Aborting Timed Loops

[eberaud]

Hi,

I am trying to abort a timed loop in order to quickly stop the VI that contains it without having to wait for the next iteration to wake up the loop. The example that ships with LabVIEW works fine, but when I do what appears to be the same thing, the loop itself returns error -816: "Timed structure aborted or attempted to execute another iteration of the following aborted Timed Loop.", and the Wakeup Reason left data node is "Normal". When I run the example, there is no error and the Wakeup Reason is "Aborted".

Does anybody know what could be different in my code? It's a complicated VI with a lot of code inside the loop, but I believe all that code is unrelated...

One difference with the example is that the Abort function is in the same VI as the loop in the example, whereas in my case it's inside a separate VI, but that should still work, right?

Thanks for your help!

[smithd]

I've never seen abort work without an error, so I think you have to be able to handle both situations.

Maybe it has something to do with how they're named, or the timing source, or something along those lines? I'd try just taking a subset of your code and fiddling until you spot the difference.Or just handle the error

[eberaud]

Actually I realize that even the NI example does generate -816 in its left data node if you add it. The error node coming out of the loop didn't have any error, but if I link the left error data node to the right error data node, then the error node coming out of the loop does have the error as well. Long story short you're right, it always produces an error.

Thanks!

[eberaud]

Our software now periodically run into critical errors where some resources (most likely DVR) are not released, and many components simply hang. I am highly suspicious that it comes from the Abort Timed Loop function. There are many operations inside the loop that function as pairs (open/release DVR in an IPE, dequeue/enqueue elements,...) and I want to be sure that one operation won't be performed without its counterpart also being performed. If we're unlucky and the abort occurs just after opening a DVR and just before releasing it, is it possible that the DVR dies?

The reason for the abort is not too shorten the duration of an iteration, which is always pretty short, but rather to force the loop to stop if it's just sleeping and waiting until it's time to perform its next iteration.

At this point the only acceptable solution I see would be to use a semaphore: the loop would acquire it when it starts a new iteration and release it when the iteration is done (before going back to sleep). The code sending the abort would only do so when the semaphore is available.

Am I on the right track?

 

[smithd]

4 hours ago, Manudelavega said:

Our software now periodically run into critical errors where some resources (most likely DVR) are not released, and many components simply hang. I am highly suspicious that it comes from the Abort Timed Loop function. There are many operations inside the loop that function as pairs (open/release DVR in an IPE, dequeue/enqueue elements,...) and I want to be sure that one operation won't be performed without its counterpart also being performed. If we're unlucky and the abort occurs just after opening a DVR and just before releasing it, is it possible that the DVR dies?

The reason for the abort is not too shorten the duration of an iteration, which is always pretty short, but rather to force the loop to stop if it's just sleeping and waiting until it's time to perform its next iteration.

At this point the only acceptable solution I see would be to use a semaphore: the loop would acquire it when it starts a new iteration and release it when the iteration is done (before going back to sleep). The code sending the abort would only do so when the semaphore is available.

Am I on the right track?

 

It doesn't actually abort the timed loop, the help says:

" If you attempt to abort a running Timed Loop, the Timed Loop immediately executes the current iteration and returns Aborted in the Wakeup Reason output of the Left Data node. "

You can see this in the form of a horrible horrible example which nobody should ever follow, here: https://zone.ni.com/reference/en-XX/help/371361J-01/lvconcepts/aborting_timed_structs/

If the code is possible to run on a windows target (potentially using remote references to the FPGA for example, or if you have a HAL) then you can use desktop execution trace to see memory leaks.

I've never heard of a DVR being unlocked but not locked again, I don't think this is possible (if it is, well thats just silly). That doesn't mean you still can't deadlock with them. I'd make sure your code doesn't have any DVR IPEs within other DVR IPEs, anywhere if you can help it. You might also check for fixed size queues or similar -- I've definitely seen people disable sections of code which read from a queue but forget to disable the enqueuer and hang their system that way. And I've also definitely seen DVR access within class property nodes which were themselves accessed by reference, leading to a deadlock. I wouldn't imagine any of this is caused by the fact that you're 'aborting' (but not really) the timed loop.

[eberaud]

Again, thanks Smithd. I will keep troubleshooting this issue later. For now I have another issue which is even worse: the "Stop Timed Structure.vi" itself hangs quite frequently and I can't figure out why. This is getting quite frustrating and I'm on the edge of giving up... Anybody knows what could be causing it?

I 100% guarantee that the timed loop itself is running when the call to the "Stop Timed Structure.vi" occurs. The name is correct, and most of the time it works fine, but after a few dozens of cycles (a cycle being start timed loop and abort timed loop 1s later), the "Stop Timed Structure.vi" hangs, with no possibility to recover. So this is worse than any error message

EDIT: I started a new thread for this purpose, please reply there instead of here.

Edited August 26, 2016 by Manudelavega