ThreadsCS-502 Fall Threads CS-502 Operating Systems (Slides include materials from Operating System Concepts, 7 th ed., by Silbershatz, Galvin, & Gagne and from Modern Operating Systems, 2 nd ed., by Tanenbaum)

ThreadsCS-502 Fall Processes – Review Fundamental abstraction in all operating systems Implementation and management of concurrent activity Also known as task, job, thread of control, etc.

ThreadsCS-502 Fall Processes – Review (continued) Process state –information maintained by OS for representing process, in PCB PSW, registers, condition codes, etc. Memory, files, resources, etc. Priority, blocking status, etc. Queues Ready Queue Semaphore queues Other kinds of queues not yet covered (e.g., for disks, communication resources, etc.)

ThreadsCS-502 Fall Processes – Review (continued) Interrupts and traps Switching contexts Saving state of one process Loading state of another process Scheduling Deciding which process to run (or serve) next More later this evening Interprocess Communication Later in the course

ThreadsCS-502 Fall Processes – Review (continued) Process creation Fork + Exec (Unix-Linux) Create Process (Windows) Relationships among processes Parent-child (Unix-Linux) No default relationship (Windows) Process termination Wait for child processes, etc.

ThreadsCS-502 Fall Questions?

ThreadsCS-502 Fall Problem – Unix/Windows Processes are “heavyweight” Lots of data in process context Even more when we study memory management More than that when we study file systems, etc. Processor caches a lot of information Memory Management information Caches of active pages Costly context switches and traps 100’s of microseconds

ThreadsCS-502 Fall Problem – Heavyweight Unix/Windows Processes (continued) Separate processes have separate address spaces Shared memory is limited or nonexistent Applications with internal concurrency are difficult Isolation between independent processes vs. cooperating activities Fundamentally different goals

ThreadsCS-502 Fall Example Web Server – How to support multiple concurrent requests One solution: –create several processes that execute in parallel –Use shared memory ( shmget() ) to map to the same address space in the processes –have the OS schedule them in parallel Not efficient –space: PCB, page tables, etc. –time: creating OS structures ( fork() ) and context switch

ThreadsCS-502 Fall Example 2 Transaction processing systems E.g, airline reservations or bank ATM transactions 1000’s of transactions per second Very small computation per transaction Separate processes per transaction are too costly Other techniques (e.g., message passing) are much more complex

ThreadsCS-502 Fall This problem … … is partly an artifact of Unix, Linux, and Windows and of Big, powerful processors (e.g., Pentium 4) … tends to occur in most large systems … is infrequent in small-scale systems PDAs, cell phones, hand-held games Closed systems (i.e., controlled applications)

ThreadsCS-502 Fall Solution:– Threads A thread is the execution of a program or procedure within the context of a Unix or Windows process I.e., a specialization of the concept of process A thread has its own Program counter, registers, PSW Stack A thread shares Address space, heap, static data All other resources with all other threads of the same process

ThreadsCS-502 Fall Threads 0x xFFFFFFFF Virtual address space code (text) static data heap thread 1 stack PC (T2) SP (T2) thread 2 stack thread 3 stack SP (T1) SP (T3) PC (T1) PC (T3) SP PC

ThreadsCS-502 Fall Single and Multithreaded Processes

ThreadsCS-502 Fall Benefits Responsiveness Resource Sharing Economy Utilization of multi-processor architectures

ThreadsCS-502 Fall Threads Linux, Windows, and various versions of Unix have their own thread interfaces Similar, not standardized Some issues E.g., ERRNO in Unix — a static variable set by system calls Semantics of fork() and exec()

ThreadsCS-502 Fall Who creates and manages threads? User-level implementation –done with function library (e.g., POSIX) –Runtime system – similar to process management except in user space –Windows NT – fibers: a user-level thread mechanism Kernel implementation – new system calls and new entity to manage –Linux: lightweight process (LWP) –Windows NT & XP: threads

ThreadsCS-502 Fall User Threads Thread management done by user-level threads library Three primary thread libraries: – POSIX Pthreads – Win32 threads – Java threads

ThreadsCS-502 Fall User Threads (continued) Can be implemented without kernel support … or knowledge! Program links with a runtime system that does thread management Operation are very efficient (procedure calls) Space efficient and all in user space (TCB) Task switching is very fast Since kernel not aware of threads, there can be scheduling inefficiencies E.g., blocking I/O calls Non-concurrency of threads on multiple processors

ThreadsCS-502 Fall User Threads (continued) Thread Dispatcher –Queues in process memory to keep track of threads’ state Scheduler – non-preemptive –Assume threads are well-behaved –Thread voluntarily gives up CPU by calling yield() – does thread context switch to another thread Scheduler – preemptive –Assumes threads may not be well-behaved –Scheduler sets timer to create a signal that invokes scheduler –Scheduler can force thread context switch –Increased overhead Application or thread library must handle all concurrency itself!

ThreadsCS-502 Fall Thread Interface E.g., POSIX pthreads API: –int pthread_create(pthread_t *thread, const pthread_attr_t *attr, void*(*start_routine) (void), void *arg) creates a new thread of control new thread begins executing at start_routine –pthread_exit(void * value_ptr) terminates the calling thread –pthread_join(pthread_t thread, void **value_ptr) blocks the calling thread until the thread specified terminates –pthread_t pthread_self() Returns the calling thread's identifier

ThreadsCS-502 Fall Kernel Threads Supported by the Kernel OS maintains data structures for thread state and does all of the work of thread implementation. Examples Windows XP/2000 Solaris Linux Tru64 UNIX Mac OS X

ThreadsCS-502 Fall Kernel Threads (continued) OS schedules threads instead of processes Benefits –Overlap I/O and computing in a process –Creation is cheaper than processes –Context switch can be faster than processes Negatives –System calls (high overhead) for operations –Additional OS data space for each thread

ThreadsCS-502 Fall Threads – supported by processor E.g., Pentium 4 with Hyperthreading™ Multiple CPU’s on a single chip True concurrent execution within a single process Requires kernel support Re-opens old issues Deadlock detection Critical section management of synchronization primitives

ThreadsCS-502 Fall Unix Processes vs. Threads On a 700 Mhz Pentium running Linux –Processes: fork()/exit () : 250 microsec –Kernel threads: pthread_create()/pthread_join(): 90 microsec –User-level threads: pthread_create()/pthread_join(): 5 microsec

ThreadsCS-502 Fall Models for Implementation Many-to-One One-to-One Many-to-Many

ThreadsCS-502 Fall Many-to-One Many user-level threads mapped to single kernel thread Examples: Solaris Green Threads GNU Portable Threads

ThreadsCS-502 Fall Many-to-One Model

ThreadsCS-502 Fall One-to-One Each user-level thread maps to kernel thread Examples Windows NT/XP/2000 Linux Solaris 9 and later

ThreadsCS-502 Fall One-to-one Model

ThreadsCS-502 Fall Many-to-Many Model Allows many user level threads to be mapped to many kernel threads Allows the operating system to create a sufficient number of kernel threads Solaris prior to version 9 Windows NT/2000 with the ThreadFiber package

ThreadsCS-502 Fall Many-to-Many Model

ThreadsCS-502 Fall Two-level Model Similar to M:M, except that it allows a user thread to be bound to kernel thread Examples IRIX HP-UX Tru64 UNIX Solaris 8 and earlier

ThreadsCS-502 Fall Two-level Model

ThreadsCS-502 Fall Threading Issues Semantics of fork() and exec() system calls for processes Thread cancellation Signal handling Thread pools Thread specific data Scheduler activations

ThreadsCS-502 Fall Semantics of fork() and exec() Does fork() duplicate only the calling thread or all threads? –Easy if user-level threads –Not so easy with kernel-level threads Linux has special clone() operation Windows XP has something similar

ThreadsCS-502 Fall 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

ThreadsCS-502 Fall 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 1.Signal is generated by particular event 2.Signal is delivered to a process 3.Signal is handled 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

ThreadsCS-502 Fall 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

ThreadsCS-502 Fall 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)

ThreadsCS-502 Fall 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 kernel threads

ThreadsCS-502 Fall Mutual Exclusion within Threads extern void thread_yield(); extern int TestAndSet(int &i); /* sets the value of i to 1 and returns the previous value of i. */ void enter_critical_region(int &lock) { while (TestAndSet(lock) == 1) thread_yield(); /* give up processor */ }; void leave_critical_region(int &lock) { lock = 0; };

ThreadsCS-502 Fall Threads inside the OS kernel Kernels have evolved into large, multi- threaded programs. Lots of concurrent activity Multiple devices operating at one time Multiple application activities at one time A useful tool Special kernel thread packages, synchronization primitives, etc. Useful for complex OS environments

ThreadsCS-502 Fall Threads – Summary Processes are very heavyweight in Unix, Linux, Windows, etc. Need for isolation between processes at odds with desire for concurrency within processes Threads provide an efficient alternative Thread implementation and management strategies depend upon expected usage Kernel support or not Processor support or not

ThreadsCS-502 Fall Break Next topic

ThreadsCS-502 Fall IA-32 Intel® Architecture Software Developer’s Manual (Figure 2-1)