• Threads: Resource ownership and execution
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• Processes have two characteristics:
– Resource ownership – a process includes an address space to hold the process image and may be allocated control or ownership of resources
– Scheduling/execution – process 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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• The unit of dispatching is referred to as a thread or lightweight process
• The unit of resource ownership is referred to as a process or task
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• The ability of an
OS to support multiple, concurrent paths of execution within a single process.
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• MS-DOS supports a single user process and a single thread
• Some UNIX, support multiple user processes but only support one thread per process
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• Java run-time environment is a single process with multiple threads
• Multiple processes and threads are found in Windows, Solaris, and many modern versions of UNIX
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• In a multithreaded environment, a process is defined as
– a unit of resource allocation: a virtual address space which holds the process image
– a unit of protection: protected access to processors, other processes (for IPC), files,
I/O resources
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• Each thread has
– An execution state (running, ready, etc.)
– Saved thread context when not running
– 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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• One way to view a thread is as an independent program counter operating
within a process.
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• The thread control block contains register values, priority, and other thread-related state information.
• All threads of a process share the state and resources of that process.
– reside in the same address space
– have access to the same data
• when one thread alters a data, other threads see the results
• when one thread opens a file, other threads can also access that file
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• Takes less time to create a new thread than a process (can be 10 times faster)
• Less time to terminate a thread than a process
• Switching between two threads takes less time that switching processes
• Threads can communicate with each other
– without invoking the kernel
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• If an application is implemented as a set of related units of execution, it is far more efficient to do so as a collection of threads rather than a collection of separate processes
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• In a file server, a new thread can be spawned for the file management program per each new file request  multiple threads within the same process can be executing simultaneously on different processors
• In a spreadsheet program, one thread could read user input while another thread executes user commands
• In a word processor, a thread can be created to do periodic backup asynchronously
• In a multithreaded process, one thread can compute one batch of data while another thread reads the next batch from an I/O device
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• Like processes, threads have execution states
– Running, Ready, and Blocked
• Some states are process-level
– Suspend: if a process is swapped out, all of its threads are necessarily swapped out because they all share the address space of the process
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• Like processes, thread may synchronize with one another.
– Any alteration of a resource by one thread affects other threads in the same process
• consider two threads each try to add an element to a linked list at the same time
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• Consider:
– A program that performs two remote procedure calls (RPCs)
• to two different hosts
• to obtain a combined result.
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The program has to wait for a response from each server in turn.
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The program waits concurrently for the two replies.
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Interleaving of multiple threads within multiple processes on a uniprocessor
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