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Threads, SMP, and Microkernels

Chapter 4

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Process: Some Info.• Motivation for threads!• Two fundamental aspects of a “process”:

• Resource ownership• Scheduling or execution

• Resource ownership - process includes a virtual address space to hold the process image

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

• These two characteristics are treated independently by the operating system

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Process• Dispatching is referred to as a thread or

lightweight process• Resource of ownership is referred to as a

process or task• So far, we have considered processes that had

just one thread: • A “process” or a “process with one thread” are kind of

similar!

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Multithreading• An attempt to define 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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What exactly is a Process now?

• 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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What exactly is a Thread now?• 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

process• All 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:• In GUI-based applications, one thread can make

display and the other can process input

• Asynchronous processing: • Daemons or services in OSes

• Speed of execution: • Make even better use of processor, especially in

multiprocessor systems

• Modular program structure

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Threads: What happens to state models?

• 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 state

• Spawn• Spawn another thread

• Block• Unblock• Finish

• 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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TOP Command

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PSTREE Command

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/proc directory on Linux

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Details of Process 2114

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Adobe PageMaker

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Categories of Thread Implementation

• User-level threads• Kernel-level threads

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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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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

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Flynn’s Taxonomy• There is no authoritative classification of parallel computers! • Flynn’s taxonomy is one such classification based on number of

instruction and data stream processed by a parallel computer: • Single Instruction Single Data (SISD)• Multiple Instruction Single Data (MISD)• Single Instruction Multiple Data (SIMD)• Multiple Instruction Multiple Data (MIMD)

• Almost all modern computers fall in this category

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Flynn’s Taxonomy• Extensions to Flynn’s taxonomy:

• Single Program Multiple Data (SPMD)—a programming model

• This classification is largely outdated!

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Symmetric Multi-Processors (SMP)• A SMP is a parallel processing system with a

shared-everything approach:• The term signifies that each processor shares the

main memory and possibly the cache

• Typically a SMP can have 2 to 256 processors• Examples include AMD Athlon, AMD Opteron

200 and 2000 series, Intel XEON etc

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Shared Memory• All processors have access to shared memory:

• Notion of “Global Address Space”

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Multiprocessor Operating System Design Considerations

• Simultaneous concurrent processes or threads: • Executing same kernel code should not result in

erroneous behavior

• Scheduling: • Opportunities exist to execute multiple threads of the

same process

• Synchronization• Memory management• Reliability and fault tolerance

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Microkernels• Small operating system core• 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• Extensibility• Flexibility• Portability• Reliability• Distributed System Support• Object-oriented Operating System

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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