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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)