Threads CSSE 332 Operating Systems Rose-Hulman Institute of Technology 1

Announcements Lab 2 due tonight at 11:59 PM Project kick-off is on 21 December Exam 1 is on 10 January First 4 chapters and chapter 6 from text C Programming, makefiles, debugging Shared memory, fork() & exec(), Pthreads 2

Thread libraries Thread library provides programmer with API for creating and managing threads Two primary ways of implementing Library entirely in user space Kernel-level library supported by the OS 3

Pthreads May be provided either as user-level or kernel-level A POSIX standard (IEEE 1003.1c) API for thread creation and synchronization API specifies behavior of the thread library, implementation is up to development of the library Common in UNIX operating systems (Solaris, Linux, Mac OS X) Explore the code from examples/threadsExample.c with class. Point students to threads links on resource page. 4

Java threads Java threads are managed by the JVM Typically implemented using the threads model provided by underlying OS Java threads may be created by: Extending Thread class Implementing the Runnable interface There is also the win32 threads library, but we will not discuss this right now. 5

Threading issues Semantics of fork() and exec() system calls Thread cancellation of target thread Asynchronous or deferred Signal handling Thread pools Thread-specific data Scheduler activations 6

Semantics of fork() and exec() If one thread calls fork(), does fork() duplicate only the calling thread or all threads? Some UNIX systems have 2 version of fork(), one that duplicates and one that does not. If the new process calls exec(), the program specified in the parameter to exec() will replace the entire process --- including all the threads. If exec is called immediately after fork() then only the calling thread needs to be replaced. If the new process does not call exec(), then all the threads should be duplicated. 7

Thread Cancellation Terminating a thread before it has finished Two general approaches: Asynchronous cancellation terminates the target thread immediately Deferred cancellation allows the target thread to periodically check if it should be cancelled Asynchronous cancellation: one thread cancels the target thread immediately. Why cancel threads? If several threads are searching a database and one thread finds the result, the other threads should be cancelled. A user presses his/her browser’s STOP button. All the threads that are loading images and text must be cancelled. 8

Signal Handling Signals are used in UNIX systems to notify a process that a particular event has occurred A signal handler is used to process signals Signal is generated by particular event Signal is delivered to a process Signal is handled A signal may be received either synchronously or asynchronously, depending on the source of and reason for the event being signaled. All signals follow the same pattern (see signal handler bullet). Examples of synchronous signals include memory access and division by zero. Some texts refer to these as exceptions or traps. Synchronous signals are received by the same process that performed the operation that caused the signal. Asynchronous signals are generated by events outside the process. E.g., Ctrl + C to terminate the process and when a timer expires. These external signals are also called interrupts. 9

Signal Handling options Deliver the signal to the thread to which the signal applies Deliver the signal to every thread in the process Deliver the signal to certain threads in the process Assign a specific thread to receive all signals for the process HIGHLIGHT THESE NOTES FIRST Every signal has a default signal handler that is run by the kernel when handling that signal. That default handler can be overwritten by a user defined signal handler that is called to handle the signal. You will get to write user defined signal handlers during lab 4. Signals are handled in different ways: Some are ignored Others are handled by terminating the program. To whom is a signal delivered in a multithreaded process? There is only one option in a single threaded program. In general, the following options exist. 10

Thread Pools Create a number of threads in a pool where they await work Advantages: Usually slightly faster to service a request with an existing thread than create a new thread Allows the number of threads in the application(s) to be bound to the size of the pool 11

Thread Specific Data Allows each thread to have its own copy of data Useful when you do not have control over the thread creation process (i.e., when using a thread pool) As opposed to sharing the global data of a process. A transaction processing system is an example of when thread specific data would be important. Associate each thread with a transaction. Most thread libraries, including Win32 and Pthreads provide some support for thread specific data. 12

Scheduler Activations Both M:M and Two-level models require communication to maintain the appropriate number of kernel threads allocated to the application Scheduler activations provide upcalls - a communication mechanism from the kernel to the thread library This communication allows an application to maintain the correct number of kernel threads The communication spoken of here is the communication between the kernel and the thread library. That coordination is accomplished by an intermediate data structure called a light-weight process, which acts as a virtual CPU for the user level thread. 13

Light weight process Text also talks about how threads are implemented in Windows XP and LINUX systems. 14