topic Re: How to know how manv times the dequeue element function is called? in LabVIEW

How to know how manv times the dequeue element function is called?

euqinom2727 — Tue, 18 Jun 2024 13:04:37 GMT

Hey, I'm developing a producer-consumer VI and I want to stop both loops when I dequeue 100 elements. How can I count how many elements have been dequeued or how many times the dequeue element function was called?

Could you help me please? Thankss

Re: How to know how manv times the dequeue element function is called?

billko — Tue, 18 Jun 2024 13:13:52 GMT

You can use the iteration counter to count to 99 (value of 99 since it is zero-based).

Re: How to know how manv times the dequeue element function is called?

Kevin_Price — Tue, 18 Jun 2024 13:30:00 GMT

Unless you're enqueuing from multiple producers, I think it'd be smarter to count on the producer side and stop enqueuing when you get to 100. That's more in the spirit of the producer / consumer pattern where the producer's job is to *defer* the needed work and the consumer's job is to complete it.

-Kevin P

Re: How to know how manv times the dequeue element function is called?

euqinom2727 — Tue, 18 Jun 2024 13:39:56 GMT

I did this actually, but when I see my tdms file I don't have 50 elements, I have 0. So I assumed that the consumer loop is called without any elements on the queue or something like that.

Re: How to know how manv times the dequeue element function is called?

pincpanter — Tue, 18 Jun 2024 14:13:36 GMT

If the timed out? output of the Dequeue Element function is true, you have no data, so there is nothing to save.

Also, you should not use a null timeout, as the loop would spin too fast even if there are no data.

Re: How to know how manv times the dequeue element function is called?

euqinom2727 — Tue, 18 Jun 2024 14:55:14 GMT

It seems that the timed out? it's never true, so I always have data, so my theory is not true. I could count how many times I write to the file, but I don't know how to do it either

Re: How to know how manv times the dequeue element function is called?

Bob_Schor — Wed, 19 Jun 2024 02:16:14 GMT

In a Producer/Consumer design, the Producer "produces", and is "responsible" for controlling (and counting, if that's important) the things being "produced". The Producer is also responsible for knowing when Production is done (since the Producer "gets the data first").

The Consumer has no idea when the data will be coming, but knows to process it as it comes, until it gets a sign from the Producer that "Production is done", in which case the Consumer can stop waiting for more data and close whatever it is doing with the data.

So, the Producer simply counts 1, 2, 3, ... until it gets to 100 (or, if you are doing this in a For Loop, the Producer has 100 written to the "N" of the For loop). When it is done, it sends a "signal" (called a "sentinel") to the Consumer saying "I'm Done".

Until 2016, LabVIEW handled Producer/Consumer Designs with Queues. You enqueue 100 "elements" in the Producer and it goes to the Consumer. How do you signal you are done? NI seems to do this by having the Producer releasing the Queue, which causes the Consumer to throw an Error. Bad! Better is for the Producer to Enqueue One More Time and send a Sentinel. Since a frequent quantity being passed in P/C designs are Arrays of Data, a great Sentinel is an Empty Array. The Producer exits the For Loop (bring the Queue with it), and enqueues an Empty Array. The Consumer is modified so when it Dequeues, it checks for the Sentinel (Empty Array? function) and uses a Case Statement to Process Data if False (meaning Not Empty Array) or Releasing the Queue (safely!) and stopping the Consumer Loop if True.

In 2016, LabVIEW got Asynchronous Channel Wires. The Stream Channel is what you use for Producer/Consumer. No more "data wire going backwards". It also has "Element Valid?" and "Last Element?" inputs in the Channel Writer (Producer), ideal for signaling the same to the Channel Reader (Consumer). Best of all, you only need Writer and Reader, as Writer also does "Obtain Stream" and Reader will do "Release Stream" when it gets "Last Element" is true.

Bob Schor

Re: How to know how manv times the dequeue element function is called?

euqinom2727 — Thu, 20 Jun 2024 12:56:51 GMT

Nice, I used the stream channel and it worked, thank u