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Operating Systems,Review,Questions,What are two functions of an OS?What “layer” is above the OS?What “layer” is below the OS?,Manages all system resourcesCPUMemoryI/OFilesObjectives: Security EfficiencyConvenience,Questions,What causes OS to change?What is multi-programming?What is time-sharing?Or, why arent we still running MS-DOS?What is a process? What is a thread?What is address space?What is a file?,Review,What is (average) waiting time? Explain how SJF,FCFS,RR etc. worksTrue or False:FCFS is optimal in terms of avg waiting timeMost processes are CPU boundThe shorter the time quantum, the better,6,Response timeEstimate by time from job submission to time to first CPU dispatchAssume all jobs submitted at same time, in order givenTurnaround timeTime interval from submission of a process until completion of the processAssume all jobs submitted at same time,7,Response Time Calculations,8,Turnaround Time Calculations,Review,What is a “race condition”?What are 3 properties necessary for a correct “critical region” solution?mutual exclusion, progress, bounded waitingWhat is Petersons Solution?Why is semaphore better than Petersons solution and TSL?,Review,What is deadlock?What is the four conditions for deadlock to happen?,Review of Bankers Algorithm,P = P0, P1, , P4; R=A(10), B(5), C(7)Snapshot at time T0:,Can request for (1,0,2) by P1 be granted?Can request for (3,3,0) by P4 be granted? Can request for (0,2,0) by P0 be granted?,Review,What is the Memory Management Unit?How does it work during a virtualphysical address translation?,Translate virtual address to physical address2041008300,Review,A paging scheme uses a Translation Loo-aside Buffer (TLB) A TLB access takes 10 ns and a main memory access takes 50 ns. What is the effective access time (in ns) if the TLB hit ratio is 90% and there is no page-fault?,Tanenbaum, Modern Operating Systems 3 e, (c) 2008 Prentice-Hall, Inc. All rights reserved. 0-13-6006639,Multilevel Page Tables,Figure 3-13. (a) A 32-bit address with two page table fields. (b) Two-level page tables.,What is the PT1 and PT2 value for a virtual address 0x00403004?,?,A computer has 32-bit virtual addresses and 4-KB pages. The program and data together fit in the lowest page (0-4095) the stack fits in the highest page. How many entries are needed in the page table if traditional paging is used? How many page table entries are needed for 2-level paging, with 10 bits in each part?,Review,What is internal fragmentation?What is external fragmentation?What is compaction?,True or False,With paging, a process logical address spaces is contiguousWith paging, a process physical address spaces is contiguousPaging reduces the size of the possible address space used by a process,Review,Does paging have fragmentation?No? Then why not?Yes? Then what kind?What are the overheads associated with paging?,Review,True or False: The logical address space cannot be bigger than the physical address spaceProcesses have big address spaces because they always need themDemand paging:Is unrelated to prepagingBrings logical pages into physical memory when requested by a processIncreases memory requirements for a system,Review,Page faultsWhat is a page fault?What does an OS do during a page fault?What is a Page Replacement Algorithm?What is “Beladys Anomaly”?How does the Optimal algorithm work?How does Enhanced Second Chance work?What is thrashing?How do we fix it?,?,A computer has four page frames. The time of loading, time of last access, and the R and M bits for each page are as shown below (the times are in clock ticks): Page Loaded Last Ref. R M 0 126 285 1 0 1 120 2650 0 2 140 270 0 1 3 110 280 1 1 (a) Which page will NRU replace? (b) Which page will FIFO replace? (c) Which page will LRU replace? (d) Which page will second chance replace?,Cause of thrashing,Degree of multiprogramming,CPU utilization,thrashing increases,?,Array A1024, 1024 of integer, each row is stored in one pageProgram 1for j := 1 to 1024 dofor i := 1 to 1024 doAi,j := 0;1024 1024 page faults Program 2for i := 1 to 1024 dofor j := 1 to 1024 doAi,j := 0;1024 page faults,First-In-First-Out (FIFO),3 Frames / Process,1,2,3,4,1,2,5,1,2,3,4,5,4,1,2,5,3,4,9 Page Faults,How man page faults would we have if we had4 Frames/Process?,?,How long does it take to load a 64KB program from a disk whose average seek time and rotation time are 10 msec, and whose tracks hold 32KB.For 2KB page size?For 4KB page size?,Figure 4-34. A UNIX i-node.,The UNIX V7 File System (2),Tanenbaum, Modern Operating Systems 3 e, (c) 2008 Prentice-Hall, Inc. All rights reserved. 0-13-6006639,I-Node,How many data blocks are there?If you added 3 more data blocks to the file, what would happen?,Disk blocks,i-node,null,null,null,null,null,62,77,Review,Directories:In what way is a directory different than a file?In what way is a directory similar to a file?Aliases:Describe a hard-linkDescribe a soft-linkFree space management:If the si