Hot spot (computer programming): Difference between revisions

Revision as of 19:20, 14 December 2011

Revision as of 16:30, 2 September 2012

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==History of hot spot detection==

The [[computer science|computer scientist]] [[Donald Knuth]] described his first encounter with what he refers to as a ''jump trace'' in an interview for ''[[Dr. Dobb's Journal]]'' in 1996, saying:

<blockquote>In the [[1960s|'60s]], someone invented the concept of a 'jump trace'. This was a way of altering the [[machine code|machine language]] of a program so it would change the next branch or [[jump instruction]] to retain control, so you could execute the program at fairly high speed instead of interpreting each instruction one at a time and record in a file just where a program diverged from sequentiality. By processing this file you could figure out where the program was spending most of its time. So the first day we had this software running, we applied it to our [[Fortran]] [[compiler]] supplied by, I suppose it was in those days, [[Control Data Corporation]]. We found out it was spending 87 percent of its time reading [[Comment (computer programming)|comments]]! The reason was that it was translating from one code system into another into another.<ref>[http://www.ntg.nl/maps/16/14.pdf Jack Woehr: An interview with Donald Knuth, April 1996.]</ref></blockquote>

===Iteration===

The example above serves to illustrate that effective hot spot detection is often an [[Iteration|iterative]] process and perhaps one that should always be carried out (instead of simply accepting that a program is performing reasonably). After eliminating all extraneous processing (just by removing all the embedded comments for instance), a new runtime analysis would more accurately detect the "genuine" hot spots in the translation. If no hot spot detection had taken place at all, the program may well have consumed vastly more resources than necessary, possibly for many years on numerous machines, without anyone ever being fully aware of this.