A program repeatedly performs a three-step process: it reads in a 4-KB block of

Question

A program repeatedly performs a three-step process: it reads in a 4-KB block of data from disk, does some processing on that data, and then writes out the result as another 4-KB elsewhere on the disk. Each block is contiguous and randomly located on a single track on the disk. The disk drive rotates at 7200 RPM, has an average seek time of 8 ms, and has a transfer rate of 20 MB/sec. The controller overhead is 2 ms. No other program is using the disk or processor, and there is no overlapping of disk operation with processing. The processing step takes 20 million clock cycles, and the clock rate is 400 MHz. What is the overall speed of the system in blocks processed per second?

Explanation / Answer

Step I : Calculate the average rotational delay

tdr = 0.5/7200 = 4.167 ms

Step II: Calculate time to read 4-KB

trd = 4 KB / 20-MB/s = 0.195 ms

Step III Calculate the total read time

ttrd = rotational delay + seek time + overhead + transfer time

= 4.167 + 8 + 2 + 0.195 = 14.36 ms

Step IV Processing of data

20 million cycles / 400 MHz = 50 ms

Step V Writing of data to disk = Time to read

Read time + Writing time + Processing = 2 x 14.36 +50 = 78.72 ms to process 1 block i.e 12.7 (13 approx) blocks per second

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