ijk@hou5e.UUCP (10/18/83)
The following are questions from a recent PhD qualifying exams that I am having difficulty in answering. I'm interested if anyone could add something to my analysis. 1). "A job to be run consists of inputting and processing a sequence of records: R1, R2, R3, ... Each odd-numbered record takes two units of time to input, and one unit of time to process. Each even-numbered recors takes one unit of time to input and two units of time to process. a). What is the maximum number of input buffers which can beneficially be used? Justify your answer by b) Drawing a timing diagram to show processor and input-device utilization for the number of buffers in your answer to part a. c). Drawing timing diagrams to show processor and input-device utilization for each smaller number of buffers, and explaining how the diagram suggests that more buffers may be more beneficial. 2). a) In a multiple-input buffer system what is the criterion for deciding how many buffers to use, i.e., for any integer N, how do we tell if N+1 buffers would be better than N buffers? b) A job to be run consists of inputting and processing a sequence of records: R1, R2,... Each odd-numbered record takes two units of time to input, and three time units to process. What is the optimum value of N for this job? Justify your answer in terms of criterion in question a. I have a hard time understanding what the problem is, especially with fixed rates input & fixed processing times (obviously, for variable rates, we get into queuing theory, but that's not the issue). Unless you have multiple I/O channels, the best performance is with TWO buffers (usually), since the CPU processes one record while the I/O channel reads in the other. More buffers would be useful only if one type of a record take very long to process, and all the others took a short time to process. Then, you would want read a sufficient number of records to make sure the processor would have something to do when it finished the number. (The number of buffers would then equal 1 + [longest time for CPU to process a recors / shortest time for I/O to read in a record: rounded up]). It seems to trivial to be on a qualifier, but it must be important since these two questions came in different years. Am I missing something? Ihor Kinal AT&TIS, Holmdel NJ hou5e!ijk (talks to most Holmdel Bell Labs machines, ariel, ho* ) Thanx in advance.