This chapter examines some more advanced concepts related to process management, which are found in a number of contemporary operating systems. We show that the concept of process is more complex and subtle than presented so far and in fact embodies two separate and potentially independent concepts: one relating to resource ownership and another relating to execution.
Chapter Threads • Threads: Resource ownership and execution Processes and Threads • 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 Processes and Threads • 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 Multithreading • The ability of an OS to support multiple, concurrent paths of execution within a single process Single Thread Approaches • MS-DOS supports a single user process and a single thread • Some UNIX, support multiple user processes but only support one thread per process Multithreading • 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 Processes • 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 One or More Threads in Process • 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) One view… • One way to view a thread is as an independent program counter operating within a process Threads vs processes 10 Threads vs processes • 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 11 Benefits of Threads • 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 12 Benefits of Threads • If an application is implemented as a set of related units of execution, it is far more efficient to so as a collection of threads rather than a collection of separate processes 13 Example Applications of Threads • 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 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 14 Threads vs processes • 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 15 Threads vs processes • 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 16 Example: Remote Procedure Call • Consider: – A program that performs two remote procedure calls (RPCs) • to two different hosts • to obtain a combined result 17 RPC Using Single Thread The program has to wait for a response from each server in turn 18 RPC Using One Thread per Server The program waits concurrently for the two replies 19 Multithreading on a Uniprocessor Interleaving of multiple threads within multiple processes on a uniprocessor 20 ... its process (all threads of a process share this) One view… • One way to view a thread is as an independent program counter operating within a process Threads vs processes 10 Threads vs processes... device 14 Threads vs processes • 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. .. 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 11 Benefits of Threads • Takes less time to create a new