CGS 3763 Operating Systems Concepts Spring 2013

1. CGS 3763 Operating Systems Concepts Spring 2013 Dan C. Marinescu Office: HEC 304 Office hours: M-Wd 11:30 - 12:30 AM

2. Last time: CPU Scheduling Process synchronization Today Answers to student questions. Process synchronization Semaphores Monitors Thread coordination with a bounded buffer. Next time Process synchronization Reading assignments Chapter 6 of the textbook Lecture 27 – Friday, March 22, 2013 Lecture 27

3. March 11th Monday: • How does the CPU decide which type of scheduling to use? In what applications would the different CPU scheduling techniques be applicable? Can a system utilize any of the algorithms or is it built with a specific one? If Round Robin is the fairest scheduler, why are there other types that are used? • How do you determine the length of the next CPU burst for one thread? Why is the waiting factor typically .5 to determine the length of the next CPU burst? • Difference between SRTF and SJF? • What is the importance of exponential averaging? Lecture 27

4. March 13th Wednesday: • Priority Inversion, how does a thread acquire a lock? How do locks work? • How is the priorities set or determined by the scheduler? How does the computer know whether a process has a higher priority than another process? • Is there an error found on the computer when starvation happens? • Can a process age to have absolutely zero priority? And if it does, does it get ignored or does it get re-sent back into waiting? • In Priority scheduling would SJF have precedence over RR? Lecture 27

5. March 15th Friday: • What methods are used for the system to determine which core to use for a specific process (since each core may finish different processes at different times, does it pre-allocate where a process will go?) • What is the average maximum temperature that a processor can function in? • Does a higher clock rate always indicate that a computer is inefficient? • What happens if one boosts a clock rate too much? • For NUMA, what happens if two processes or threads try to access the same data at the same time? • What is a system contention scope? • What are the drawbacks of Fair Share Scheduling? • Discuss the purpose of the Light Weight Process (LWP). • What is a homogeneous processor? • What is a soft and hard processor affinity? Lecture 27

6. Contention scope • Contention scope  which threads compete with one another for CPU. • User level threads, many-to-one and many-to-many • Scheduled by the thread library • Process contention scope (PCS)  competition among the threads belonging to the same process • Kernel-level threads • The system scheduler • System contention scope (SCS) competition is among all threads in the system. Lecture 27

7. Lecture 27

8. System model • Resource types R1, R2, . . ., Rm (CPU cycles, memory space, I/O devices) • Each resource type Ri has Wi instances. • Resource access model: • request • use • release Lecture 27

9. Wait-for-graph  directed graph (an edge connected one vertex to another has a direction associated with it). The vertices are the locks and the threads. Lecture 27

10. 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 {P0, P1, …, P0} of waiting processes such that P0 is waiting for a resource that is held by P1, P1 is waiting for a resource that is held by P2, …, Pn–1 is waiting for a resource that is held by Pn, and P0 is waiting for a resource that is held by P0. The circular wait is reflected by a cycle in the Wait-for-Graph Lecture 27

11. Lecture 27

12. 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 a resource with multiple copies. 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 27

13. P and V counting semaphore operations • The value of the semaphore S is the number of units of the resource that are currently available. • ThePoperation forces a thread tosleep until a resource protected by the semaphore becomes available, at which time the resource is immediately claimed. • wait(): Decrements the value of semaphore variable by 1. If the value becomes negative, the process executing wait() is blocked, i.e., added to the semaphore's queue. • TheV operation is the inverse: it makes a resource available again after the thread has finished using it. • signal(): Increments the value of semaphore variable by 1. After the increment, if the pre-increment value was negative (meaning there are threads waiting for a resource), it transfers a blocked thread from the semaphore's waiting queue to the ready queue. Lecture 27

14. The Wait (P) and Signal (V) 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 27