1. THREADS CSC 8320 Advanced Operating Systems Neelima Jullapelli

2. Contents • Introduction to threads • Characteristics of Threads • Advantages of Threads • User Threads • Kernel Threads • Threads in distributed environment • Programming with Threads • RPCS and Threads

3. Two types of processes • “Heavyweight” process normal process that does not share memory with other processes • “Lightweight” process: Threads memory is shared with other processes maintains own state ,program counter, stack • A Thread is dispatchable unit of work executes sequentially and is interruptable. • A process is collection of one or more threads.

4. Thread • Characteristics All threads have exactly the same address space - Therefore has access to same global variables - can also wipe out other thread’s work(overwrite variables,etc) - All threads share the same set of open files, child processes, timers , and signals ,semaphores, ports etc. - Threads can be in any one of several states: ready, running, blocked or terminated

5. Advantages of Threads • Sharing data is easy -reduction in interprocess communication overhead -just synchronize critical sections in shared data • If the server had multiple threads of control, a second thread could run while the first one was sleeping the net result would be higher throughput and better performance. • Ease of programming -only one address space to worry about

6. User Threads • Thread package runs on top of kernel - referred to as ‘user-space runtime system’ - kernel maintains processes -user program maintains threads the runtime system • Blocking system calls -if a blocking system call goes to kernel the kernel suspends the process -wants to call another thread when one becomes blocked use non-blocking calls (some OS’s don't support these) Unix select()- use instead of read –if read() would block, call thread manager instead

7. User threads (contd…) • Preemption of processes - no good way to perform preemptive scheduling clock interrupts are at process level -thread must voluntarily give up CPU -can lead to deadlock for example waiting for a semaphore with a blocking system call. • Advantages - can run threads on process-oriented systems(like unix) -each process can have customised scheduling algorithm -context switch is minimized

8. Kernel Threads • Kernel schedules threads not processes -when a thread blocks,kernel can schedule: another thread from the same process A thread from different process -context switch is more expensive kernel has to maintain process tables with thread entries • user-level problems with page faults, deadlock don’t occur

9. Threads in distributed Environment • Multiprocessors -task environment located in shared memory -threads can run in parallel on different CPU’S -just need to synchronize shared memory access • Threads for the client/server model -each client makes a request to the server -server executes identical procedure for all calls -threads reduce overhead -make server program easier

10. Programming with threads • Thread management -static threads: number of threads fixed -dynamic threads: threads can be created and destroyed at runtime. • Mutex -essentially a binary semaphore. -in C threads package: a)mutex_lock(mutexId) b)mutex_unlock(mutexId) -deadloack can occur with semaphore locks.

11. Cont….. • Condition variables (CV) -use for long term waiting -using signals on CV reduces probablity of deadlock. -condition_wait(conditionId, mutexId) a) queue request on conditionId b) then mutex is released. -condition_signal(conditionId) a)if a thread is queued in conditionId, unlock it.

12. RPC’S and threads • Use shared memory to pass parameters,results -when started kernel given interfaces for both client and server. -kernel creates an argument stack shared by both -when client calls server client puts info into stack,trap to kernel. Server reads directly from it address spaceand the results are passed in the same manner • Pop-up threads the thread that is created spontaneously to handle an incoming RPC is occasionally referred to as pop-up thread

13. Current research • Dynamic thread resizing for speculative multithreaded processorsZahran, M.; Franklin, M.;Computer Design, 2003. Proceedings. 21st International Conference • Dynamically Controlled Resource Allocation in SMT ProcessorsCazorla, F.J.; Ramirez, A.; Valero, M.; Fernandez, E.;Microarchitecture, 2004. MICRO-37 2004. 37th International Symposium on04-08 Dec. 2004 Page(s):171 - 182

14. References: • Distributed operating systems by Andrew s Tanenbaum • Operating system concepts by silberscatz Galvin • http://ieeexplorer.ieee.org