Pipelining

In computing, a pipeline is a set of data processing elements connected in series, where the output of one element is the input of the next one. The elements of a pipeline are often executed in parallel or in time-sliced fashion; in that case, some amount of buffer storage is often inserted between elements.

Computer-related pipelines include:

A linear pipeline processor is a series of processing stages and memory access.

A non linear pipelining (also called dynamic pipeline) can be configured to perform various functions at different times. In a dynamic pipeline there is also feed forward or feedback connection. Non-linear pipeline also allows very long instruction words.

As the assembly-line example shows, pipelining doesn't decrease the time for processing a single datum; it only increases the throughput of the system when processing a stream of data.

"High" pipelining leads to increase of latency - the time required for a signal to propagate through a full pipe.

A pipelined system typically requires more resources (circuit elements, processing units, computer memory, etc.) than one that executes one batch at a time, because its stages cannot reuse the resources of a previous stage. Moreover, pipelining may increase the time it takes for an instruction to finish.

A variety of situations can cause a pipeline stall, including jumps (conditional and unconditional branches) and data cache misses. Some processors have a instruction set architecture with certain features designed to reduce the impact of pipeline stalls -- delay slot, conditional instructions such as FCMOV and branch prediction, etc. Some processors spend a lot of energy and transistors in the microarchitecture on features designed to reduce the impact of pipeline stalls -- branch prediction and speculative execution, out-of-order execution, etc. Some optimizing compilers try to reduce the impact of pipeline stalls by replacing some jumps with branch-free code, often at the cost of increasing the binary file size.

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