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:  Processor sharing among multiple threads Today: Conditions for thread coordination – Safety, Liveness, Bounded-Wait, Fairness Critical sections – a solution to critical section problem Locks and Before-or-After actions. Hardware support for locks Deadlocks Signals Semaphores Monitors Thread coordination with a bounded buffer.  WAIT  NOTIFY  AWAIT  ADVANCE  SEQUENCE  TICKET Next Time  Scheduling Algorithms Lecture 18 – Tuesday March 29, 2011 Lecture 182

3. Thread coordination Critical section  code that accesses a shared resource Race conditions  two or more threads access shared data and the result depends on the order in which the threads access the shared data. Mutual exclusion  only one thread should execute a critical section at any one time. Scheduling algorithms  decide which thread to choose when multiple threads are in a RUNNABLE state  FIFO – first in first out  LIFO – last in first out  Priority scheduling  EDF – earliest deadline first Preemption  ability to stop a running activity and start another one with a higher priority. Side effects of thread coordination  Deadlock  Priority inversion  a lower priority activity is allowed to run before one with a higher priority Lecture 183

4. Solutions to thread coordination problems must satisfy a set of conditions Lecture 184 1.Safety: The required condition will never be violated. 2.Liveness: The system should eventually progress irrespective of contention. 3.Freedom From Starvation: No process should be denied progress for ever. That is, every process should make progress in a finite time. 4.Bounded Wait: Every process is assured of not more than a fixed number of overtakes by other processes in the system before it makes progress. 5.Fairness: dependent on the scheduling algorithm FIFO: No process will ever overtake another process. LRU: The process which received the service least recently gets the service next. For example for the mutual exclusion problem the solution should guarantee that: Safety  the mutual exclusion property is never violated Liveness  a thread will access the shared resource in a finite time Freedom for starvation  a thread will access the shared resource in a finite time Bounded wait  a thread will access the shared resource at least after a fixed number of accesses by other threads.

5. Thread coordination problems Dining philosophers Critical section Lecture 185

6. A solution to critical section problem Applies only to two threads T i and T j with i,j ={0,1} which share  integer turn  if turn=i then it is the turn of T i to enter the critical section  boolean flag[2]  if flag[i]= TRUE then T i is ready to enter the critical section To enter the critical section thread T i  sets flag[i]= TRUE  sets turn=j If both threads want to enter then turn will end up with a value of either i or j and the corresponding thread will enter the critical section. T i enters the critical section only if either flag[j]= FALSE or turn=i The solution is correct  Mutual exclusion is guaranteed  The liveliness is ensured  The bounded-waiting is met But this solution may not work as load and store instructions can be interrupted on modern computer architectures Lecture 186

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8. 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 188

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11. Deadlocks Happen quite often in real life and the proposed solutions are not always logical: “When two trains approach each other at a crossing, both shall come to a full stop and neither shall start up again until the other has gone.” a pearl from Kansas legislation. Deadlock jury. Deadlock legislative body. Lecture 1811

12. Examples of deadlock Traffic only in one direction. Solution  one car backs up (preempt resources and rollback). Several cars may have to be backed up. Starvation is possible. Lecture 1812

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14. Thread deadlock Deadlocks  prevent sets of concurrent threads/processes from completing their tasks. How does a deadlock occur  a set of blocked threads each holding a resource and waiting to acquire a resource held by another thread in the set. Example  locks A and B, initialized to 1 P 0 P 1 wait (A);wait(B) wait (B);wait(A) Aim  prevent or avoid deadlocks Lecture 1814

15. System model Resource types R 1, R 2,..., R m (CPU cycles, memory space, I/O devices) Each resource type R i has W i instances. Resource access model:  request  use  release Lecture 1815

16. Simultaneous conditions for deadlock Mutual exclusion: only one process at a time can use a resource. Hold and wait: a process holding at least one resource is waiting to acquire additional resources held by other processes. No preemption: a resource can be released only voluntarily by the process holding it (presumably after that process has finished). Circular wait: there exists a set {P 0, P 1, …, P 0 } of waiting processes such that P 0 is waiting for a resource that is held by P 1, P 1 is waiting for a resource that is held by P 2, …, P n–1 is waiting for a resource that is held by P n, and P 0 is waiting for a resource that is held by P 0. Lecture 1816

17. Wait for graphs Lecture 1817

18. Semaphores Abstract data structure introduced by Dijkstra to reduce complexity of threads coordination; has two components  C  count giving the status of the contention for the resource guarded by s  L  list of threads waiting for the semaphore s Counting semaphore – for an arbitrary resource count. Supports two operations: V - signal() increments the semaphore C P - wait() P decrements the semaphore C. Binary semaphore: C is either 0 or 1. Lecture 1818

19. The wait and signal operations P (s) (wait) { If s.C > 0 then s.C − −; else join s.L; } V (s) (signal) { If s.L is empty then s.C + +; else release a process from s.L; } Lecture 1819

20. Monitors Semaphores can be used incorrectly  multiple threads may be allowed to enter the critical section guarded by the semaphore  may cause deadlocks Threads may access the shared data directly without checking the semaphore. Solution  encapsulate shared data with access methods to operate on them. Monitors  an abstract data type that allows access to shared data with specific methods that guarantee mutual exclusion Lecture 1820

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22. Asynchronous events and signals Signals, or software interrupts, were originally introduced in Unix to notify a process about the occurrence of a particular event in the system. Signals are analogous to hardware I/O interrupts:  When a signal arrives, control will abruptly switch to the signal handler.  When the handler is finished and returns, control goes back to where it came from After receiving a signal, the receiver reacts to it in a well-defined manner. That is, a process can tell the system (OS) what they want to do when signal arrives:  Ignore it.  Catch it and deliver it. In this case, it must specify (register) the signal handling procedure. This procedure resides in the user space. The kernel will make a call to this procedure during the signal handling and control returns to kernel after it is done.  Kill the process (default for most signals). Examples: Event - child exit, signal - to parent. Control signal from keyboard. Lecture 1822

23. Signals state and implementation A signal has the following states:  Signal send - A process can send signal to one of its group member process (parent, sibling, children, and further descendants).  Signal delivered - Signal bit is set.  Pending signal - delivered but not yet received (action has not been taken).  Signal lost - either ignored or overwritten. Implementation: Each process has a kernel space (created by default) called signal descriptor having bits for each signal. Setting a bit is delivering the signal, and resetting the bit is to indicate that the signal is received. A signal could be blocked/ignored. This requires an additional bit for each signal. Most signals are system controlled signals. Lecture 1823

24. Back to thread coordination with a bounded buffer The bounded buffer is a shared resource thus it must be protected; the critical section is implemented with a lock. The lock must be released if the thread cannot continue. Spin lock  a lock which involves busy wait. The thread must relinquish control of the processor, it must YIELD. Lecture 1824

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27. Coordination with events and signals We introduce two events  p_room  event which signals that there is room in the buffer  p_notempty  event which signals that there is a new item in the buffer We also introduce two new system calls  WAIT(ev)  wait until the event ev occurs  NOTIFY(ev)  notify the other process that event ev has occurred. SEND will wait if the buffer is full until it is notified that the RECIVE has created more room  SEND  WAIT(p_room) and RECIVE  NOTIFY(p_room) RECEIVE will wait if there is no new item in the buffer until it is notified by SEND that a new item has been written  RECIVE  WAIT(p_notempty) and SEND  NOTIFY(p_notempty) Lecture 1827

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29. NOTIFY could be sent before the WAIT and this causes problems Lecture 1829 The NOTIFY should always be sent after the WAIT. If the sender and the receiver run on two different processor there could be a race condition for the notempty event. Tension between modularity and locks Several possible solutions: AWAIT/ADVANCE, semaphores, etc

30. AWAIT - ADVANCE solution A new state, WAITING and two before-or-after actions that take a RUNNING thread into the WAITING state and back to RUNNABLE state. eventcount  variables with an integer value shared between threads and the thread manager; they are like events but have a value. A thread in the WAITING state waits for a particular value of the eventcount AWAIT(eventcount,value)  If eventcount >value  the control is returned to the thread calling AWAIT and this thread will continue execution  If eventcount ≤value  the state of the thread calling AWAIT is changed to WAITING and the thread is suspended. ADVANCE(eventcount)  increments the eventcount by one then  searches the thread_table for threads waiting for this eventcount  if it finds a thread and the eventcount exceeds the value the thread is waiting for then the state of the thread is changed to RUNNABLE Lecture 1830

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32. Implementation of AWAIT and ADVANCE Lecture 1832

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34. Solution for a single sender and single receiver Lecture 1834

35. Supporting multiple senders: the sequencer Sequencer  shared variable supporting thread sequence coordination -it allows threads to be ordered and is manipulated using two before-or-after actions. TICKET(sequencer)  returns a negative value which increases by one at each call. Two concurrent threads calling TICKET on the same sequencer will receive different values based upon the timing of the call, the one calling first will receive a smaller value. READ(sequencer)  returns the current value of the sequencer Lecture 1835

36. Multiple sender solution; only the SEND must be modified Lecture 1836