The control-flow module for GeoServer allows the administrator to control the amount of concurrent requests actually executing inside the server, as well as giving an opportunity to slow down users making too many requests. This kind of control is useful for a number of reasons:
Performance: tests show that, with local data sources, the maximum throughput in GetMap requests is achieved when allowing at most 2 times the number of CPU cores requests to run in parallel.
Resource control: requests such as GetMap can use a significant amount of memory. The WMS request limits allow to control the amount of memory used per request, but an OutOfMemoryError is still possible if too many requests run in parallel. By controlling also the amount of requests executing it’s possible to limit the total amount of memory used below the memory that was actually given to the Java Virtual Machine.
Fairness: a single user should not be able to overwhelm the server with a lot of requests, leaving other users with tiny slices of the overall processing power
From the Control Flow module page:
See: http://docs.geoserver.org/latest/en/user/extensions/controlflow/index.html