Pipelining

Pipeline (computing)

A technique used in advanced microprocessors where the microprocessor begins executing a second instruction before the first has been completed. That is, several instructions are in the pipeline simultaneously, each at a different processing stage.
The pipeline is divided into segments and each segment can execute its operation concurrently with the other segments. When a segment completes an operation, it passes the result to the next segment in the pipeline and fetches the next operation from the preceding segment. The final results of each instruction emerge at the end of the pipeline in rapid succession.

Although formerly a feature only of high-performance and RISC -based microprocessors, pipelining is now common in microprocessors used in personal computers. Intel's Pentium chip, for example, uses pipelining to execute as many as six instructions simultaneously.

Pipelining is also called pipeline processing.

(2) A similar technique used in DRAM, in which the memory loads the requested memory contents into a small cache composed of SRAM and then immediately begins fetching the next memory contents. This creates a two-stage pipeline, where data is read from or written to SRAM in one stage, and data is read from or written to memory in the other stage.

DRAM pipelining is usually combined with another performance technique called burst mode. The two techniques together are called a pipeline burst cache.


For the downloading technique, see HTTP pipelining.

Instruction scheduling on the Intel Pentium 4.In computing, a pipeline is a set of data processing elements connected in series, so that 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:

Instruction pipelines, such as the classic RISC pipeline, which are used in processors to allow overlapping execution of multiple instructions with the same circuitry. The circuitry is usually divided up into stages, including instruction decoding, arithmetic, and register fetching stages, wherein each stage processes one instruction at a time.
Graphics pipelines, found in most graphics cards, which consist of multiple arithmetic units, or complete CPUs, that implement the various stages of common rendering operations (perspective projection, window clipping, color and light calculation, rendering, etc.).
Software pipelines, consisting of multiple processes arranged so that the output stream of one process is automatically and promptly fed as the input stream of the next one. Unix pipelines are the classical implementation of this concept.

Concept and motivation

Pipelining is a natural concept in everyday life, e.g. on an assembly line. Consider the assembly of a car: assume that certain steps in the assembly line are to install the engine, install the hood, and install the wheels (in that order, with arbitrary interstitial steps). A car on the assembly line can have only one of the three steps done at once. After the car has its engine installed, it moves on to having its hood installed, leaving the engine installation facilities available for the next car. The first car then moves on to wheel installation, the second car to hood installation, and a third car begins to have its engine installed. If engine installation takes 20 minutes, hood installation takes 5 minutes, and wheel installation takes 10 minutes, then finishing all three cars when only one car can be operated at once would take 105 minutes. On the other hand, using the assembly line, the total time to complete all three is 75 minutes. At this point, additional cars will come off the assembly line at 20 minute increments.

Costs, drawbacks, and benefits

As the assembly line example shows, pipelining doesn't decrease the time for a single datum to be processed; 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.