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Why is indexing conditional tunnel 3x faster than shift registers


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Indexing tunnels (conditional or not) follow a preallocation strategy of filling a larger-than-initially-needed array (later cutting the unneeded elements), while the “Build Array” primitive allocates a new array of exactly the right size on each iteration.  So there is a lot fewer calls to the memory manager with an indexing tunnel.

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21 hours ago, drjdpowell said:

Indexing tunnels (conditional or not) follow a preallocation strategy of filling a larger-than-initially-needed array (later cutting the unneeded elements), while the “Build Array” primitive allocates a new array of exactly the right size on each iteration.  So there is a lot fewer calls to the memory manager with an indexing tunnel.

Was aware of this but was not aware that this was also the case in initialize/replace scenarios.

More info on the subject here http://forums.ni.com/t5/LabVIEW/Why-is-indexing-conditional-tunnel-3x-faster-than-shift/m-p/3570173#M999382

Thanks :)

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On 1/15/2017 at 2:30 PM, drjdpowell said:

Indexing tunnels (conditional or not) follow a preallocation strategy of filling a larger-than-initially-needed array (later cutting the unneeded elements), while the “Build Array” primitive allocates a new array of exactly the right size on each iteration.  So there is a lot fewer calls to the memory manager with an indexing tunnel.

I thought build array did some preallocation but not very much. I rearranged the code with manual preallocation and you had to allocate thousands and thousands of elements for it to be faster.

I had a stray thought about allocating a percent of the existing array with a baseline and tried it out. For elements 0-10000 it allocates 1000 when it runs out, then for elements 10000+ its 10% of current array size. That made it about 30% faster than the indexing terminals. But, on the other hand, who cares in this situation? This use case doesn't seem like a big performance bottleneck.

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