Speed up time of AI Create Channel.vi task containing thousands of channels

Question

I am creating an AI task containing a total of 8000 channels of different channels with each channel being acquired many times before the next channel is acquired.  The channel list is something like 0,0,0,...,1,1,1,...,2,2,2...3,3,3... where each channel number would be acquired 2000 times.  The purpose is to perform averaging.  I am seeing that DAQmx Create Channel.vi takes about 100 msec to create 1000 channels for a total of 800 msec to create all channels (DAQmx 9.4, LV 11, WIN 7, CoreDuo P8700 2.5 GHz).  I would like to speed this up by a factor of 10 if possible.  Test vi attached.
Attachments:
Create Multiple Channels duration.vi ‏17 KB

Answer 1

Hi SteveP,
The DAQmx task contains data structures representing channels, which in turn contain other data structures representing channel properties and such. Most of the execution time of DAQmx Create Channel is spent allocating and initializing these data structures. They are separate from the task's input buffer (where data is stored until DAQmx Read is called). The input buffer is allocated when you reserve the task (which is normally done by DAQmx Start Task), and it is indeed allocated as a single large array.
DAQmx support for repeated channels focused on the SCXI use case, where hundreds of milliseconds of channel creation time seemed to be an acceptable tradeoff for measuring thousands of thermocouples. During development of NI-DAQmx 7.0, we used 3072 channels as our large system benchmark for channel creation, since that's the maximum number of channels supported by an SCXI task: 8 chassis x 12 modules x 32 channels (ignoring CJC channels). Repeated channels also make your use case possible, but this wasn't the original goal, otherwise we would have tried to make it easier to use for this purpose.
When replying to your previous thread on this topic (DAQmx AI Task with many samples CH0, then many CH2, then many CH3), I assumed that you were doing this because you were having problems when you took the straightforward approach to averaging, but Ben is right to point out that there are ways to deal with ghosting, cross talk, etc. What is the source impedance of the signals that you are measuring? How Do I Eliminate Ghosting From My Measurements? lists some options for dealing with high source impedance.
Failing that, is it possible for you to reduce the frequency at which your program creates and destroys tasks? If you can extend the task lifetime by reusing the same task more than once, you can avoid the channel creation overhead, or at least amortize it over the lifetime of the task. Note that you don't have to clear one task before you create a second one; you only have to make sure that two tasks don't have the same hardware reserved at the same time--if you do, you will get a reservation error. In most cases, stopping the first task before starting the second is sufficient to avoid this. It is possible to create multiple tasks up front, then switch between them throughout the rest of your program.
Brad
Brad Keryan
NI R&D