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2.2. The Multicore Revolution
2.2. The Multicore Revolution
Performance improvements for sequential processors are tailing off. Before
2005, processor designers could use more transistors and higher clock
frequencies to continuously increase the performance of new processor generations.
Moore’s Law, which states that with improved manufacturing techniques
and the resulting miniaturization the number of transistors doubles
approximately every two years without extra cost, enabled them to increase
the amount of logic that processors could contain. They used the additional
logic to improve techniques such as out-of-order execution, branch prediction,
memory caching schemes, and cache hierarchies. Unfortunately, these
techniques are optimizations for specific usage patterns and eventually, their
returns diminish [Hennessy and Patterson, 2007; Michaud et al., 2001]. 8 With
the shrinking transistor sizes, it was possible to steadily increase the clock
frequency as well.
Higher clock speeds become impractical. Around 2005, processor designers
reached a point were it was no longer feasible to keep increasing the clock
speed with the same pace as in previous decades. An increase in clock speed
corresponds directly to higher energy consumption and more heat dissipation
[Hennessy and Patterson, 2007, p. 18]. However, handling the lost heat
beyond a certain limit requires cooling techniques that are impractical for
commodity devices. For mobile devices, the necessary increase in power consumption
and the resulting heat make clock speed increases beyond a certain
limit prohibitive. Processor designers worked around the issue by applying
various engineering techniques to handle the heat. This led to processors that
can over-clock themselves when the thermal budget permits it. However, none
of these techniques could deliver the performance improvements software developers
have gotten accustomed to over the decades.
Increasing transistor budgets led to multicore processors. Still, the transistor
budget for processors keeps increasing. In an attempt to satisfy the
software industry’s need for constant performance improvement, processor
designers started to explore the design space of parallel processors. The increasing
transistor budget can be spent on a wide range of different features
of a modern processor [Hill and Marty, 2008]. Duplicating functional units,
8 Welcome to the Jungle, Herb Sutter, access date: 27 June 2012
http://herbsutter.com/welcome-to-the-jungle/
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