CS630 Operating System Design Second Exam, Spring 2015,

Transcription

1 CS630 Operating System Design Second Exam, Spring 2015, Problem 1. (10 Points) A technique, called swap prefetch, preloads a process s nonresident pages that are likely to be referenced in the near future. Such strategies attempt to reduce the number of page faults a process experiences. What s the advantage and disadvantage of this strategy? Problem 2. (35 Points) Assume that our system prefetches an extra page each time a page is referenced if the targeted extra page is not found in the page table. Assume that PA and PB are two distinctive programs. When PA is executed, a page reference string of {4, 5, 10, 10, 10, 6, 6, 7, 8} will be obtained; and when PB is executed, a page reference string of {4, 4, 4, 5, 5, 6} will be obtained. Assume that each page fault needs 4 ticks to be handled with a FCFS strategy. Assume that two processes P1 and P2 have arrived before t=0 and P1 is executing PA and P2 is executing PB. Draw a Gantt chart illustrating the procedure if a round-robin scheduling with a time quantum of 3 is used and each process has a working set of 3 frames by using a FIFO page replacement strategy. Problem 3. (45 Points) Assume that PA is a program. When this program is executed as PA n, a page reference string such as {4, [5, 8, 9] n, 6, 7, 7} will be generated where n is an integer. This means that a part of instructions will be repeated n times. Assume that the first instruction on page #8 forks a new process which executing PA again with an input argument of m where m is set to n-1 initially and it will reduce by 1 if the process forks again. For example, a process executes PA 2 will produces a page reference string of {4, 5, 8, 9, 5, 8, 9, 6, 7, 7}, and its first child process will be executed as PA 1 and its second child process will be executed as PA 0. Assume that one process P1 executing PA 3 has arrived before t=0. Draw a Gantt chart illustrating the scheduling if a round-robin scheduling with a time quantum of 3 is used and each process has a working set of 3 frames by using a FIFO page replacement strategy. Assume that I/O requests will be handled in a FCFS manner and each I/O request can be completed within 3 ticks. Mark on the chart when a new process is forked and a process has done its execution.

2 CS630 Operating System Design, Second Exam, Fall 2014 Problem 1. (25 Points) Assume that a process executes the following pseudo codes: #5 #6 #7 main (int argc, char *argv[ ]) { int i, keyin; /* the last line of 1st half of page #5 */ for (i=1; i < argc; i++) { /* the beginning of 2nd half of page #5 */ keyin = atoi (argv[i]); /* convert i-th argument to integer */ if (keyin is even) sub( ); /* the last line of 2nd half of page #5 */ } /* the last line of 1st half of page #6 */ } /* the last line of 1st half of page #7 */ /* the 2nd half of page #7 is intentionally left blank */ #10 #11 sub ( ) { } /* the last line of 1st half of page #11 */ /* the 2nd half of page #11 is intentionally left blank */ Assume that this program is executed as pseudo Firstly, use a Gantt chart to illustrate how the created process is really executed page by page; and then find the page reference string by assuming that the kernel is referencing a page at every tickmark as the process is executing. Show related work to claim credits. Problem 2. (13 Points) Consider a computer system with 3 individual resources {R1, R2, R3}. Let s assume there are 4 processes {P1, P2, P3, P4} make requests in the following order: {P1 R1, P3 R2, P2 R1, P1 R2, P4 R3, P3 R3, P4 R1}. Assume Pi gets Rj if Rj is currently available. Ignore a request if it cannot be preceded. Is there a deadlock and if so at what point did it occur and which processes did it involve? If there is deadlock, then how to avoid such a deadlock by rearrange the order of requests. (Turn Over)

3 Problem 3. (40 Points) Consider the Demand Paging problem. PA and PB are two programs in a file system. Assume that a page reference string is made by recording the virtual page number every tick as the process is executing. When PA is executed, the page reference string will be given as: {8, 9, 9, 21, 9, 10}, and when PB is executed, the page reference string will be given as: {4, 4, 5, 5, 6}. Assume that there are two processes in the system just before t=0: P1 executes PA and P2 executes PB. Assume that there may have new processes created at some special moments if at any one of those moments there is a scheduled process; say it starts at t=4.5 (i.e. 4 and half ticks) and then every 5 ticks after that). Assume that all the new processes will be executing PB. Assume that each I/O needs 4 ticks to be handled, and I/O requests are handled on a FCFS fashion. Draw a Gantt chart to illustrate the scheduling of these processes until the process P1 is done if a round-robin scheduling with a time quantum of 3 ticks is used. Assume that the working set contains 3 pages and uses the FIFO page replacement method. Show your work. Problem 4. (12 Points) Given a page reference string as: Does this page reference string suffer from Belady s Anomaly when the page replacement algorithm is FIFO? Note: you will need to check the window size from 2 up to 4. Problem 5. (10 Points) Why LRU doesn't suffer Belady's Anomaly in general?

4 CS630: Operating System Design, Second Exam, Spring 2014 Q 1. (10 Points) Assume that a process executes the following pseudo codes: #4 #5 #6 #7 #10 main (int argc, char *argv[ ]) { int I, *input; n = argc 1; input = (int *)malloc (n); for (i=0; i < n; i++) input[i] = atoi (argv[i+1]); statement_block_1; for (i=0; i < n; i++) { statement_block_2; if (input[i]%2 == 0) continue; /* last instruction */ If (!fork()) execv(); /* first instruction */ statement_block_3; sub (input[i]%3); } /* last instruction */ statement_block_4; } sub (int n) { statement_block_5; if (n > 1) sub (n 1); } /* last instruction */ Assume that this program is executed as pseudo , find the page reference string. Note that system calls malloc() allocates dynamic memory space, atoi() converts a string to an integer, and the expression (a%b) returns the reminder of a/b. Show related work to claim credits. Q 2. (10 Points) Assume that R1 has only one instance, both R2 and R3 have two instances and a resource allocation graph is given as the following: {P1 R1, P2 R3, P3 R2, R1 P2, R2 P2, R3 P1, R3 P3}. Does this system contain deadlock? Show your reasoning. If the answer is No then can we create a deadlock situation and how? Q 3. (20 Points) Given a page reference string as: {1, 3, 2, 4, 1, 3, 5, 1, 3, 2, 4, 5}. Does this page reference string suffer from Belady s anomaly when the page replacement algorithm is a) FIFO, and b) LRU? Note: you will need to check the window size from 3 up to 5. (Turn Over)

5 Q 4. (15 Points) In the code below, four processes are competing for four resources labeled A to D where get() is same as wait() and release() is the same as signal(). a) Using a RAG (resource allocation graph, show the possibility of a deadlock in this implementation. b) Modify the order of some of the get requests to prevent the possibility of any deadlock. You cannot move requests across programs, only change the order inside each program. Use a RAG to justify your answer. Q 5. (45 Points) Consider the Demand Paging problem. P A and P B are two programs in a file system. Assume that a page reference string is made by recording the virtual page number every 2 ticks as the process is executing. When P A is executed, the page reference string will be given as: {4, 5, 6, 10, 10, 5, 6, 10, 7}, and when P B is executed, the page reference string will be given as: {4, 4, 4, 5, 5}. Assume that there are two processes in the system just before t=0: P 1 executes P A and P 2 executes P B. For P A, assume that the first instruction in page#10 fork a new process which executing P B. Assume that each page fault needs 4 ticks to be handled, and page faults are handled on a first-come-first-serve fashion. Draw a Gantt chart to illustrate the scheduling of these processes and their child processes and to find the number of page faults for each process if a round-robin, non-preemptive scheduling with a time quantum of 4 ticks is used. Assume that the working set contains 3 pages and uses the FIFO page replacement method. Show your work.