# Trouble understanding Break-Even Point for upgrade

Suppose the daytime processing load consists of 60% CPU activity and 40% disk activity. Your customers are complaining that the system is slow. After doing some research, you have learned that you can upgrade your disks for 8,000 dollars to make them 2.5 times as fast as they are currently. You have also learned that you can upgrade your CPU to make it 1.4 as fast for 5,000 dollars.

a. Which would you choose to yield the best performance improvement for the least amount of money?

b. Which option would you choose if you don't care about the money, but want a faster system?

c. What is the break-even point for the upgrades? That is, what price would be charged for both upgrades to make their cost and performance improvement equal?

Ans.

Fraction of work: 60% CPU, 40% disk.

SCPU = 1/((1-f)+(f/k)) = 1/((1-0.60)+(0.60/1.4)) = 120.69%

SDISK= 1/((1-f)+(f/k)) = 1/((1-0.40)+(0.40/2.5)) = 131.58%

a. Choose the CPU upgrade: 5000/120.69% = $41.43 8000/131.58% =$38.00 b. The disk option gives a better performance improvement.

c. All things are equal when you pay 41.43 * 131.58 = \$5451.16 for the disk upgrade.

I don't quite understand the break-even logic.. how are things equal when you do 41.43 * 131.58 ? maybe I'm approaching it wrong.

• If you're going to copy-paste material, please credit the original source. Also, what are your thoughts? How do you think it should be approached? Please edit the question to show us your attempt to solve that part of the question, and the reasoning/approach you took. The more you tell us, the more likely we can give you a useful answer. – D.W. Mar 6 '17 at 17:31