The Mach System
Abraham Silberschatz, Peter Baer Galvin, Greg Gagne
Presentation By: Agnimitra Roy
Agenda
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Microkernel – What is it?
What is Mach?
Design Goals
Mach Primitives (2 slides)
Key Mach Characteristics
Support for Higher Level OS Functionality
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Access by Applications
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Process Management
Inter-process Communication (3 slides)
Memory Management (2 slides)
System Calls
Summary
Microkernel – What is it?
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Highly modular collection of OS-neutral abstractions
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Modularity enables scalability, extensibility, and portability
not typically found in monolithic or conventional operating
systems
Moves many OS services into "user space" that on
other operating systems are kept in the kernel
User Programs
User Programs
UNIX System Call
Interface
Microkernel System
Call Interface
Privileged Mode
Non Privileged Mode
TH
Trap Handler
Operating
System
IPC
TH
Microkernel
Hardware
Microkernel Based OS
TH
Hardware
Monolithic OS
Microkernel – Advantages &
Disadvantages
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Advantages
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Robustness – Services can be started independently
Security – Services run as lower level user processes have
restricted access to system resources
Configurability – Can change service without restarting
system
Easier Coding – Easier to write user mode code
Lower memory footprint – user mode service code can be
moved out of memory
Near real time performance – interrupts turned off in kernel
mode – less code in kernel implies less “interrupts are
interrupted”
Disadvantages
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Most microkernels are not tiny, despite the name
Need formalized message-passing mechanisms to be used
New kinds of deadlocks and other error conditions are
possible between system components
What is Mach?
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Earliest example of microkernel
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Simple abstraction over the hardware
Set of primitives to implement minimal OS services
Design Vision
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OS will ultimately consist of minimal kernel that
runs in privileged mode
Supported by larger collection of OS servers
(modules) that runs in non-privileged mode
Microkernel is protected from all server modules,
which are protected from each other
Design Goals
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Simple kernel with few extensible abstractions
Support diverse architecture – uni-processor &
multiprocessor
Network speed independence
Support distributed operation
Integrated memory management & inter-process
communication
Simple API
Portability
Full compatibility with UNIX BSD
Mach Primitives
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Task
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Thread
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Group of ports sharing message queue
Message
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Object reference mechanism
Secure pipe for IPC between tasks
Operations on objects are requested by sending messages to ports
Port Set
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Unit of execution – runs in the context of task
Single unit of code running on a processor
Port
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Execution environment supporting basic resource allocation
Set of resources that enable "threads" to run
Method of communication between threads
Passed between tasks on ports
Memory Object
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Source of memory
Mach Primitives (contd.)
Key Mach Characteristics
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Blends memory and inter-process communication features
Each server module contained in its own address space called
Task
Server modules support parallelism
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Supports message passing primitives
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Within themselves – using threads
Across modules – using micro-kernel system calls
Lower level than RPC
Can be used in combination to build RPC – not as efficient as
optimized RPC
Primitives include send, recv, message queues etc
Passes messages by moving memory pointers to shared memory
objects where possible – avoids object copy
Uses virtual memory re-mapping to transfer large messages –
also known as virtual copy or copy-on-write.
Memory management is based on the use of memory objects
Memory objects may reside on remote systems and accessed
transparently
Process Management
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Task primitive is equivalent to traditional process without
instruction pointer or register set
Task by itself does nothing unless threads execute on it
Threads can be in two states
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(1/1)
Running
Suspended
Provides thread synchronization primitives
C threads package provides low level, flexible C routines for
process management
CPU Scheduling enabled via 32 global run queues with locking
facility
Varies size of time quantum inversely with number of threads
Supports two granularities for exception handling
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Per thread
Per task
Inter-process Communication
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Components: Ports & Messages
Depends on ports & messages for
communication
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(1/3)
Delivers location independence
Delivers secure communication
Security ensured with rights (port name
+ capability) for senders and receivers
Interprocess Communication
(2/3)
Ports, Messages, NetMsg Server
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Ports
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Protected, bounded queue in the kernel where objects reside
Enables one thread to send data to another
Several ports created by kernel when a task or thread is
created
Can be collected into port sets
Messages
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Header (destination port + reply port + message length) +
Typed data objects (variable count)
Inline Data: Mach 3 – No limit; Mach 2.5 < 8KB
Message passing to Receiver
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On same computer: virtual memory management using
pointers
On different computers: NetMsgServer
Interprocess Communication
(3/3)
Ports, Messages, NetMsg Server
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NetMsgServer
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Location transparent naming & transport to
extend IPC across multiple computers
Maintains distributed database for port rights
Uses type information stored in messages to
translate data from sender’s to receiver’s format
Synchronization
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Port can be used as synchronization variable
Can be used for threads in same task – not among
different tasks
Memory Management
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Memory Object
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(1/2)
Principle abstraction in Mach
Used to manage secondary storage; represents
files, pipes or other data mapped into VM for read
& write
Backed by user level memory managers
Virtual address space for task is generally
sparse
Maintains cache of memory resident pages of
all mapped objects
Can use user level memory managers
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However, supports a default memory manager
Memory Management
(2/2)
Shared Memory
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Mach uses shared memory to reduce
complexity of system services
Supports consistent shared memory for tasks
running on processors that share memory
Tasks # Shared memory
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OS constructs such as fork
Parent tasks can declare which regions of memory
can be inherited by children
No copy-on-write inheritance
External memory managers
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Handles shared read-write from different machines
willing to share memory
Programmer Interface
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System Call flow
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Traps to the kernel
Upcalls into emulation library
Switch to thread waiting on a port
Returns to emulation library
Returns from trap
Calls are slower compared to traditional
systems
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Handling single system call might involve several
IPC exchanges
Each IPC exchange requires trap to kernel &
various scheduler overheads
Summary
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Satisfies critical goals
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Supports multiprocessing & parallel execution
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Multiple threads of execution within one task
Complete & efficient security mechanisms
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Can execute UNIX executables
Supports many memory models + parallel & distributed
computing
Extensible kernel
Messages are the only communication methods
Integrates messages with virtual memory system
Reduced size of kernel – but allows user level
emulation
Backup
History of Mach
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Traces root to Accent developed at CMU
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Initially developed inside 4.2 BSD kernel
Evolved further with 4.3 BSD release
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Mach 2 capabilities made kernel size larger
Mach 3 moved BSB code outside of kernel
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Pioneered several OS concepts
Unable to execute UNIX applications
Difficult to port – tied to HW architecture
Resulted in smaller microkernel
Only basic Mach features in kernel – UNIX code evicted
Allowed execution of multiple OS on top of microkernel
In 1989 OSF used Mach as basis for new OS OSF/1
Mach research continues at CMU and OSF