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

CS533 Concepts of Operating Systems, Spring 2011

OUTLINE

IntroductionMACH ArchitectureMotivation System ComponentsProcess ManagementInterprocess CommunicationMemory ManagementProgrammer 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 ?

(Source: http://en.wikipedia.org/wiki/File:OS-structure.svg )

Near minimum amount of software that can provide the following mechanisms needed to implement an OS:low-level address space managementthread managementinter-process communication (IPC) (Source: Wikipedia)

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 ManagementBasic Structure:

•Tasks & ThreadsCreate 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

2nd Call: Thread moved off the run queue till event

Alert: Thread Block

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:

VictimVictimThread

Handler

RPC message: (exception info, thread, task)Wait

routine

Clears exception → Resume/Terminate Victim

Exception Handling Supports BSD style signals BSD expects hardware exceptions as signals Flow:

HardwareExceptions

Exception RPC

MACH exception handling

In-kernel Task

receives

Signal

Exception causingThread (Blocked)

clears

Exception causingThread (Run)

Signal handling code

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: PortsImplemented as protected, bounded queue within the kernel of the system on which object resides

If a queue is full

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

Sender may abort Wait for a slot

Kernelask Deliver message

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 ManagementUser-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