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Threads, SMP, and Microkernels

Chapter 4

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Process

Resource ownership: process includes a virtual address space to hold the process image (fig 3.16)

Scheduling/execution: follows an execution path that may be interleaved with other processes

These two characteristics can be treated independently by the operating system

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Process

Unit of dispatching is referred to as a thread or lightweight process

Unit of resource ownership is referred to as a process or task

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Multithreading

Operating system supports multiple threads of execution within a single process

MS-DOS supports a single thread UNIX supports multiple user processes but

only supports one thread per process Windows, Solaris, Linux, Mach, and OS/2

support multiple threads

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Process

Have a virtual address space which holds the process image

Protected access to processors, other processes, files, and I/O resources

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Thread

An execution state (running, ready, etc.) Saved thread context when not running Has an execution stack Some per-thread static storage for local

variables Access to the memory and resources of its

processall threads of a process share this

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Benefits of Threads

Takes less time to create a new thread than a process

Less time to terminate a thread than a process Less time to switch between two threads within

the same process Since threads within the same process share

memory and files, they can communicate with each other without invoking the kernel

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Uses of Threads in a Single-User Multiprocessing System Foreground to background work Asynchronous processing Speed of execution Modular program structure

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Threads

Suspending a process involves suspending all threads of the process since all threads share the same address space

Termination of a process, terminates all threads within the process

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Thread States

States associated with a change in thread stateSpawn

Spawn another thread

BlockUnblockFinish

Deallocate register context and stacks

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Remote Procedure Call Using Single Thread

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Remote Procedure Call Using Threads

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Multithreading

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Adobe PageMaker

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User-Level Threads

All thread management is done by the application

The kernel is not aware of the existence of threads

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User-Level Threads

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Advantages of User-Level Threads

Thread-switching does not require kernel mode privileges and switching

Thread scheduling can be tailored to application

ULTs can run on any OS. They can be implemented with a thread library that runs within the app.

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Disadvantages of ULTs

If blocking system call (eg IO call), whole process blockedcan use ‘jacketing’ – call other process to

check state of resource Cannot use multiprocessing since OS

sees all threads as a single process.

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Kernel-Level Threads

Windows is an example of this approach Kernel maintains context information for

the process and the threads Scheduling is done on a thread basis

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Kernel-Level Threads

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VAX Running UNIX-Like Operating System

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Combined Approaches

Example is Solaris Thread creation done in the user space Bulk of scheduling and synchronization of

threads within application

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Combined Approaches

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Relationship Between Threads and Processes

1:M and M:N approaches are experimental.

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Symmetric Multiprocessing (SMP)

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Symmetric Multiprocessing

Tradition: Single sequence of instructionsvon Neumann machine concept

Real machines do instruction pipelining, DMA and other parallel operationsPipeline explanation and movieHyperthreading

With cheaper HW can extend this to multiple processors

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Categories of Computer Systems Single Instruction Single Data (SISD) stream

Single processor executes a single instruction stream to operate on data stored in a single memory.

‘Normal’ computing

Single Instruction Multiple Data (SIMD) stream Each instruction is executed on a different set of data

by the different processors. Eg DSP or vector processing

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Categories of Computer Systems Multiple Instruction Single Data (MISD) stream

A sequence of data is transmitted to a set of processors, each of which executes a different instruction sequence.

Never implemented!

Multiple Instruction Multiple Data (MIMD) A set of processors simultaneously execute different

instruction sequences on different data sets

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Symmetric Multiprocessing

Kernel can execute on any processor Typically each processor does self-

scheduling from the pool of available process or threads

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Multiprocessor Operating System Design Considerations Simultaneous concurrent processes or threads

Re-entrant kernel code, deadlock, etc. Scheduling (see ch 10 for more detail) Synchronization

Mutual exclusion, event ordering, locks etc (ch 5) Memory management

Coordination in paging systems when multiple processors accessing pages (ch 7)

Reliability and fault tolerance Graceful degradation on processor failure

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Distributed Processing

Another alternative is distributed processing

Each node has its own CPU, memory and communication channelSlower communications, more independence

EG. Cluster computing, Beowulf etc. See Ch 14, 15

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Microkernels

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Microkernels

Small operating system core (how small?)

Concept: Contains only essential core operating systems functions

Many services traditionally included in the operating system are now external subsystems Device drivers File systems Virtual memory manager Windowing system Security services

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Benefits of a Microkernel Organization Uniform interface on request made by a process

Don’t distinguish between kernel-level and user-level services

All services are provided by means of message passing

Extensibility Allows the addition of new services

Flexibility New features added Existing features can be subtracted

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Benefits of a Microkernel Organization Portability

Changes needed to port the system to a new processor is changed in the microkernel - not in the other services

ReliabilityModular designSmall microkernel can be rigorously tested

Exhaustive testing vs. module testing

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Benefits of Microkernel Organization Distributed system support

Message are sent without knowing what the target machine is

Object-oriented operating systemComponents are objects with clearly defined

interfaces that can be interconnected to form software

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Microkernel Design

Low-level memory management Mapping each virtual page to a physical page frame

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Microkernel Design

Interprocess communication I/O and interrupt management

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MS-Windows Processes

Implemented as objects An executable process may contain one or

more threads Both processes and thread objects have

built-in synchronization capabilities

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Windows Process Object

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Windows Thread Object

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MS-Windows Thread States

Ready Standby Running Waiting Transition Terminated

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Solaris

Process includes the user’s address space, stack, and process control block

User-level threads Lightweight processes (LWP) Kernel threads

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Solaris Lightweight Data Structure

Identifier Priority Signal mask Saved values of user-level registers Kernel stack Resource usage and profiling data Pointer to the corresponding kernel thread Pointer to the process structure

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Linux Task Data Structure State Scheduling information Identifiers Interprocess communication Links Times and timers File system Address space Processor-specific context

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Linux States of a Process

Running Interruptible Uninterruptible Stopped Zombie

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Linux Threads

Traditionally no threadsULTs with pthread library

Modern Linux supports KLT Process = thread (cloning)