message: (exception info,
thread, task)
Victim
Thread
Handler
Clears exception →
Resume/Terminate Victim
Exception Handling

Supports BSD style signals

BSD expects hardware exceptions as signals

Flow:
MACH exception handling
Hardware
Exceptions
Exception
RPC
receives
clears
In-kernel Task
Exception causing
Thread (Blocked)
Signal handling
code
Exception causing
Thread (Run)
Signal
Interprocess Communication

Location independent message passing

All objects addressed via communications ports

Message senders & receivers must have rights
•Right = port name + capability(send/receive) on that port
•Only 1 task with receive rights to a port
•Multiple tasks with send rights
•Rights passed in messages by object creator/kernel
•Message Receiver gains rights, Sender loses it
•Destruction of port/receive right holder → revocation of all rights
Component of IPC: Ports

Implemented as protected, bounded queue within the kernel of the
system on which object resides
Sender may abort

If a queue is full
ask

Wait for a slot
Kernel
Deliver message
System calls to provide port functionality:
•Allocate a new port (port_allocate + task_self)
•Deallocate a task's access rights to a port
•Get current status of a task's port
•Create backup port

Port sets:
•When 1 thread has to service multiple objects
•Not passed in messages
•1 port member of only 1 port set
Component of IPC: Messages
MESSAGE:
Header (fixed length)
Destination port name
Reply port name
Length of the message
Data Objects (variable length)
In-line data (data in message, less than 8K)
Pure typed data
Port rights
Out-of-line data
Pointers to data exceeding 8K
Transfers entire address space of a task in one message
Address map of receiving task is modified to include
copy-on-write copy of message pages
Note: Message also stores the type information of data!!
NetMsgServer


User-level, forwards messages between hosts
MACH Tenets: All objects are location independent & location is
transparent to the user
●
Provides Name Service Primitive
Allows tasks networkwide to register ports for lookup
Transfers 1st port that allows cross-computer IPC
Subsequent IPC interactions are fully transparent
●
Maintains a distributed database of ports and port rights
●
Uses type information of data
Solves the problem of cross-computer data format
NetMsgServer
Network IPC forwarding
Memory Management

Memory Objects
Manage secondary storage
Files/pipes/data mapped into virtual memory
Backed by user-level memory managers
Has a port associated with it
Manipulated by messages being sent to the port
Independent of kernel (no knowledge of content)

Default Memory Managers
Where user-level memory managers are insufficient
When user-level fails to pageout

Shared Memory
Between tasks running on processors that share memory
Changes made to the same copy
Thread synchronization: critical sections/ mutex
Separate Machines → Use External Memory Managers
Same external memory manager for unrelated tasks accessing
same memory section
Memory Management
User-level Memory Managers
Memory objects mapped into virtual address space of task
Maintains cache of memory-resident pages of mapped objects
Memory can be paged by user-written memory managers
Paging algorithm based on the object it is backing
System Calls:
vm_map
memory_manager_init (routine)
memory_object_set_attributes
get & set attributes
page-level locking
memory_object_init
memory_object_data_request
memory_object_data_provided
precious pages
memory_object_data_write
locking & modification of protection information
Programmer Interface

System call Interface
Emulation libraries (run at user level)
OS calls translated to subroutine calls to library
Server (run at user level)
For system calls that cannot be implemented in library
Multithreaded

C Threads package
Run-time library provides C language interface
Provides access to Mach thread primitives
Fork, Join
Mutex
Condition variables

MIG
Interface / Stub generator
Coding send/receive messages
Compiler
Input = Interface definition (declarations of variables, types & procedures)
Output = RPC interface code
Summary
Micro kernel

Operating system emulation at user level

Message: only communications method

Provides low level system calls

Supports many memory models, parallel & distributed
computing

References
Operating Systems Concepts, Sixth Edition" by
Abraham Silberschatz, Peter Baer Galvin, and Greg
Gagne, published by J Wiley, 2002.
http://en.wikipedia.org/wiki/File:OS-structure.svg

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