Understanding Operating Systems: Concurrent Processes PDF

CHAPTER 6 UNDERSTANDING OPERATING LECTURE 7: CONCURRENT PROCESSES  After completing this chapter, you should be able to describe:  The critical difference between processes and processors, and their connection  The differences among common configurations of multiproc...

CHAPTER 6 UNDERSTANDING OPERATING LECTURE 7: CONCURRENT PROCESSES  After completing this chapter, you should be able to describe:  The critical difference between processes and processors, and their connection  The differences among common configurations of multiprocessing systems  The significance of a critical region in process synchronization  The basic concepts of process synchronization software: test-and- set, WAIT and SIGNAL, and semaphores 2  The need for process cooperation when several processes work together  How several processors, executing a single job, cooperate  The similarities and differences between processes and threads  The significance of concurrent programming languages and their applications 3 Parallel processing  Multiprocessing  Two or more processors operate in unison  Two or more CPUs execute instructions simultaneously  Processor Manager  Coordinates activity of each processor  Synchronizes interaction among CPUs First of all, understand what is a Multiprocessing system, click on https://www.youtube.com/watch?v=IZfWjg3U3mA 4  Parallel processing development  Enhances throughput  Increases computing power  *Benefits  Increased reliability  More than one CPU  If one processor fails, others take over  Not simple to implement  Faster processing  Instructions processed in parallel two or more at a time 5  Faster instruction processing methods  CPU allocated to each program or job  CPU allocated to each working set or parts of it  Individual instructions subdivided  Each subdivision processed simultaneously  Concurrent programming  -Two major challenges  Connecting processors into configurations  Orchestrating processor interaction  Example: six-step information retrieval system  Synchronization is key 6 Understanding Operating Systems, Sixth Edition 7  Developed for high-end midrange and mainframe computers  Each additional CPU treated as additional resource  Today hardware costs reduced (Moore’s law)  Multiprocessor systems available on all systems  Multiprocessing occurs at three levels  Job level  Process level  Thread level  Each requires different synchronization frequency Understanding Operating Systems, Sixth Edition 8 Understanding Operating Systems, Sixth Edition 9  Multi-core processing  Several processors placed on single chip  Problems  Heat and current leakage (tunneling)  Solution  Single chip with two processor cores in same space  Allows two sets of simultaneous calculations  80 or more cores on single chip  Two cores each run more slowly than single core chip Understanding Operating Systems, Sixth Edition 10   Multiple processor configuration impacts systems  *Three types  Master/slave  Loosely coupled  Symmetric To understand more onTypes of Multiprocessing system, click on https://www.youtube.com/watch?v=So9SR3qpWsM 11  Asymmetric multiprocessing system  Single-processor system  Additional slave processors  Each managed by primary master processor  Master processor responsibilities  Manages entire system  Maintains all processor status  Performs storage management activities  Schedules work for other processors  Executes all control programs Understanding Operating Systems, Sixth Edition 12 Understanding Operating Systems, Sixth Edition 13  Advantages  Simplicity  Disadvantages  Reliability  No higher than single processor system  Potentially poor resources usage  Increases number of interrupts Understanding Operating Systems, Sixth Edition 14  Several complete computer systems  Each with own resources  Maintains commands and I/O management tables  Independent single-processing difference  Each processor  Communicates and cooperates with others  Has global tables  Several requirements and policies for job scheduling  Single processor failure  Others continue work independently  Difficult to detect Understanding Operating Systems, Sixth Edition 15 Understanding Operating Systems, Sixth Edition 16  Decentralized processor scheduling  Each processor is same type  Advantages (over loosely coupled configuration)  More reliable  Uses resources effectively  Can balance loads well  Can degrade gracefully in failure situation  Most difficult to implement  Requires well synchronized processes  Avoids races and deadlocks Understanding Operating Systems, Sixth Edition 17 Understanding Operating Systems, Sixth Edition 18  Decentralized process scheduling  Single operating system copy  Global table listing  Interrupt processing  Update corresponding process list  Run another process  More conflicts  Several processors access same resource at same time  Process synchronization  Algorithms resolving conflicts between processors Understanding Operating Systems, Sixth Edition 19  Successful process synchronization  Lock up used resource  Protect from other processes until released  Only when resource is released  Waiting process is allowed to use resource  Mistakes in synchronization can result in:  Starvation  Leave job waiting indefinitely  Deadlock  If key resource is being used Understanding Operating Systems, Sixth Edition 20   Critical region  Part of a program  Critical region must complete execution  Other processes must wait before accessing critical region resources  Processes within critical region  Cannot be interleaved  Threatens integrity of operation To understand more on Process Synchronization Critical Region, click on https://www.youtube.com/watch?v=eKKc0d7kzww Understanding Operating Systems, Sixth Edition 21  Synchronization  Implemented as lock-and-key arrangement:  Process determines key availability  Process obtains key  Puts key in lock  Makes it unavailable to other processes  Types of locking mechanisms  Test-and-set  WAIT and SIGNAL  Semaphores Understanding Operating Systems, Sixth Edition 22  Indivisible machine instruction (TS)  Executed in single machine cycle  If key available: set to unavailable  Actual key  Single bit in storage location: zero (free) or one (busy)  Before process enters critical region  Tests condition code using TS instruction  No other process in region  Process proceeds  Condition code changed from zero to one  P1 exits: code reset to zero, allowing others to enter Understanding Operating Systems, Sixth Edition 23  Advantages  Simple procedure to implement  Works well for small number of processes  Drawbacks  Starvation  Many processes waiting to enter a critical region  Processes gain access in arbitrary fashion  Busy waiting  Waiting processes remain in unproductive, resource-consuming wait loops Understanding Operating Systems, Sixth Edition 24  Modification of test-and-set  Designed to remove busy waiting  Two new mutually exclusive operations  WAIT and SIGNAL  Part of process scheduler’s operations  WAIT  Activated when process encounters busy condition code  SIGNAL  Activated when process exits critical region and condition code set to “free” Understanding Operating Systems, Sixth Edition 25  Nonnegative integer variable  Flag  Signals if and when resource is free  Resource can be used by a process  Two operations of semaphore  P (proberen means “to test”)  V (verhogen means “to increment”) Understanding Operating Systems, Sixth Edition 26 Understanding Operating Systems, Sixth Edition 27  Let s be a semaphore variable  V(s): s: = s + 1  Fetch, increment, store sequence  P(s): If s > 0, then s: = s – 1  Test, fetch, decrement, store sequence  s = 0 implies busy critical region  Process calling on P operation must wait until s > 0  Waiting job of choice processed next  Depends on process scheduler algorithm Understanding Operating Systems, Sixth Edition 28 Understanding Operating Systems, Sixth Edition 29  P and V operations on semaphore s  Enforce mutual exclusion concept  Semaphore called mutex (MUTual EXclusion) P(mutex): if mutex > 0 then mutex: = mutex – 1 V(mutex): mutex: = mutex + 1  *Critical region  Ensures parallel processes modify shared data only while in critical region  Parallel computations  Mutual exclusion explicitly stated and maintained To understand more on Semaphore, click on https://www.youtube.com/watch?v=DvF3AsTglUU&t=130s Understanding Operating Systems, Sixth Edition 30  Several processes work together to complete common task  Each case requires  Mutual exclusion and synchronization  Absence of mutual exclusion and synchronization  Results in problems  Examples  Producers and consumers problem  Readers and writers problem  Each case implemented using semaphores Understanding Operating Systems, Sixth Edition 31  One process produces data  Another process later consumes data  Example: CPU and line printer buffer  Delay producer: buffer full  Delay consumer: buffer empty  Implemented by two semaphores  Number of full positions  Number of empty positions  Mutex  Third semaphore: ensures mutual exclusion To understand more on Producer and Consumer problem, click on https://www.youtube.com/watch?v=LRiN3DJdskA Understanding Operating Systems, Sixth Edition 32 Understanding Operating Systems, Sixth Edition 33  Two process types need to access shared resource  Example: file or database  Example: airline reservation system  Implemented using two semaphores  Ensures mutual exclusion between readers and writers  Resource given to all readers  Provided no writers are processing (W2 = 0)  Resource given to a writer  Provided no readers are reading (R2 = 0) and no writers writing (W2 = 0) Understanding Operating Systems, Sixth Edition 34  Concurrent processing system  One job uses several processors  Executes sets of instructions in parallel  Requires programming language and computer system support Understanding Operating Systems, Sixth Edition 35  A = 3 * B * C + 4 / (D + E) ** (F – G) Understanding Operating Systems, Sixth Edition 36  A = 3 * B * C + 4 / (D + E) ** (F – G) Understanding Operating Systems, Sixth Edition 37 SUMMARY  Multiprocessing  Single-processor systems  Interacting processes obtain control of CPU at different times  Systems with two or more CPUs  Control synchronized by processor manager  Processor communication and cooperation  System configuration  Master/slave, loosely coupled, symmetric Understanding Operating Systems, Sixth Edition 38  Multiprocessing system success  Synchronization of resources  Mutual exclusion  Prevents deadlock  Maintained with test-and-set, WAIT and SIGNAL, and semaphores (P, V, and mutex)  Synchronize processes using hardware and software mechanisms Understanding Operating Systems, Sixth Edition 39  Avoid typical problems of synchronization  Missed waiting customers  Synchronization of producers and consumers  Mutual exclusion of readers and writers  Concurrent processing innovations  Threads and multi-core processors  Requires modifications to operating systems Understanding Operating Systems, Sixth Edition 40  Threads  Small unit within process  Scheduled and executed  Minimizes overhead  Swapping process between main memory and secondary storage  Each active process thread  Processor registers, program counter, stack and status  Shares data area and resources allocated to its process Understanding Operating Systems, Sixth Edition 41 Understanding Operating Systems, Sixth Edition 42  Operating system support  Creating new threads  Setting up thread  Ready to execute  Delaying or putting threads to sleep  Specified amount of time  Blocking or suspending threads  Those waiting for I/O completion  Setting threads to WAIT state  Until specific event occurs Understanding Operating Systems, Sixth Edition 43  Operating system support (cont'd.)  Scheduling thread execution  Synchronizing thread execution  Using semaphores, events, or conditional variables  Terminating thread  Releasing its resources Understanding Operating Systems, Sixth Edition 44   Information about current status and characteristics of thread Understanding Operating Systems, Sixth Edition 45  Ada  First language providing specific concurrency commands  Developed in late 1970’s  Java  Designed as universal Internet application software platform  Developed by Sun Microsystems  Adopted in commercial and educational environments Understanding Operating Systems, Sixth Edition 46