Re: Parallel Loops (was More then one active Vi with open Front panel?)

[Bill Gilbert]

Sometimes they are needed, but I have seen one really egregious LV app 
in particular where the author had about 7 while loops checking local 
and global flag  variables to see if they should run the code they 
contained. The program automated a highly sequential process, and would 
have been much better off with a state machine exerting strict control 
over the flow. As it was, it was like a car full of kids yelling "are we 
there yet?" every few milliseconds. This generated lots of useless cpu 
activity running loops that had nothing to do. A state machine only 
executes the code that needs to be running.

Flow control was abandoned to the task manager, and it took me two days 
to find one bug that wouldn't show when execution highlighting was on, 
but would appear as soon as it was turned off. I think some famous comic 
said "Timing is everything".

In addition, that program construction rendered the LV debugging really 
hard to use. When using execution highlighting, or single stepping,the 
point of action shifted at random around this large diagram, turning it 
into a software version of Whack-a-Mole. With a state machine, you know 
where to look.

I generally avoid them, but then, since I came here I've mostly been 
automating fairly slow test sequences, not pushing the envelope of high 
sample rates etc. The only reason I've used an independent loop is to 
poll an Abort button to update lots of local variables used to shut down 
a long process with lots of nested loops.

-BG

Bill Gilbert, EM Tech
UMN School of Physics and Astronomy
Tel 612 624 4870
Fax 612 624 4578        
Paul F. Sullivan wrote:
Michael,

You wrote:

...I enjoy discussions on methodology and implementation techniques... 
The issue lately is: single loop good, multiple loops bad.


Without multiple loops you lose the parallelism that is the hallmark of 
LabVIEW. Why would multiple loops be bad? If it's the block diagram real 
estate taken up by the loop structure, you could hide the loops in subVIs.



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