LabVIEW
what would cause one read of a global be slower than another?
Hi All,
After some trouble shooting the engineer I was working with looked at me and said "WHY?"
"Why questions are always the hardest Q's to answer.
Warning! I am going to be asking about normal LV globals NOT LV2 globals. Using LV 7.1.
I have to admit that I have very little experience using globals and this is probably my first attempt to use them.
I am using them in this application because I found that a simple boolean global can be read a lot faster than the best LV2 version I could think of. In my application, the "Stop Boolean" global is only written one place but read in about 250 sepearte loops to indicate it is time to stop.
About 30 of those loops are timed loops and we noticed that two of the timed loops were asserting the "Finished Late" flag. The actual finish time was about 1 msec after what was expected.
We found that if we ran the application without the two loops that were flaging the "Finished Late", none of the other loops had problems. If we added either of the two loops that were finishing late, they would again finish late but the others never had trouble.
Running either of the two loops by themselves never give an error. Add any of the other non-late-finishers, BINGO we start seeing the finished late again. Oh bother!
I removed ALL of the code inside the loops that finished late except the read from the global boolean and still the loops finish late.
At this point I had to question the global!
I replaced the global boolean read with a LV2 style boolean and the finished late errors stopped!
We could not make (dragging a window around on the desktop helped produce the error) that loop to finish late as long as it was using the LV2.
I restored the global boolean and >>>> Finished late!
More background info.
The application dynamically loads all the VI's required to operate. A timed loop is used in every loaded module. This is how I am quickly including and excluding the offending code. Continuing....
Changing the order in which the modules (timed loops) are loaded did not make a difference.
So.....
Why would 2 timed loops always take longer to read a global boolean than any of the other timed loops?
Do gloabl reads have to execute in the UI thread?
Thank you for reading,
Ben
After some trouble shooting the engineer I was working with looked at me and said "WHY?"
"Why questions are always the hardest Q's to answer.
Warning! I am going to be asking about normal LV globals NOT LV2 globals. Using LV 7.1.
I have to admit that I have very little experience using globals and this is probably my first attempt to use them.
I am using them in this application because I found that a simple boolean global can be read a lot faster than the best LV2 version I could think of. In my application, the "Stop Boolean" global is only written one place but read in about 250 sepearte loops to indicate it is time to stop.
About 30 of those loops are timed loops and we noticed that two of the timed loops were asserting the "Finished Late" flag. The actual finish time was about 1 msec after what was expected.
We found that if we ran the application without the two loops that were flaging the "Finished Late", none of the other loops had problems. If we added either of the two loops that were finishing late, they would again finish late but the others never had trouble.
Running either of the two loops by themselves never give an error. Add any of the other non-late-finishers, BINGO we start seeing the finished late again. Oh bother!
I removed ALL of the code inside the loops that finished late except the read from the global boolean and still the loops finish late.
At this point I had to question the global!
I replaced the global boolean read with a LV2 style boolean and the finished late errors stopped!
We could not make (dragging a window around on the desktop helped produce the error) that loop to finish late as long as it was using the LV2.
I restored the global boolean and >>>> Finished late!
More background info.
The application dynamically loads all the VI's required to operate. A timed loop is used in every loaded module. This is how I am quickly including and excluding the offending code. Continuing....
Changing the order in which the modules (timed loops) are loaded did not make a difference.
So.....
Why would 2 timed loops always take longer to read a global boolean than any of the other timed loops?
Do gloabl reads have to execute in the UI thread?
Thank you for reading,
Ben
Hi Shane,
I use queues to pass data between all of those loops. They give better performance that my best round-robbin LV2 buffers.
The Globals in question is just a boolean that is false while I am running anf true when it is time to stop.
The queue does not seem to suit that use case well.
"the contents of the multiple loops (late finishers versus not late finishers) are different?"
At the tail end of my troubleshooting, the offending loops ONLY had the global and they STILL finished late. 
Ben
