Computing Science 300 Sample Final Examination

Transcription

1 Computing Science 300 Sample Final Examination 1. [10 points] Generally speaking, input and output operations can be done using two different methods, busy-waiting and interrupt-driven (using DMA or single word transfer). Give a brief step by step description of how each method works (point form). When answering this problem assume that you are using I/O hardware that transfers 1 word at a time in both cases. 2. One process is running on a single CPU computer system. The process has two threads that run concurrently. The code in pseudo-c for the two threads is as follows: x is a global variable shared by the two threads. Suppose that x is initialized to be 0. int x 0 ; /* a global variable shared by Thread A and Thread B */ /* Thread A */ for(i 0; i < 4; i++) x x + 5; /* Thread B */ for(j 0; j < 3; j++) x x -2; /* integer subtract*/ Suppose that the computer uses a load-store architecture, which means that the operations x x + 5 or x x - 2 are done in three steps. (a) x is loaded into a register; (b) 5 is added to the register or 2 is subtracted from the register; and (c) the register is stored back to x. Each loop consists of three instructions a, b, c Use A0a, A0b, A0c, A1a, A1b, A1c, A2a, A2b, A2c, A3a, A3b, A3c to denote the instructions within the four loops Thread A executes. Similarly, use B0a, B0b, B0c, B1a, B1b, B1c, B2a, B2b, B2c to denote the instructions within the three loops Thread B executes. a) [3 points] What is the minimum value of x that can result. b) [12 points] Give a series of steps that will produce the result x3. Be sure to indicate when swapping between processes occurs and to keep track of the value of x and the register R at each step. 3. [10 points ] Suppose in a computer system, there are five batch jobs, J1, J2, J3, J4, and J5 arriving at almost the same time. These jobs arrive in the order specified in the previous sentence. These jobs have the estimated run times of 10, 6, 2, 4, and 8 units, respectively. Their priorities are 4, 1, 2, 5 and 2, respectively, with 5 being the highest priority. Now suppose that all the five jobs are CPU-bound and only one job can run at a time. Also suppose that there is only one CPU in the system. Compute the average process turnaround time for each of the following algorithms. a) Priority scheduling (non-preemptive) b) Priority scheduling (preemptive)

2 4. [5 points] Give a step by step (point form) description of the translation of a virtual address in a specified segment to a physical address. Base your description on the diagram below virtual address Seg 1 (code) Seg 2 (data) Segment # Offset Physical memory Seg 3 (stack) Virtual memory as in paging: valid, modified, protection, etc. Seg 3 (stack) MMU Segment table Base Limit Other Seg 1 (code) no offset < limit? yes STB STL Seg 2 (data) memory access fault After Dominik Raub Segment Base + Offset physical address 0x00

3 PART 2 (60 points): PROGRAMMING PROBLEMS Choose 3 of the following 4 problems 5. Consider a filesystem. Assume that the computer system using this filesystem uses a memory block size of 4Kbytes. Answer each of the following questions a) [3 points] If the filesystem is a LINUX filesystem. Draw a diagram showing how blocks of memory referenced using the double indirect entry in the inode structure are accessed. An inode structure is illustrated the right. b) [5 points] How many pages of memory can be accessed by a LINUX inode structure? Assume that each block of memory will hold 1024 addresses. Explain, step by step, how you determined your answer. c) [2 points] Consider the LINUX file system from parts a) and b). This filesystem presently includes 5000 files. Twenty of these files are presently being used by active processes. How may inodes are in main memory? How many inodes are in the inode table? Briefly (~1sentence) explain each answer. d) [1 point ] How large is the largest file that can be used in the LINUX system discussed in a), b) and c)? e) [7 points] Now consider the disk is part of a windows system. Instead of Inodes the windows system uses a FAT table to keep track of memory blocks. A FAT table has 1 entry for each block in the system. Assume each block contains 1024 words. Explain, using a File Information Address block 0 Address block 1 Address block 2 Address block 3 Address block 4 Address block 5 Address block 6 Address block 7 Address block 8 Address block 9 Single indirect.. Double indirect. Triple indirect detailed example (including addresses and block numbers), how a fat table is used to keep track of the blocks in a file, and of the order of the blocks in the file.. You may use a diagram to help you explain. A diagram alone is not sufficient. f) [2 point]assume you have a disk in your Windows computer system that has been formatted to include 3 partitions. Briefly (1-2 sentences) explain how these partitions relate to the windows file system discussed above. 6. (20 points) Consider a hard disk. The disk has 8 platters each with N128 tracks and 8 sectors per track. There is one head that reads from each platter. The seek time per track is 1 ms. The rotation speed of the disk is 7200rpm. a) [2 points] Draw a diagram of one of the platters of the hard disk. Be sure to label the tracks and the sectors. [3 points] What is rotational latency? Assume that once the head is in position above the appropriate track you need to wait an average of half a rotation of the disk before the head can read your data. What are the maximum and minimum rotational latency of this disk? What is the average rotational latency of this disk?

4 b) [3 points] What are the seek time and seek length? Given that the probability of a seek of length K is P k 3( N K ) N 2 and mean seek length ipk 2 N 1 i 0 What is the mean seek time on a platter of the disk? You may use the following formulae. N 1 T 0 T N *( first term + last term) / 2 N ( N 1) N 1 2 N( N 1)(2 N 1) T T 0 6 c) [1 point] How long, on average, does it take to reach the correct place on the disk so you can begin reading a piece of requested data? d) [5 points] Assume that the disk is striped. This means that the data is loaded into tracks in the following order. Track 0 disk 0, Track 0 disk 1, Track 0 disk 2,, Track 0 disk 7, Track 1 disk 0, Track 1 disk 1,, Track 1 disk 7,, Track 127 disk 0,, Track 127 disk 7. ( this is illustrated in the diagram below). Assume that the disk controller is capable of simultaneously accessing all disks (simultaneously reading from all disks). Assume that i. The DMA transfer rate is 400 Mbytes per second (DMA from buffer to memory) ii. The DMA setup time five milliseconds (5*10-3 s). iii. One DMA transfer is used to transfer one track iv. Transferring one track of data from the disk to the controller buffer takes 3 s v. The data being read is contiguous beginning at the start of Track 0 on disk 0 and ending at the end of on disk 3 vi. Each track holds 100 Mbytes of data. How long does it take to transfer the file from the disk to memory? How long would it take to transfer the file if the data were stored on track 0 to 9 on disk 0? Why is striping useful? Disk 0 Disk 1 Disk 7 track 0 track 3 Track0 Track3... track 0 track 3 e) f) [ 6 points] The following sequence of disk access requests have been received by the computer systems. 45, 74, 52, 121, 11, 44, 2, 99, 76 All the access requests are on platter 0. The numbers are the track numbers on platter 0. Determine the actual average seek length for this particular series of accesses based on each of the following algorithms. Assume the read head starts at track 45. i. Shortest seek first (move the fewest number of tracks to the next entry) ii. Elevator algorithm(you are presently moving toward smaller track numbers each time you seek)

5 7. [20 points] Consider a system in which there are two kinds of atoms, oxygen and hydrogen. Atoms are added to the system randomly (with a uniform random distribution). The maximum time between arriving oxygen atoms is twice the maximum time between arriving hydrogen atoms. In order to assemble these atoms into water molecules we have to wait until two hydrogen atoms and one oxygen atom are present. Then the three atoms can combine to create a water molecule by creating a bond (each atom must execute function bond() to create the bond). After the bond has been created the atoms cease to exist and the resulting complete molecule leaves the system. You must guarantee that all the atoms from one molecule create a bond before any of the atoms from the next molecule do. In other words: If an oxygen atom arrives when no hydrogen threads are present, it has to wait for two hydrogen atoms. If a hydrogen atom arrives when no other atoms are present, it has to wait for an oxygen atom and another hydrogen atom. Write pseudo code for the functions representing the oxygen and hydrogen molecules that enforces these constraints using processes and semaphores. 8. Explain in detail the following aspects of paging. You may use diagrams to help you explain. Diagrams alone will not receive full points. a) [8 points] Working set clock page replacement algorithm b) [6 points ] Translation of virtual address to physical address using a paging table as illustrated below. c) [6 points] Purpose and use of a TLB. (first tell me what it is) Page Table entries TOP LEVEL is a list of pages containing page table entries. LOWEST LEVEL is a page of page table entries