Computer performance: Difference between revisions

Revision as of 12:57, 18 November 2020

Revision as of 15:13, 2 December 2020

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{{Main|Channel capacity}}

Channel capacity is the tightest upper bound on the rate of [[information]] that can be reliably transmitted over a [[channel (communications)|communications channel]]. By the [[noisy-channel coding theorem]], the channel capacity of a given channel is the limiting information rate (in units of [[information entropy|information]] per unit time) that can be achieved with arbitrarily small error probability.<ref>{{cite web |url=http://www.cs.ucl.ac.uk/staff/S.Bhatti/D51-notes/node31.html |author=Saleem Bhatti |title=Channel capacity |work=Lecture notes for M.Sc. Data Communication Networks and Distributed Systems D51 -- Basic Communications and Networks |url-status=dead |archive-url=https://web.archive.org/web/20070821212637/http://www.cs.ucl.ac.uk/staff/S.Bhatti/D51-notes/node31.html |archive-date=2007-08-21 |df= }}</ref><ref>{{cite web | url = http://www.st-andrews.ac.uk/~www_pa/Scots_Guide/iandm/part8/page1.html | title = Signals look like noise! | author = Jim Lesurf | work = Information and Measurement, 2nd ed.}}</ref>

[[Information theory]], developed by [[Claude E. Shannon]] during [[World War II]], defines the notion of channel capacity and provides a mathematical model by which one can compute it. The key result states that the capacity of the channel, as defined above, is given by the maximum of the [[mutual information]] between the input and output of the channel, where the maximization is with respect to the input distribution.<ref>{{cite book| author = Thomas M. Cover, Joy A. Thomas | title = Elements of Information Theory | publisher = John Wiley & Sons, New York |year=2006}}</ref>

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{{Main|Performance per watt}}

System designers building [[parallel computing|parallel computers]], such as [[Google search technology#Production hardware|Google's hardware]], pick CPUs based on their speed per watt of power, because the cost of powering the CPU outweighs the cost of the CPU itself.<ref>{{cite web |url=http://www.eembc.org/benchmark/consumer.asp?HTYPE=SIM |title=Archived copy |accessdate=2009-01-21 |url-status=dead |archive-url=https://web.archive.org/web/20050327005323/http://www.eembc.org/benchmark/consumer.asp?HTYPE=SIM |archive-date=2005-03-27 |df= }}[http://news.cnet.com/Power+could+cost+more+than+servers,+Google+warns/2100-1010_3-5988090.html]</ref>

=== Compression ratio ===