Designing and Maintaining a High-Performance Architecture
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In distributed embedded applications, data-transfer has to be modelled explicitly because network communication channels are slow relative to processing and therefore need to operate asynchronously. The challenge in modelling data-transfer explicitly is defining an interface that handles all the possible implementations. Sending data over a TCP link looks very different from reading a memory mapped register.
Fortunately, applications tend to follow patterns based on the way they access and transfer data. Two key properties of those patterns are whether a processing element uses every value from a data source (e.g. FFT of a waveform) or only the latest (e.g. reporting a temperature) and whether the application is most sensitive to data latency or throughput. Based on this, three common “paradigms” were identified and labelled as follows:
- “Tags”: read latest value, low latency
- “Streams”: read every value, high-throughput
- “Messages”: read every value, low latency
Within each paradigm, the access to the data was largely the same. That is the read and write operations were consistent in terms of semantics and parameterization. That suggests that code that only performs reads or writes can be reused with any implementation of the same paradigm.
The configuration of different data-transfer mechanisms tended to be very different even within each paradigm. However, these differences were localized to the part of applications that was least often reused: the system level configuration of data-transfer connections. This means the application specific code can be separated from the reusable data-transfer and data-access components.
Control applications read inputs (feedback signal), process them and write outputs (control signal). Acquisition of real-world inputs and generation of outputs are a form of data-transfer. The transfer is between the digital and real worlds, and so only one “end” of the transfer is visible to the software. Because of this, inputs and outputs can be modelled by the same paradigms above. A control application then naturally divides into data-transfer and data-processing code, components which can be reused in different applications. The missing piece is the system level composition code which is application specific and thus not reusable.
Data-transfer requires drivers and system level development, knowledge of hardware, and knowledge of bus protocols. Data-processing code tends to be application domain specific such as control algorithms. While the skill sets are transferrable, there is reason to want to allow people or teams to specialize. The componentization described above enables this.
A specific mechanism for data-transfer is referred to as a channel. To reflect the fact that the processing logic at each end of a data-transfer link needs local memory access, each end of a channel must have an endpoint which represents a local buffer or copy of the data. The endpoints provide one or more accessors which are implementations of the standardized paradigm data-access interfaces. The accessors determine how the endpoints are accessed and ensure the semantics of the particular paradigm are followed. The accessors are primarily read and write operations but include other operations as necessary (e.g. status).
System configuration code creates endpoints and then links them to form channels. The channels and endpoints have configuration options specific to the implementation being selected and thus implementation specific configuration interfaces. The system configuration code is thus specific to the particular channels begin used. It is appropriate for system configuration code to be non-portable in this way. To deliver maximum performance, implementation specific options have to be accessible. If desired, higher level abstractions can automate the generation of channel configuration.
System configuration code then requests accessors which it passes to the processing components. By restricting processing components to using the access APIs, it is possible to ensure reuse of the processing code without sacrificing configurability of the data-transfer mechanism, and thus with the performance of the system. As long as the accessor semantics are followed, the processing logic will be unaffected by changes to this configuration.
Processing elements communicate by writing to and reading from endpoints on a channel via the accessors and relying on asynchronous data-transfer channels to move the data. To provide tight coordination, a timing interface has to be added that configures when the data-transfer and data-processing components operate. Generating this configuration is the responsibility of the system configuration code.