V22.0436 - Prof. Grishman

Practice Final Exam Questions (1): Performance Issues

Time calculations

  1. A processor runs at 100 Mhz. What is the length of its clock cycle?
  2. A disk has an access time of 10 ms. Assuming the time for data transfer is negligible, how many disk accesses can be performed each second?

Processor Performance

  1. Suppose that when program Zippo runs, it executes 200,000 loads, 100,000 stores, 699,998 R-type instructions, and 2 multiply instructions. Consider 2 machine designs: in design M1, the clock rate is 100Mhz; loads and stores take 2 cycles, R-type instructions take 1 cycle, and multiplies take 5 cycles. In design M2, the clock rate is 66Mhz and all instructions take 1 cycle. Which machine is faster?

Memory

  1. Consider two alternative caches, each of which has a capacity of 8 words and a block size of one word. Cache D is a direct mapped cache, and cache T is a two-way set associative cache. Suppose the cache is initially empty and we fetch the words at the following addresses in sequence: 1, 2, 9, 3, 1, 5, 9. Which of these fetches will result in cache hits?
  2. Suppose that we have a 10 ns cache (it takes 10 ns to access the data or identify a miss), and a memory system with a 100 ns access time. What is the average memory access time if the cache hit rate is 97%? If we built a larger cache, with a 12 ns access time but a hit rate of 98%, would the average memory access time increase or decrease?

IO

  1. Suppose we have a floppy disk which transfers 50 KB and interrupts the CPU each time a byte is available. The CPU executes approximately 50 mips, and the interrupt routine takes 25 instructions to transfer a byte to memory. What fraction of the CPU time will be occupied doing IO with the floppy disk?