Amdahl's law
For example, assume that we are given a serial task which is split into four consecutive parts, whose percentages of execution time are , , , and respectively. Then we are told that the 1st part is not sped up, so , while the 2nd part is sped up 5 times, so , the 3rd part is sped up 20 times, so , and the 4th part is sped up 1.6 times, so . By using Amdahl's law, the overall speedup is