Operating system 22 threading issues
- 1. Operating System 22 Threading Issues Prof Neeraj Bhargava Vaibhav Khanna Department of Computer Science School of Engineering and Systems Sciences Maharshi Dayanand Saraswati University Ajmer
- 2. Threading Issues • Semantics of fork() and exec() system calls • Signal handling – Synchronous and asynchronous • Thread cancellation of target thread – Asynchronous or deferred • Thread-local storage • Scheduler Activations
- 3. Semantics of fork() and exec() • Does fork()duplicate only the calling thread or all threads? – Some UNIXes have two versions of fork • exec() usually works as normal – replace the running process including all threads
- 4. Signal Handling n Signals are used in UNIX systems to notify a process that a particular event has occurred. n A signal handler is used to process signals 1. Signal is generated by particular event 2. Signal is delivered to a process 3. Signal is handled by one of two signal handlers: 1. default 2. user-defined n Every signal has default handler that kernel runs when handling signal l User-defined signal handler can override default l For single-threaded, signal delivered to process
- 5. Signal Handling (Cont.) n Where should a signal be delivered for multi-threaded? l Deliver the signal to the thread to which the signal applies l Deliver the signal to every thread in the process l Deliver the signal to certain threads in the process l Assign a specific thread to receive all signals for the process
- 6. Thread Cancellation • Terminating a thread before it has finished • Thread to be canceled is target thread • Two general approaches: – Asynchronous cancellation terminates the target thread immediately – Deferred cancellation allows the target thread to periodically check if it should be cancelled • Pthread code to create and cancel a thread:
- 7. Thread Cancellation (Cont.) • Invoking thread cancellation requests cancellation, but actual cancellation depends on thread state • If thread has cancellation disabled, cancellation remains pending until thread enables it • Default type is deferred – Cancellation only occurs when thread reaches cancellation point • I.e. pthread_testcancel() • Then cleanup handler is invoked • On Linux systems, thread cancellation is handled through signals
- 8. Thread-Local Storage • Thread-local storage (TLS) 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) • Different from local variables – Local variables visible only during single function invocation – TLS visible across function invocations • Similar to static data – TLS is unique to each thread
- 9. Scheduler Activations • Both M:M and Two-level models require communication to maintain the appropriate number of kernel threads allocated to the application • Typically use an intermediate data structure between user and kernel threads – lightweight process (LWP) – Appears to be a virtual processor on which process can schedule user thread to run – Each LWP attached to kernel thread – How many LWPs to create? • Scheduler activations provide upcalls - a communication mechanism from the kernel to the upcall handler in the thread library • This communication allows an application to maintain the correct number kernel threads
- 10. Operating System Examples • Windows Threads • Linux Threads
- 11. Windows Threads • Windows implements the Windows API – primary API for Win 98, Win NT, Win 2000, Win XP, and Win 7 • Implements the one-to-one mapping, kernel- level • Each thread contains – A thread id – Register set representing state of processor – Separate user and kernel stacks for when thread runs in user mode or kernel mode – Private data storage area used by run-time libraries and dynamic link libraries (DLLs) • The register set, stacks, and private storage area are known as the context of the thread
- 12. Windows Threads (Cont.) • The primary data structures of a thread include: – ETHREAD (executive thread block) – includes pointer to process to which thread belongs and to KTHREAD, in kernel space – KTHREAD (kernel thread block) – scheduling and synchronization info, kernel-mode stack, pointer to TEB, in kernel space – TEB (thread environment block) – thread id, user-mode stack, thread-local storage, in user space
- 13. Windows Threads Data Structures
- 14. Linux Threads • Linux refers to them as tasks rather than threads • Thread creation is done through clone() system call • clone() allows a child task to share the address space of the parent task (process) – Flags control behavior • struct task_struct points to process data structures (shared or unique)
- 15. Assignment • Explain any two issues related to thread management.