Network I/O

At the network level, many things can affect performance. The bandwidth (the amount of data that can be carried by the network) tends to be the first culprit checked. Assuming you have determined that bad performance is attributable to the network component of an application, there are more likely causes for the poor performance than the network bandwidth. The most likely cause of bad network performance is the application itself and how it is handling distributed data and functionality. I consider distributed-application tuning in several chapters (notably Chapter 12), but this section provides lower-level information to assist you in tuning your application, and also considers nonapplication causes of bad performance.

The overall speed of a particular network connection is limited by the slowest link in the connection chain and the length of the chain. Identifying the slowest link is difficult and may not even be consistent: it can vary at different times of the day or for different communication paths. A network communication path can lead from an application, through a TCP/IP stack (which adds various layers of headers, possibly encrypting and compressing data as well), then through the hardware interface, through a modem, over a phone line, through another modem, over to a service provider’s router, through many heavily congested data lines of various carrying capacities and multiple routers with differing maximum throughputs and configurations, to a machine ...

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