Amdahl's Law
Amdahl's Law: "The overall speedup of a system is limited by the fraction of the task that cannot be parallelized". If eight people together clean a room, but only one holds the key to a door that takes thirty minutes to find, the whole job can never finish faster than thirty minutes.
Origin
In 1967, Gene Amdahl, chief architect of IBM's System/360, presented a three-page paper, "Validity of the Single Processor Approach to Achieving Large Scale Computing Capabilities," at the AFIPS Spring Joint Computer Conference. The prevailing optimism held that adding ever more processors would yield ever more speed. Amdahl objected: every task contains a portion that must run sequentially, and no matter how many processors are added, that portion never gets faster. The ceiling on total speedup is set not by the parallelized part, but by that small unparallelizable fragment.
Meaning
If eight people together clean a room, but only one holds the key to a door that takes thirty minutes to find, the whole job can never finish faster than thirty minutes. That is the whole of Amdahl's Law. Scale a program to 100 cores, and if 5% of it must run sequentially, the theoretical maximum speedup never exceeds 20x. This discovery — that hardware spending alone cannot dissolve a bottleneck — became the founding premise of multicore and cloud system design.
Lesson — Meeting Eastern Classics
I Ching, Commentary on the Judgment for Hexagram 28 (大過, Great Preponderance): "棟橈, 本末弱也" — the ridgepole sags because the root and the tip are weak. However thick and sturdy the central beam, if the two ends supporting it are weak, the whole beam bends. This 3,000-year-old insight overlaps exactly with Amdahl's Law: a system's limit is set not by its strongest part, but by its weakest.
"弱" combines two bent bows (弓) with hair-like strokes (彡彡) — bows too soft to hold tension, bent and weak, giving "feeble, weak." In Amdahl's Law, 弱 names the unparallelizable sequential fraction. However much the 強 (strong) portion is scaled up, the 弱 portion holds the whole system back — the same logic by which the I Ching's ridgepole sags.
