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Amdahl's Law for Tail Latency


Amdahl's Law for Tail Latency, illustration

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Translating the impact of Amdahl's Law on tail latency provides new insights on what future generations of data-center hardware and software architectures should look like. The emphasis on latency, instead of just throughput, puts increased pressure on system designs that improve both parallelism and single-thread performance.

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Computer architecture is at an inflection point. The emergence of warehouse-scale computers has brought large online services to the forefront in the form of Web search, social networks, software-as-a-service, and more. These applications service millions of user queries daily, run distributed over thousands of machines, and are concerned with tail latency (such as the 99th percentile) of user requests in addition to high throughput.6 These characteristics represent a significant departure from previous systems, where the performance metric of interest was only throughput, or, at most, average latency. Optimizing for tail latency is already changing the way we build operating systems, cluster managers, and data services.7,8 This article investigates how the focus on tail latency affects hardware designs, including what types of processor cores to build, how much chip area to invest in caching structures, how much resource interference between services matters, how to schedule different user requests in multicore chips, and how these decisions interact with the desire to minimize energy consumption at the chip or data-center level.2


 

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