Amdahl’s Law Example #2（Amdahl法则2的例子）.pdfVIP

Amdahl’s Law Example #2 • Protein String Matching Code –4 days execution time on current machine • 20% of time doing integer instructions • 35% percent of time doing I/O –Which is the better tradeoff? • Compiler optimization that reduces number of integer instructions by 25% (assume each integer inst takes the same amount of time) • Hardware optimization that reduces the latency of each IO operations from 6us to 5us. Amdahl’s Corollary #2 • Make the common case fast (i.e., x should be large)! – Common == “most time consuming” not necessarily “most frequent” – The uncommon case doesn’t make much difference – Be sure of what the common case is – The common case changes. • Repeat… – With optimization, the common becomes uncommon and vice versa. Amdahl’s Corollary #2: Example Common case 7x = 1.4x 4x = 1.3x 1.3x = 1.1x Total = 20/10 = 2x • In the end, there is no common case! • Options: – Global optimizations (faster clock, better compiler) – Find something common to work on (i.e. memory latency) – War of attrition – Total redesign (You are probably well-prepared for this) Amdahl’s Corollary #3 • Benefits of parallel processing • p processors • x% is p-way parallizable • maximum speedup, Spar S = 1 . par (x/p + (1-x)) x is pretty small for desktop applications, even for p = 2 Example #3 • Recent advances in process technology have quadruple the number transistors you can fit on