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Questions about multithreading

Question asked by snicolay on Sep 21, 2015
Latest reply on Dec 11, 2017 by Stan Bischof

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Hello,
I encounter some issues using Agilent Vee Pro 9.32 and the documentation is not clear about multithreading, could someone help me please :
I want to have 2 threads, each generating random data and storing the generated data in a global variable. This global variable is a record with several fields, Thread 1 stores data in one field, Thread 2 stores data in another field (not the same as Thread 1). When I run the program, it starts running ok, but after a few seconds, it crashes. So I'm wondering:

- Is possible to use global variables in thread objects ?
- If I declare a global record with 2 fields, is it possible that 2 threads access different fields of the same record at the same time, or should I protect the access to the record ?
- Is there a way to protect the data such as a semaphore ? (cannot find any semaphore object)

Thank you.

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Mike Sep 29, 2015 11:19 AM

It sounds as though you have answered some of your own questions and could answer more with a couple of experiments :-)

Without trying it I would say that you _have_ to use global variables ( okay you could use a file or the like ) to communicate between threads and I would caution against a record as it is data container and as such changing one part in one thread and another in another thread seems fraught with danger.

If you want to compartmentalise the data you could create a 'structure' library - a VEE file which only declares ( and maybe initialises ) a set of data items. You can then access them by importing the library and accessing the various data values in the imported lib ( that also restricts scope to 'library' ).

Making your own semaphone using a boolean flag or 2 should be safe and straightforward enough. I thought ther might be more examples provided but there aren't... maybe we should make one. Why not post how you get on including your example and if appropriate I'll have have a hack at it as well.

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Mike Dec 6, 2017 7:50 AM

I recently had occasion to want to use this in an application and so have revisited this issue. After looking at ideas for semaphores and the like ( mutex ) I thought I'd try it first. 3 threads: main plus two from thread objects all writing to and reading from the same global record without issue.

So what's with the OP's problem? I think we've been looking in the wrong place and the issue is actually due to VEE's 'feature' of hogging all of the CPU when running in a loop. What we know of that is that for the typical VEE program there's no problem but everything else on the PC slows down. Well I think the same is happening where 2 threads, running delay or similar 'time delay' mechanisms, are each hogging the CPU so one eventually crashes. You can get a hint for this as it doesn't always error on the write to the global.

The solution for the CPU hogging is to put a thread sleep in the loop somewhere: include mscorlib in the .NET references, System.Threading namespace. Then in a formula box System.Threading.Thread.Sleep(10) for a 10 ms pause, which effectively yields control of the thread so other processes can use it. By experiment less than 10 ms does nothing and more doesn't buy you any more 'ceding' of the processor.

I tried this with the experiment suggested by Stan Bischof and it seems to fix the problem. No flags needed, just read & write to the global as needed.

I'd add a file but the forum won't allow that. Video, yes. A picture no problem. A VEE file? What would you want with one of those?

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Stan Bischof @ Mike on Dec 11, 2017 11:06 AM

"I'd add a file but the forum won't allow that. Video, yes. A picture no problem. "

Very simple solution since VEE source is plain text. Just in-line the file and you're done. Can't get stripped or "lost" and doesn't depend upon any sort of packaging (MIME, etc).

Stan

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Mike Sep 29, 2015 11:19 AM
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It sounds as though you have answered some of your own questions and could answer more with a couple of experiments :-)

Without trying it I would say that you _have_ to use global variables ( okay you could use a file or the like ) to communicate between threads and I would caution against a record as it is data container and as such changing one part in one thread and another in another thread seems fraught with danger.

If you want to compartmentalise the data you could create a 'structure' library - a VEE file which only declares ( and maybe initialises ) a set of data items. You can then access them by importing the library and accessing the various data values in the imported lib ( that also restricts scope to 'library' ).

Making your own semaphone using a boolean flag or 2 should be safe and straightforward enough. I thought ther might be more examples provided but there aren't... maybe we should make one. Why not post how you get on including your example and if appropriate I'll have have a hack at it as well.
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Mike Dec 6, 2017 7:50 AM
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I recently had occasion to want to use this in an application and so have revisited this issue. After looking at ideas for semaphores and the like ( mutex ) I thought I'd try it first. 3 threads: main plus two from thread objects all writing to and reading from the same global record without issue.

So what's with the OP's problem? I think we've been looking in the wrong place and the issue is actually due to VEE's 'feature' of hogging all of the CPU when running in a loop. What we know of that is that for the typical VEE program there's no problem but everything else on the PC slows down. Well I think the same is happening where 2 threads, running delay or similar 'time delay' mechanisms, are each hogging the CPU so one eventually crashes. You can get a hint for this as it doesn't always error on the write to the global.

The solution for the CPU hogging is to put a thread sleep in the loop somewhere: include mscorlib in the .NET references, System.Threading namespace. Then in a formula box System.Threading.Thread.Sleep(10) for a 10 ms pause, which effectively yields control of the thread so other processes can use it. By experiment less than 10 ms does nothing and more doesn't buy you any more 'ceding' of the processor.

I tried this with the experiment suggested by Stan Bischof and it seems to fix the problem. No flags needed, just read & write to the global as needed.

I'd add a file but the forum won't allow that. Video, yes. A picture no problem. A VEE file? What would you want with one of those?
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- Stan Bischof @ Mike on Dec 11, 2017 11:06 AM
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  "I'd add a file but the forum won't allow that. Video, yes. A picture no problem. "
  
  Very simple solution since VEE source is plain text. Just in-line the file and you're done. Can't get stripped or "lost" and doesn't depend upon any sort of packaging (MIME, etc).
  Stan
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