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The Mach System
Presented by Catherine Vilhauer
CS533 - Concepts of Operating Systems
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What is Mach?
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First Generation micro-kernel
Builds operating system above minimal kernel
Kernel provides only fundamental services
These services basic but powerful enough to be used
on wide range of architectures
Aids distributed computing and multiprocessing
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Design Principles: Based on BSD Unix
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Advantages of BSD Unix
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Simple programmer interface
Easy portability
Extensive library
Combine utilities via pipes
Disadvantages
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Redundant features in kernel
Lack of support for multi-processing
Too many fundamental abstractions to choose from
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Mach Goals
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Support diverse architectures
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With varying degrees of shared memory
Function with varying intercomputer network speeds
Simplified kernel structure
Distributed operation
Integrated memory management and IPC
Heterogenous system support
CS533 - Concepts of Operating Systems
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How Mach Works
BSD Unix
Tasks and
Threads
Another
OS
IPC
Windows
Virtual
Memory
Maybe
another
OS
Database
System
Scheduling
Software
Emulation
Layer
Microkernel
MACH
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Mach: Basic Abstractions
Task: execution environment
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virtual address space
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protected access to system resources via ports
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may contain one or more threads
Thread: basic unit of execution
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must run in context of a task
Port: basic object reference mechanism
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kernel-protected communication channel
protected by port rights
Port Set: group of ports sharing common message queue
Message: basic method of communication
Memory Object: source of memory
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May be managed by external memory manager
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Mach’s Basic Abstractions
Text Region
Port
Program
counter
Task
Message
Data Region
Port Set
Secondary Storage
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Key Features of the Mach System
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Blending of memory management and communication
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Memory management
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based on use of memory objects
Memory object represented by port
IPC messages sent to request operations (pagein / pageout)
Because IPC used, memory objects can live on remote systems
Kernel caches contents of memory objects in local memory
Communication
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Message-based kernel so message handling must be efficient
Uses virtual memory remapping (large messages)
Message transfer modifies receiving task’s address space to
include a copy of the message contents
Copy on Write
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Advantages of Mach Approach
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Increased flexibility in memory management to user
programs
Greater generality
Improved performance over Unix message passing
Easier task migration
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Since ports are location independent, a task and all its ports
can be moved from one machine to the other
All tasks previously communictated with the moved task can
continue to do so because they reference a task by only its
ports and communicate via messages
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Process Management
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Tasks main execution environment
Can contain no or many kernel-supported threads
Thread synchronization minimal but sufficient
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Threads synchronized via IPC
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Messages used to queue requests for resources
Thread receives message if resource available
To return resource thread can send a message to the port
Equivalent to send and receive
Suspend count kept for thread - multiple suspend calls on a
thread but only when an equal number of resume calls made
can it wake up
Cthreads
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CPU Scheduling
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Only threads scheduled (not tasks)
Priority scheduling
Run-queues
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No central dispatcher
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32 global run-queues, searched in priority order
Per-processor run-queues for threads bound to individual
processor
When idle each processor consults run queues
Local run queue has absolute priority
No fixed-length quantum
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Varies inversely with number of threads on system
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Interprocess Communication
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Two components to Mach IPC
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Objects addressed by communications ports
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PORTS and MESSAGES
Allows location independence and security of communication
Security established by means of ‘rights’
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‘Right’ = port name and capability (send / receive) on that
port
Only one task with ‘receive’ rights
Many with ‘send’ rights
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Interprocess Communication
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Ports
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Port sets - a collection of ports.
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Protected, bounded queue within kernel
Useful if one thread is to service requests on multiple ports
Messages
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Fixed-length header (destination and reply port, msg length)
Variable number of typed data objects
Data (in-line data) sent in message or as pointer
Pointers allow transfer of task address space in one message
No copying if on same system! Copy-on-write.
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Interprocess Communication
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Need way to extend IPC across multiple computers
NetMsgServer
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Mach’s kernel IPC transfers message to NetMsgServer
NetMsgServer then uses network protocol to transfer
NetMsgServer on receiving computer uses that kernel’s IPC
to send message to correct destination task.
Uses type info in message to transfer data to heterogenous
machines in understandable way
Alternative in form of NORMA
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Memory Management
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Memory Object principle abstraction
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Used to manage secondary storage
Represent files, pipes and other data mapped into virtual
memory
Instead of traditional approach where kernel provides
management of secondary storage Mach treats it like all
other objects
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Memory Management (cont’d.)
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Mach virtual address space sparse
No regular page table
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Less memory storage required
Simpler address-space maintenance
User-level memory managers can be used instead
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User-Level Memory Managers
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Mach takes care of basics only
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Acts as interface between hardware and user-level
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e.g. receives page faults from hardware
Notifies relevant task (via port) of page fault
Implements pageout policy (FIFO)
Supplies default Memory Manager in some cases
User-Level Memory Managers
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Handle majority of memory management - can page memory
System calls used to communicate with kernel for memory
mapping / page-in / page-out / provide page-level locking
Responsible for consistency of the contents of a memory
object mapped by tasks on different machines
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Shared Memory
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Shared memory advantages
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Provides fast IPC
Reduces file management overhead
Supports multiprocessing and database management
Mach supports lean version
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Threads in a task share memory
Difficult for separate machines to share memory and
maintain data consistency
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Consistent shared memory only supported when tasks share
processor - in other cases parent can declare inherited
memory to be readable-writable - coordinated through locks
For external machines, Mach allows external memory managers
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Programmer Interface
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System calls
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Equivalent to Unix system call interface
How does it do this?
With emulation libraries
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Live in a read-only part of program’s address space
OS calls are translated into subroutine calls
Have emulated MS-DOS and Macintosh OS’s
Lives in address space of program needing its functionality
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Summary
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Unix can run on top of Mach in ‘User Space’
Uses lightweight processes
Supports multiprocessing and parrallel computing
Messages as communication method
Integration of messages with virtual memory
Integration of virtual memory with messages
Size of kernel reduced
Permits OS emulation at user-level
CS533 - Concepts of Operating Systems
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