By far the biggest reason I have seen for the performance discrepancy above is not due to a faulty test but due to the stress test being executed on wildly different data sets than what is in production. - http://saasinterrupted.com/2009/12/16/the-most-common-flaw-in-software-performance-testing/

http://www.jayphilips.com/2010/01/07/50-open-source-performance-testing-tools/

Discuss about performance management, a process to monitor and control performance details - http://blog.dynatrace.com/2011/09/15/why-do-you-do-apm-in-production/