THE MACH SYSTEM
"Operating Systems Concepts, Sixth Edition" by Abraham
Silberschatz, Peter Baer Galvin, and Greg Gagne
Presentation by Betsy Kavali
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Mach is a Microkernel OS
Picture from Wikipedia
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History of Mach
Derived its communication system and philosophy
from Accent.
BSD Unix support
➢
Originally constructed inside 4.2BSD kernel.
➢
Replaced one piece at a time.
Started with an effort to support multiprocessors.
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Goals of Mach
➢
Support diverse architectures
- UMA, NUMA, NORMA
➢
Simplified kernel structure
➢
Compatibility with UNIX, Ease of use.
➢
Integrate memory management and IPC
Distributed Operation and Varying network speed
➢
Heterogeneous System support.
➢
Object-oriented design
➢
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System Components
message
text region
threads
port
task
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•
•
•
•
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Task
Thread
Port
Port set
Message
Memory object
port set
data region
secondary
storage
memory
object
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System Components
Task:
 Execution environment
 Contains one or more threads
 Provides a protection domain, a protected access to
system resources via ports
Thread:
 unit of computation (execution)
 must run in the context of a task
 all threads in a task share ports, memory, etc.
 process = task + thread
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System Components
Port: Kernel protected communication channel.
Mechanism to reference an object.
Port set:
Group of ports sharing a common message queue
Message:
Basic method of communication between threads in
different tasks.
Memory objects: storage unit,
Map all or part of object into address space
They are accessed by tasks using ports
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Process Management
Tasks
➢ Parent task creates children tasks
➢ New Tasks contain one thread initially.
➢ Suspending a task, suspends threads in the
task.
Threads
➢ Suspending/resuming a thread does not
suspend/resume the task.
➢ Threads share the address space of the task , hence
the need for synchronization
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Process Management - Threads
User Level Threads
➢
Mach provides a basic kernel interface for managing
threads
➢
C threads package is built on top of Mach's
primitives.

Influenced POSIX P threads standard.
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Process Management - Threads
Thread control routines :
Create : give function to execute and its parameters
Destroy : Destroys the thread and returns a value to
the creating thread.
Wait : for a specific thread to terminate then continue
the calling thread
Yield : Thread yields use of a processor.
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Process Management - Threads

Mutual Exclusion using spin locks.

Mutual exclusion Routines are:
Mutex_lock, mutex_unlock, mutex_alloc, mutex_free.

Synchronization through condition variables(wait,
signal), the associated routines are:

Condition_alloc, condition_free
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CPU Scheduler
➢
Only threads are scheduled, tasks are ignored.
➢
Each thread will have a priority number(0 -127)
➢
Dynamic thread priority - The lowest priority thread is the
one with the most recent large CPU usage.
➢
Global run queues + per processor local run queues
➢
Processors consult run queues to select next thread:
the local queue first, then the global queue
➢
Thread time quantum varies inversely with total
number of threads
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Exception Handling
➢
The exception handler is just another thread in the task.
➢
RPC messages: synchronize & communicate between victim
and handler.
Two different granularities of exception handling.
Error Handlers: Perform recovery actions in response to an
exception and resume execution of the thread.
Debuggers: Examine the state of an entire application to
investigate why an exception occurred and/or why the
program is misbehaving.
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Exception handling proceeds as follows.
1. Victim: raise – RPC message sent to the handler.
2. Victim: wait -- synchronize with completion of exception
handling.
3. Handler: catch -- receive notification, identifies the
exception and the victim
4. Handler: take actions
• clear -- clear exception causing victim to return from wait.
• terminate -- cause termination of victim thread.
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InterProcess Communication - IPC
Location Independent IPC
The two components of IPC are
1. Ports
2. Messages
Ports :
Protected bounded queue within the kernel
Capability: send or receive ``right‘’
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InterProcess Communication - IPC
System calls for port functionality.
Allocate: new port in a task(task decides the rights of the port)
Deallocate: revoke tasks access rights to a port.
Get current port status.
Create a back up port
-port sets : Useful when one thread has to service requests
coming on multiple ports.
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InterProcess Communication - IPC
Messages :
Header + one or more typed data objects
Header : contains destination port name, reply port
name, message length
In-line message data : typed data, port rights
Out-of-line data: pointers to data
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InterProcess Communication - IPC
NetMsgServer

Used when receiver port is not on the kernel’s
computer

User-level daemon that forwards messages
between hosts

Provides Name Service Primitive -Allows tasks
networkwide to register ports for lookup

It is protocol independent.
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NetMsgServer
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Memory Management.
Memory Object:

Mach's basic abstraction of physical memory, an
object.

Secondary storage or data that are mapped into
virtual-memory (Files, pipes)

Served by user-Level memory managers.
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Memory management
User-Level memory manager Memory can be paged by user-written memory
managers
Mach has no knowledge of memory object contents.
Default memory manager Used in circumstances when there is no local
manager.
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Blend of Memory and IPC
This is an unique feature of Mach, and key to the system's
efficiency.
Memory management using IPC.
A memory object is represented as a port.
To request operations on this object, IPC messages are sent to
this port.
Because IPC is used, memory object may reside on remote
systems, Kernel caches the contents.
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Blend of Memory and IPC
IPC using memory management techniques:
Here messages are passed by moving pointers to
shared-memory objects.
Virtual-memory remapping to transfer large contents
using virtual copy / copy-on-write techniques
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Virtual copy/ copy on write
http://www.sci.csuhayward.edu/~billard/cs4560/node23.html
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Programmer Interface
System-call level:
Implemented via emulation libraries and servers
C Threads package:
C language interface to Mach threads primitives
Not suitable for NORMA systems
Interface/Stub generator (MIG):
Input = Interface definition (declarations of variables,
types & procedures)
Output = RPC interface code
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Conclusion
➢
Micro kernel
➢
Few simple abstractions
➢
Higher level OS functionality built in user level servers
➢
Focus on communication facilities
➢
Mach pioneered many concepts.
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Thank You
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