05-09-2008 05:15 PM
Keywords: buffer overwritten, 200279, -200279, timed while loops, input channels, ReadMX, PCI 6229 DAQ, Windows XP, fast acquisition
My problem is related to a buffer overwritten error (200279). I have 8 concurrent tasks running (i.e. 8 while loops):
The data acquisition is set on: continuous acquisition mode, acquire 20 samples, at 9kHz and there are 13 channels (analog inputs) that are being read. For the data generation there are 4 voltages being output every 2 ms.
The symptoms are: the program runs for a while normally but then it returns a 200279 error (there's no consistent time for when the error shows up) and stops. Just before (or at the same time) as the 200279 error is created the timed while loops appear to stop for 5 seconds (+/- 0.5 sec). I've found this out by time-stamping every iteration of all the loops in my program. Interestingly enough the non-timed while loops seem to be running (this includes the while loop that contains the wait which keeps on running) normally during this 5 second stop. I changed the while loop that contains the wait to a timed loop and that results in it also not running during the 5 second stop. Moreover if I change the timed-while loop that performs the data acquisition into a non timed-while loop (with a delay) I still get the 200279 error but this time LabView crashes!!!
My problem is that I don't know what causes the buffer to be overwritten. I am reading 20 samples every 2ms (with very few loops running late (i.e. 1 or 2)) which translates to reading 10kSamples every second. My point here is that if my acquisition is running at 9kHz I am reading 10kSamples then I'm reading more samples than I'm acquiring--- presumably I need to wait to acquire those samples --- and that should mean I can't be filling up the buffer. Note: I also tried reading all the samples in the buffer (by setting number of samples read to -1) but this doesn't help either.
From the debugging I've done I have a feeling that having 5 timed-while loops (of which 3 are running at 2 ms) might be creating problems. Additionally I'm using a queue to store the samples I read (this might lead to some priority-inversion problems … but I don't think it should be resulting in waiting times beyond a couple of hundred ms).
The next thing I've done was to go for N-samples N-channels and this option works without returning any error (as expected). However every now and then the program hangs for about 5 seconds (similar behavior that was causing the buffer overflow error during acquisition mode).
Running LabView 8.0, DAQ: M-series 6229 on PCI slot, Windows XP SP3, Intel Core2Duo @ 2.33 GHz, 2 GB of RAM.
My questions for you guys are:
Or any pausing of the timed-while loops?
Thank you for your input!
05-12-2008 12:31 PM
I believe that the source of this error is that you're trying to run you're loops at 500Hz. On a Windows operating system the fastest you can comfortably run an empty loop is several kHz, but when you add in other processing into this loop this rate decreases dramatically. In order to avoid buffer overflow errors it is recommended that you set your samples to read to be at least 10% of your sample rate in Hz. When you call a DAQmx Read LabVIEW asks Windows to move data from the DAQ card's first in first out (FIFO) queue in RAM into LabVIEW's memory space in RAM. The largest source of delay in this process is waiting for Windows to perform this task--in general the response is on the order of a few ms. You can reduce the effect of this delay by reading more samples each time you call a DAQmx Read. For instance, on an average computer you can comfortably acquire at 1 MHz if your DAQmx Read is reading at least 100,000 Samples/sec. However if you run the same program acquiring at 1 kHz, but only reading 1 sample at a time the same machine would likely encounter a buffer overflow error.
I would recommend modifying your code so that all the loops run at about 10 Hz by increasing the number of samples to read to be 10% of your sample rate. If you have multiple input and output tasks in your program it’s entirely possible that it isn't the same buffer overflowing each time so tracking down exactly which task is causing the problem is a moot point. The real issue is that the CPU is being worked too hard to read from all the buffers fast enough. Also, in general, timed while loops are more useful if you're using a deterministic or real time operating system. In these operating systems software execution always takes the same amount of time where as in a non-real time operating system (like Windows) other processes, including background processes, can change the amount of time it takes for code or a VI to execute. For these applications I would usually just use a while loop with a Wait (ms) VI in it. It is important to put the wait in the loop so that the CPU doesn't continually run the loop as fast as it can (this is another tactic for reducing the CPU's work load).
I hope this helps get you started, and have a great day!
05-28-2008 07:15 PM - edited 05-28-2008 07:19 PM
05-29-2008 05:37 PM
This is interesting behavior that you're seeing. I think that it may be caused by the loop priority settings--while this feature is enabled for Windows, Windows is not designed to handle these priorities so in general it is not recommended. I would suggest removing the priorities and trying it that way.
Also, there are a couple benefits to using timed loops over while loops with a wait VI including the ability to set which processor each loop executes on, however, as far as timing goes both of these loops will be identical. On a Real-Time system these loops operate deterministically, but on a Windows systems processes, including timed loops can be interrupted anytime so the timing between these two types of loops is equivalent.
Just to make sure we're on the same page, when you use a while loop you should put a Wait (ms) VI in the loop to control the loop timing. If you specify 100 ms on the input of this VI then each loop iteration will take either 100 ms or the length of time it takes to execute the other code in the loop, whichever is longer. As long as the rest of your code can execute in 100 ms each iteration will take 100 ms and you can control the timing in this way.
I hope this helps, and I'd be interested to hear what you find out. Have a great night!
06-03-2008 05:31 AM
I thought I might give you some of my experience regarding having two timed-while-loops.
I had the problem with stalling application (~5-6 secs.) due to having two timed while loops.
I've been trying for some days to find what could be the problem, changing priority, cpu#,
timings etc. However, today I discovered that when I replaced one of the timed-loops with
"a standard while with a wait-statement inside", everything worked out really good. No more
stalling, and everything is working as expected.
I use a Core2Duo @ 2.4Ghz, 2GB ram, Labview 8.2.1 and Labview 8.5.1 on WinXP Pro SP3.
As already stated in the comments above, I also think windows is not as capable of handling
these time-loops as one can think (anyway, as I thought :-) ).
Attached is a picture showing my current solution (one timed loop and one standard while loop).
06-03-2008 12:21 PM
06-16-2008 03:53 PM
06-17-2008 04:53 PM
As far as I know there are no known issue with LabVIEW and/or DAQmx having to do with hyper threading. Would it be possible for you to post your VI so that I can look into the issue that you're seeing? If you're not comfortable posting your code on the public forum would it be alright for me to look up your email and contact you directly?