Operating Systems Course Outline

4. Threads

a light weight process. It contains thread ID, a program counter a register set and a stack. But it only shares with its parent its code section, data section, os resources like open files.
a process consisting of more than one thread. Theoritcally allows for parallel processing. A webserver may create a new process for each connection , but it is less overhead and time consuming to use threads, especially since each thread uses the same code anyway. A web browser could be multi-process or multithreaded. Either way , it would then be able to let the user do several tasks simultaneously.
benefits of multithreading
1. better application responsiveness 2. by default resources are shared allowing simpler IPC than processes 3. Faster creation time than processes 4. allows full utilization of a multi-CPU machine or multi-core machines.
Considerations when designing software to be run in parallel
1.Divide application into part that can be run in parallel. 2.Ensure these part are equally time consuming and valuable to eachother. 3.Make sure data access is not conflicting 4.Synchronized threads that need to be. 5.Determine how to test it.
[User Thread : Kernel thread] Relationship
Kernel threads are managed by the OS, user threads are managed by the program. When a user creates a thread there needs to be a kernel thread in the OS which will correspond to it and actually do the work. The relationship between a number of user threads and the kernel threads can be either many to one, one to one or many to (<=) many. n:1 permits programmer to create as many threads as he desires, but does not provide true parallelism. One blocked thread will block others. 1:1 provides true parallelism but limits programmer to to finite number of threads since each user thread will create one new kernel thread. Best is n:n, then the OS can decide when to create new kernel threads and when not to.
POSIX standardwhich defines, specifies an API for thread creation and synchronization. Many implementations exist, Solaris, Mac, Linux, even third party software for Windows.

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