Consider three different processors P1, P2, and P3 executing the same instruction set. P1 has a 4GHz clock rate and a CPI of 2. P2 has a 2.5GHz clock rate and a CPI of 1.0, P3 has a 3GHz and a CPI of 1.2.
Which processor has the highest performance expressed in instructions per second?
If the processors each execute a program in 10 seconds, find the number of cycles and the number of instructions.
We are trying to reduce the execution time by 20% but this leads to an increase of 15% in the CPI. What clock rate should we have to get this time reduction?
Solution:
P1: 4 GHz, CPI = 2.0 → 4GHz/ 2.0 = 2 x 109 instructions/sec
P2: 2.5 GHz, CPI = 1.0 → 2.5GHZ/1.0 = 2.5 x 109 instructions/sec
P3: 3 GHz, CPI = 1.2 → 3GHz/1.2 = 2.5 x 109 instructions/sec
P2 has the highest performance at 2.5 GHz, CPI = 1.0 → 2.5GHZ/1.0 = 2.5 x 109 instructions/sec.
P1: 40 x 109 cycles
20 x 109 instructions
P2: 25 x 109 cycles
20 x 109 instructions
P3: 30 x 109 cycles
25 x 109 instructions
Execution time is reduced by 20% à t=? s
CPI is increased by 15%