Operating Systems: Internals and Design Principles, 6/E William Stallings Chapter 4 Threads Patricia Roy Manatee Community College, Venice, FL 2008, Prentice Hall
Processes and Threads 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
Processes and Threads Dispatching is referred to as a thread or lightweight process Resource ownership is referred to as a process or task Multithreading To support multiple, concurrent paths of execution within a single process
Multithreading MS-DOS supports a single user process and thread Some variants of UNIX support multiple user processes but only one thread per process Java run-time environment is a single process with multiple threads Windows, Solaris, and other modern
versions of Unix support multiple processes and multiple threads per process Threads and Processes Processes A virtual address space which holds the process image Protected access to processors, other processes (for interprocess
communication), files, and I/O resources One or More Threads in Process An execution state (running, ready, etc.) Saved thread context when not running An execution stack One or More Threads in Process
Some per-thread static storage for local variables Access to the memory and resources of its process all threads of a process share this Threads Uses of Threads in a SingleUser Multiprocessing System Modular program structure
Foreground and background work One thread for user-interface, another for data processing Speed of execution Asynchronous processing For example, a thread to do backup for a word processor Remote Procedure Call Using
Single Thread Download and display a web page in a web browser RPC Using One Thread per Server Benefits of Threads Takes less time to create a new thread
than a process Experiment shows that it is 10 times faster Less time to terminate a thread than a process Less time to switch between two threads within the same process No memory reallocation is involved Benefits of Threads
Since threads within the same process share memory and files, they can communicate with each other without invoking the kernel 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 Thread States States associated with a change in thread state Spawn Spawn another thread Block
Unblock Finish Deallocate register context and stacks Thread Implementation - Packages Threads are provided as a package, including operations to create, destroy, and synchronize them
A package can be implemented as: User-level threads Kernel threads User-Level Threads All thread management is done by the application
The kernel is not aware of the existence of threads User-Level Threads User-Level Threads
Thread library entirely executed in user mode Kernel is not involved! Cheap to manage threads Create: setup a stack Destroy: free up memory Cheap to do context switch Just save CPU registers
Done based on program logic A blocking system call blocks all peer threads Kernel-Level Threads
Kernel is aware of and schedules threads A blocking system call, will not block all peer 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 Kernel-Level Threads Kernel-Level Threads
Kernel is aware of and schedules threads A blocking system call, will not block all peer threads More expensive to manage threads More expensive to do context switch
Kernel intervention, mode switches are required Thread/Process Operation Latencies processes user-level threads kernel-level threads
null fork 34 usec 948 usec 11,300 usec signal-wait
37 usec 441 usec 1,840 usec Operation User vs. Kernel-Level Threads Users-level threads
Cheap to manage and to do context switch A blocking system call blocks all peer threads Kernel-level threads A blocking system call will not block all peer threads Expensive to manage and to do context switch Light-Weight Processes (LWP)
Support for hybrid (user-level and kernel) threads, example is Solaris A process contains several LWPs In addition, the system provides user-level threads
Developer: creates multi-threaded applications System: Maps threads to LWPs for execution Thread Implementation LWP Combining kernel-level lightweight processes and userlevel threads Thread Implementation LWP
Each LWP offers a virtual CPU LWPs are created by system calls They all run the scheduler, to schedule a thread
Thread table is kept in user space Thread table is shard by all LWPs LWPs switch context between threads Thread Implementation LWP
When a thread blocks, LWP schedules another ready thread Thread context switch is completely done in user mode When a thread blocks on a system call, execution mode changes from user to kernel but continues in the context of the current LWP When current LWP can no longer
execute, context is switched to another LWP Thread Implementation LWP Combining kernel-level lightweight processes and userlevel threads LWP Features
Cheap thread management A blocking system call may not suspend the whole process LWPs are transparent to the application LWPs can be easily mapped to different CPUs Managing LWPs is expensive (like kernel
threads) A Short Survey On awareness of CSE Bits & Bytes
