英语学习笔记：2017-05-27

Parallel computing

并行计算

Parallel computing is a type of computation in which many calculations or the execution of processes are carried out simultaneously. Large problems can often be divided into smaller ones, which can then be solved at the same time. There are several different forms of parallel computing: bit-level, instruction-level, data, and task parallelism. Parallelism has been employed for many years, mainly in high-performance computing, but interest in it has grown lately due to the physical constraints preventing frequency scaling. As power consumption (and consequently heat generation) by computers has become a concern in recent years, parallel computing has become the dominant paradigm in computer architecture, mainly in the form of multi-core processors.

Dennard scaling

Dennard scaling, also known as MOSFET scaling, is a scaling law based on a 1974 paper co-authored by Robert H. Dennard, after whom it is named. Originally formulated for MOSFETs, it states, roughly, that as transistors get smaller their power density stays constant, so that the power use stays in proportion with area: both voltage and current scale (downward) with length.

Dennard在1974发了篇论文，预测说晶体管尺寸变小，功耗会同比变小，换句话说相同面积下功耗不变。

根据Moore’s Law，晶体管数随尺寸缩小按接近平方关系增长，和Dennard scaling放在一起就是说单位面积晶体管数不断增加而功耗保持不变。过去他们一对好基友一起统治了近30年，芯片性能随工艺稳定提升，芯片产业一片繁荣昌盛；不幸的是，到2005年为止，Dennard scaling提前终结了，还记得倒霉的奔4吗？也就是说，单位晶体管功耗改进速度比尺寸缩小速度慢，这下好了，步调不一致就扯到蛋了。单位面积功耗持续上升导致的一个问题就是Dark Silicon。

Dark silicon

In electronics industry, dark silicon is the amount of circuitry of an integrated circuit that cannot be powered-on at the nominal operating voltage for a given thermal design power (TDP) constraint. This is a challenge in the era of nanometer semiconductor nodes, where transistor scaling and voltage scaling are no longer in line with each other, resulting in the failure of Dennard scaling. This discontinuation of Dennard scaling has led to sharp increases in power densities that hamper powering-on all the transistors simultaneously at the nominal voltage, while keeping the chip temperature in the safe operating range. According to recent studies, researchers from different groups have projected that, at 8 nm technology nodes, the amount of Dark Silicon may reach up to 50%-80% depending upon the processor architecture, cooling technology, and application workloads. Dark Silicon may be unavoidable even in server workloads with abundance of inherent client request-level parallelism.

现在处理器设计中的十分普遍的现象——dark silicon，也叫“暗硅”。意思是说，由于功耗的限制，一个很高端的处理器，比如多核的，其实同一时刻只能有很少的一部分门电路能够工作，其余的大部分处于不工作的状态，这部分不工作的门电路，就叫做“暗硅”。

Kotlin

Google 官方推荐的 Andriod 开发语言

Kotlin is a statically-typed programming language that runs on the Java Virtual Machine and also can be compiled to JavaScript source code or uses the LLVM compiler infrastructure. Its primary development is from a team of JetBrains programmers based in Saint Petersburg, Russia (the name comes from Kotlin Island, near St. Petersburg). While the syntax is not compatible with Java, Kotlin is designed to interoperate with Java code and is reliant on Java code from the existing Java Class Library, such as the collections framework. Kotlin is similar to Apple's Swift.

Amdahl's law

In computer architecture, Amdahl's law (or Amdahl's argument) is a formula which gives the theoretical speedup in latency of the execution of a task at fixed workload that can be expected of a system whose resources are improved. It is named after computer scientist Gene Amdahl, and was presented at the AFIPS Spring Joint Computer Conference in 1967.

G.M.Amdahl在1967年提出了Amdahl’s law，针对并行处理的scalability给出了一个模型，指出使用并行处理的提速由问题的可并行的部分所决定。这个模型为并行计算系统的设计者提供了指导。