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Architectural considerations

Designing and Maintaining a High-Performance Architecture

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Examples of Channels

The simplest possible channel is a local memory tag. In this case, the endpoints are a shared region of memory and the channel is virtual; the behavior of the memory is sufficient. To ensure updates are coherent, the accessor can rely on capabilities of the CPU such as interlocked memory instructions, or can implement a software coherency protocol such as [8]. This choice is determined by machine capabilities and the size of the data, options that are specific to the particular system configuration and not the processing element accessing the tag. These would be selected automatically by the channel implementation or explicitly with a channel specific interface used only when composing the system and not in the processing element.

Two processing elements using tags over a packet network would rely on a channel using a network protocol. The endpoints would be data-buffers for packet oriented stacks. The channel would comprise the protocol stack and network hardware on and between each network end station. The selection of protocol would determine the specific implementation of the data-transfer, but the semantics would be the same. Updates of the tag would be propagated over the network from one endpoint to the other. The protocol could easily be changed without affecting the processing logic.

The ultimate source and destination of most data is I/O, which itself can be modelled as a data-transfer channel. I/O can be seen as transferring data between the real world and the digital domain with the I/O device itself acting as the channel. In the proposed architecture, the choice of I/O conversion device is a system composition decision that doesn't impact the processing elements as long as the semantics of the data accessors are respected.

Any I/O for which only the latest value matters can be modelled as a tag. Control systems that sample at fixed intervals fit this model well. Any I/O device that can make it's I/O available as a CPU accessible register or write directly to RAM can the modelled as one end of a tag channel.

Here the reuse possibilities begin to be apparent: The processing logic need not concern itself with whether it is talking directly to an I/O device. Instead, any chain of channels and processing loops could be the data source and determined when the system is composed, without modifying the processing elements themselves.

Examples of Systems

Many reuse scenarios are enabled by this architecture. Here, three are highlighted:

During development, it is preferable to validate individual units of an application before composing them into a system. This architecture allows a simulated channel to take the place of a real one, by making the semantics to be emulated explicit. Test sequences of data can be applied to a processing element and the output verified in a way that closely matches the eventual behavior of a channel, provided the channel implementations being used have been properly verified to follow the same semantics.

For an application to scale, it is often necessary for the output of a processing element to be propagated to additional destinations. In this architecture, channels would be free to support multiple receive endpoints which would take advantage of multi-cast protocols. If that isn't possible in a particular topology, a processing element can be added that takes one input and produces the required number of outputs. This processing element itself would be reusable.

Often, as an application changes and I/O needs evolve, it can be necessary to introduce filtering. Filtering can be added to an existing system by adding a processing step between an existing channel and the processing elements that reads from it. Again, this filtering operation itself becomes reusable

Conclusion and Future Work

The research has revealed a useful software architecture for embedded systems. To realize this effectively, hardware support for the channels must be specified and implemented in standard components. One technology that seems to fit well here is time sensitive networking as defined by the IEEE 802.1 TSN working group. In this case, time needs to be incorporated into the channel model in a way that allows the channels and processing elements to remain independent. This adds to the most complex part of system design when using this architecture: the system composition and configuration code. Future work will include techniques to simplify and automate generating this code using system modelling languages.

References

[1] Eeles, Peter. (2006, February 15). What is a software architecture? http://www.ibm.com/developerworks/rational/library/feb06/eeles/

[2] Chandhoke S., Hayles T., Kodosky J., Guoqiang Wang, "A model based methodology of programming cyber-physical systems," in Proceedings of 7th International Conference on Wireless Communications and Mobile Computing (IWCMC), 2011, pp. 1654-1659.

[3] Bakic, A.M.; Mutka, Matt W., "A compiler-based approach to design and engineering of complex real-time systems," Distributed Computing Systems, 1999. Proceedings. 19th IEEE International Conference on , vol., no., pp.306,313, 1999.

[4] Chandhoke S., Sescila G., Hons E., “Controlling Data Transfer to build Predictable Real Time Systems,” unpublished.

[5] Graham, S.; Baliga, G.; Kumar, P.R., "Abstractions, Architecture, Mechanisms, and a Middleware for Networked Control," Automatic Control, IEEE Transactions on , vol.54, no.7, pp.1490,1503, July 2009.

[6] Molka, D.; Hackenberg, D.; Schone, R.; Muller, M.S., "Memory Performance and Cache Coherency Effects on an Intel Nehalem Multiprocessor System," Parallel Architectures and Compilation Techniques, 2009. PACT '09. 18th International Conference on , vol., no., pp.261,270, 12-16 Sept. 2009.

[7] Alan Burns and David Griffin. Predictability as an emergent behaviour. In the 4th Workshop on Compositional Theory and Technology for Real-Time Embedded Systems, 2011.

[8] Alberto Sangiovanni-Vincentelli, Defining platform-based design in EETimes, 5 Feb. 2002.

[9] Simpson, H.R., "Four-slot fully asynchronous communication mechanism," Computers and Digital Techniques, IEE Proceedings E , vol.137, no.1, pp.17,30, Jan 1990


* Erik Hons is a principal software architect within National Instruments Research and Development team. For more than ten years he has focused on embedded system architectures, FPGAs and software development. Hons holds a bachelor’s degree in Mathematics and Computer Science from Waterloo University. In addition, he received his Master of Science degree in digital communications. Previously he has been a speaker at technical conferences, most recently at Embedded World.

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