COT 5611 Operating Systems Design Principles Spring 2012

1. COT 5611 Operating SystemsDesign Principles Spring 2012 Dan C. Marinescu Office: HEC 304 Office hours: M-Wd 5:00-6:00 PM

2. Lecture 19 – Wednesday March 21, 2012 • Reading assignment: • Chapter 9 from the on-line text • Last time – • All-or-nothing and before-or after atomicity • Atomicity and processor management • Processes, threads, and address spaces • Thread coordination with a bounded buffer – the naïve approach • Thread management • Address spaces and multi-level memories • Kernel structures for the management of multiple cores/processors and threads/processes Lecture 19

3. Today • Locks and before-or-after actions; hardware support for locks • YIELD • Conditions for thread coordination – Safety, Liveness, Bounded-Wait, Fairness • Critical sections – a solution to critical section problem • Deadlocks • Signals • Semaphores • Monitors • Thread coordination with a bounded buffer. • WAIT • NOTIFY • AWAIT • ADVANCE • SEQUENCE • TICKET Lecture 19

4. Locks; Before-or-After actions • Locks shared variables which acts as a flag to coordinate access to a shared data. Manipulated with two primitives • ACQUIRE • RELEASE • Support implementation of before-or-after actions; only one thread can acquire the lock, the others have to wait. • All threads must obey the convention regarding the locks. • The two operations ACQUIRE and RELEASE must be atomic. • Hardware support for implementation of locks • RSM – Read and Set Memory • CMP –Compare and Swap • RSM (mem) • If mem=LOCKED then RSM returns r=LOCKED and sets mem=LOCKED • If mem=UNLOCKED the RSM returns r=LOCKED and sets mem=LOCKED Lecture 19

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7. Important facts to remember • Each thread has a unique ThreadId • Threads save their state on the stack. • The stack pointer of a thread is stored in the thread table. • To activate a thread the registers of the processor are loaded with information from the thread state. • What if no thread is able to run  • create a dummy thread for each processor called a processor_thread which is scheduled to run when no other thread is available • the processor_thread runs in the thread layer • the SCHEDULER runs in the processor layer • We have a processor thread for each processor/core. • We can use spin locks only if the two processes (the producer and the consumer) run on different CPUs; we need an active process to release a spin lock…. Lecture 19

8. Switching threads with dynamic thread creation • Switching from one user thread to another requires two steps • Switch from the thread releasing the processor to the processor thread • Switch from the processor thread to the new thread which is going to have the control of the processor • The last step requires the SCHEDULER to circle through the thread_tableuntil a thread ready to run is found • The boundary between user layer threads and processor layer thread is crossed twice • Example: switch from thread 0 to thread 6 using • YIELD • ENTER_PROCESSOR_LAYER • EXIT_PROCESSOR_LAYER Lecture 19

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10. The control flow when switching from one thread to another • The control flow is not obvious as some of the procedures reload the stack pointer (SP) • When a procedure reloads the stack pointer then the place where it transfers control when it executes a return is the procedure whose SP was saved on the stack and was reloaded before the execution of the return. • ENTER_PROCESSOR_LAYER • Changes the state of the thread calling YIELD from RUNNING to RUNNABLE • Save the state of the procedure calling it , YIELD, on the stack • Loads the processors registers with the state of the processor thread, thus starting the SCHEDULER • EXIT_PROCESSOR_LAYER • Saves the state of processor thread into the corresponding PROCESSOR_TABLE and loads the state of the thread selected by the SCHEDULER to run (in our example of thread 6) in the processor’s registers • Loads the SP with the values saved by the ENTER_PROCESSOR_LAYER Lecture 19

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13. In ENTER PROCESSOR_LAYER instead of SCHEDULER() should be SP processor_table[processor].topstack Lecture 19

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15. Implicit assumptions for the correctness of the implementation • One sending and one receiving thread. Only one thread updates each shared variable. • Sender and receiver threads run on different processors to allow spin locks • in and out are implemented as integers large enough so that they do not overflow (e.g., 64 bit integers) • The shared memory used for the buffer provides read/write coherence • The memory provides before-or-after atomicity for the shared variables in and out • The result of executing a statement becomes visible to all threads in program order. No compiler optimization supported Lecture 19

16. In practice….. Threads run concurrently  Race conditions may occur  data in the buffer may be overwritten  a lock for the bounded buffer the producer acquires the lock before writing the consumer acquires the lock before reading Lecture 19

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18. We have to avoid deadlocks If a producer thread cannot write because the buffer is full it has to release the lock to allow the consumer thread to acquire the lock to read, otherwise we have a deadlock. If a consumer thread cannot read because the there is no new item in the buffer it has to release the lock to allow the consumer thread to acquire the lock to write, otherwise we have a deadlock. Lecture 19

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20. In practice… We have to ensure atomicity of some operations, e.g., updating the pointers Lecture 19

21. One more pitfall of the previous implementation of bounded buffer • If in and out are long integers (64 or 128 bit) then a load requires two registers, e.,g, R1 and R2. int “00000000FFFFFFFF” L R1,int /* R1  00000000 L R2,int+1 /* R2  FFFFFFFF • Race conditions could affect a load or a store of the long integer. Lecture 19

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23. In practice the threads may run on the same system…. We cannot use spinlocks for a thread to wait until an event occurs. That’s why we have spent time on YIELD… Lecture 19

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