MACROMEDIA COLDFUSION 5-ADVANCED ADMINISTRATION Manual page 241

What is Scalability?
Linear scalability
Perfect scalability—excluding cache initializations—is linear. Linear scalability,
relative to load, means that with fixed resources, performance decreases at a
constant rate relative to load increases. Linear scalability, relative to resources,
means that with a constant load, performance improves at a constant rate relative to
additional resources.
Caching and resource management overhead affect an application server's ability to
approach linear scalability. Caching allows processing and resources to be reused,
alleviating the need to reprocess pages or reallocate resources. Disregarding other
influences, efficient caching can result in superior linear application server
scalability.
Resource management becomes more complicated as the quantity of resources
increases. The extra overhead for resource management, including resource reuse
mechanisms, reduces the ability of application servers to scale linearly relative to
constraining resources. For example, when an extra processor is added to a single
processor server, the operating system incurs extra overhead in synchronizing
threads and resources across processors to provide Symmetric Multi-Processing. Part
of the additional processing power that the second processor provides is used by the
operating system to manage the additional processor and is not available to help
scale the application servers.
It is important to note that application servers can only hope to scale relative to
resources when the resource changes affect the constraining resources. For example,
adding processor resources to an application server that is constrained by network
bandwidth will provide, at best, minor performance improvements. When discussing
linear scalability relative to server resources, it is implied that it is relative to the
constraining server resources.
Understanding linear scalability in relation to your site's performance is important
because it not only affects your application design and construction but also
indirectly related concerns, such as capital equipment budgets.
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