COP 4600 Operating Systems Spring 2011

1. COP 4600 Operating Systems Spring 2011 Dan C. Marinescu Office: HEC 304 Office hours: Tu-Th 5:00-6:00 PM

2. Last time: Virtualization for the three abstractions Threads Virtual Memory Bounded buffer The kernel of an operating syste Today: Threads State Processor switching Semaphores Deadlocks Next time Communication with a bounded buffer Lecture 16 – Tuesday, March 22, 2011 Lecture 16

3. Thread and VM management – virtual computer • The kernel supports thread and virtual memory management • Thread management: • Creation and destruction of threads • Allocation of the processor to a ready to run thread • Handling of interrupts • Scheduling – deciding which one of the ready to run threads should be allocated the processor • Virtual memory management  maps virtual address space of a thread to physical memory. • Each module runs in own address space; if one module runs multiple threads all share one address space. • Thread + virtual memory  virtual computer for each module. Lecture 16

4. Threads and the Thread Manager • Thread  virtual processor - multiplexes a physical processor • a module in execution; • a module may have several threads. • sequence of operations: • Load the module’s text • Create a thread and lunch the execution of the module in that thread. • Scheduler system component which chooses the thread to run next • Thread manager implements the thread abstraction. • Interrupts processed by the interrupt handler which interacts with the thread manager • Exception  interrupts caused by the running thread and processed by exception handlers • Interrupt handlers run in the context of the OS while exception handlers run in the context of interrupted thread. Lecture 16

5. The state of a thread; kernel versus application threads • Thread state: • Thread Id  unique identifier of a thread • Program Counter (PC) -the reference to the next computational step • Stack Pointer (SP) • PMAR – Page Table Memory Address Register • Other registers • Threads • User level threads/application threads – threads running on behalf of users • Kernel level threads – threads running on behalf of the kernel • Scheduler thread – the thread running the scheduler • Processor level thread – the thread running when there is no thread ready to run Lecture 16

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7. Virtual versus real; the state of a processor • Virtual objects need a physical support. • In addition to threads we should be concerned with the processor or cores running a thread. • A system may have multiple processors and each processor may have multiple cores • The state of the processor or core: • Processor Id/Core Id  unique identifier of a processor /core • Program Counter (PC) -the reference to the next computational step • Stack Pointer (SP) • PMAR – Page Table Memory Address Register • Other registers Lecture 16

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9. Processor and thread tables – control structures that must be locked for serialization Lecture 16

10. Primitives for processor virtualization Lecture 16

11. Thread states and state transitions Lecture 16

12. The state of a thread and its associated virtual address space Lecture 16

13. Switching the processor from one thread to another • Thread creation: thread_idALLOCATE_THREAD(starting_address_of_procedure, address_space_id); • YIELD  function implemented by the kernel to allow a thread to wait for an event. • Save the state of the current thread • Schedule another thread • Start running the new thread – dispatch the processor to the new thread • YIELD • cannot be implemented in a high level language, must be implemented in the machine language. • can be called from the environment of the thread, e.g., C, C++, Java • allows several threads running on the same processor to wait for a lock. It replaces the busy wait we have used before. Lecture 16

14. Implementation of YIELD • Apply the principle of least astonishment; deal first with a simpler case: • A fixed number of threads (7). • All threads run on the same core and in the same address space. When switching states we do not need to update the PMAR (Page Memory address Register) • When we enter the processor layer • change the state of the current thread from RUNNING to RUNNABLE • Save the stack pointer in the thread table • Invoke the scheduler • The SCHEDULER • Searches the thread table for a thread in RUNNABLE state and changes its state to RUNNING • Update the processor table to mark this thread as running on that processor • EXITS the Processor layer Lecture 16

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