# Calculate Speedup

What is the overall speedup of a system spending 65% of its time on I/O with a disk upgrade that provides for 50% greater throughput.

The formula should be Amdahl's law: overall speedup = 1 / (1 -fraction enhanced) + fraction enhanced/ speedup enhanced

However, I'm not sure if I am setting up the equation right and if the answer would be 60% faster.

  1 / (1 - .65) + .65/.5
1 / .35 + 1.3
1 / 1.65
= .6

• Well, for one thing, "50% greater throughput" does NOT mean that the new disk system is twice as fast as the old one. It means that in a given amount of time, it processes 150% (3/2) as much information as the original system, or that the same amount of work takes 2/3 the time (not 1/2). – Dave Tweed Nov 17 '14 at 2:10
• So, would the answer be .65 * 2/3 + .35 * 1 = .43 + .35 = .78 = 22% faster? – Carlo Nov 17 '14 at 2:52
• There isn't a simple answer to your question. If the workload has lots of short reads, better throughput may not help. – Eric Gunnerson Nov 17 '14 at 3:55
• There really isn't any other information or description other than this question. I think the workload is supposed to be "constant" if that makes sense. – Carlo Nov 17 '14 at 4:06
• Perhaps I can provide some more details. So the new speed would be .78 -- a ratio of 1/.78 and to get the percent we would subtract 1 which would give me 28%. Can anyone confirm this? – Carlo Nov 17 '14 at 16:40 