I am having some performance issues with my code and I am wondering if the way I have designed it is causing memory issues. The code is a state machine design in a timed loop. The state machine logic is inside a subVI. The main VI stores data in shift registers and passes the data into the subVI via a cluster (defined in a strict type def control). The images below show an example of what I mean. The code is in a subVI strictly for the purpose of making it more organized. It does not need to be a sub VI. My real code is very large however with multiple subVI state machines in the same loop. Some run in parallel while some have data dependencies on each other so must run sequentially (there are no dependency loops). On the first iteration of the loop, some of the state machine VIs load calibration data from csv files (and I am sure it only happens on the first loop iteration, not repeatedly). They store some moderately large 1D and 2D arrays of double values in the data cluster that is stored in the shift register. Without loading the csv files the VI runs just fine. When the cal loading is enabled the CPU usage goes up and I loose communication with the RIO. Even if I am not even doing anything with the data, just having them loaded causes a large increase in CPU load. What I am wondering is if this is a memory/cpu inefficient way of storing a large buffer of data. Is the memory buffer that stores the clusters in the shift register being recopied every time the subVI is called? Would it help to make the VI inline or reentrant? After reading the "developing efficient data structures" article here it seems like just bundling and unbundling the array is potentially causing the data to be recopied. This would be very bad as this happens multiple times each loop iteration. I would not have thought this copied the data as it is just a wiring operation. I only used clusters to keep things more organized. Without the clusters each subVI would have many wires in and out. It would be very difficult to decipher which signal is which. It seems that both practices that I do just for the sake of organization and readability (using subVI's and using clusters) have a detrimental effect of performance. Is there a way to have the organizational effects of subVIs and clusters without the data overhead? In a related question, are global variables the most efficient way to pass data between parallel running loops? I have read that local and global variables copy the data into another buffer when reading or writing. Is there a way to get around this? In my code I use variables and there is always a single writer and multiple readers. For each reader to have to copy the data into their own buffers seems inefficient. Thanks for any helpful comments on this issue.

LabVIEW

State machine architecture, shift registers, subVI memory handling

The behavior you're seeing with the array being copied to/from the subVI is an unfortunate limitation of LabVIEW. What version of LabVIEW are you using? If you're using LabVIEW 2010 you can specify your subVI to be 'inline" meaning it actually doesn't act like a subVI call... it is as if the subVI's code is placed in the callers code. This is significantly better performance and avoids the copy. Another thing to look at is if you have any VIs being used in multiple different locations or in different loops, you will probably want to set these VI's as reentrant so it can be used by multiple callers at the same time.

DVRs are OK but they might not be a good idea for an RT application unless carefully used as that data set you are referencing between multiple loops is a shared resource. So if one loop is dereferencing the data... the the other loop tries to dereference the data... it has to wait for the first to finish. On RT, the best thing is really to have each loop have its own copy of the data in order to avoid shared resource issues. Then, data that is important to get between loops... use an RT FIFO. So loop A can push some data into an RT FIFO for loop B to read. RT FIFOs are not shared resources and are very high performance. I've attached an example cRIO project that uses CAN and RT FIFOs in multiple loops. Note that the code attached uses a functional global VI to stop all the loops when requested, but since that is a shared resource between loops (RT no-no) I have it set to subroutine priority and the right click option "skip if busy" set.

For your question on the in place element structure... this thing is awesome. I use it all the time to force labview to not make memory copies in time critical sections of my RT code. When you enter the in place element structure... items that are being unbundled or indexed are operated on by pointer not by value. If you just want to do a get... you can just usually do an unbundle but a get/operate/set is best for inplace element strucutre. If you just want a get though, do this:

Note that the subVI call should be inlined for best performance.

Stephen B

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LabVIEW

State machine architecture, shift registers, subVI memory handling

Re: State machine architecture, shift registers, subVI memory handling