1. LabVIEW Performance Issues David Thomson Droplet Measurement Technologies May 20, 2010
Droplet Measurement Technologies, 2545 Central Avenue, Boulder, CO 80301, ph: , fax:

2. SP2 4.0
Data acquisition program for the Droplet Measurement Technologies Single Particle Soot Photometer
Updated recently from older program
LV 7.1 to LV 8.6.1
Traditional DAQ to DAQmx
Droplet Measurement Technologies, 2545 Central Avenue, Boulder, CO 80301, ph: , fax:

3. SP2 4.0 Main control program Housekeeping program
Opens other top-level modules
Displays data
User interaction, control set-points
Monitors Alarms
Housekeeping program
Reads A/D channels
Logs A/D channels and instrument state
Droplet Measurement Technologies, 2545 Central Avenue, Boulder, CO 80301, ph: , fax:

4. SP2 4.0 – Data Acq. Module Separate top-level module
Continuously acquires data at 5MS/s, 4 ch.
Scans data for randomly occurring events (e.g. particles)
Saves data to file
Under high particle loading, the scanning/saving cannot keep up with the continuous data rate
Droplet Measurement Technologies, 2545 Central Avenue, Boulder, CO 80301, ph: , fax:

5. SP2 4.0 Data Acq. Module Two loops
Main “Scan” loop scans acquired data for events, saves data
Data Acq loop continuously acquires “buffers” of data and makes them available to the main loop
Data Acq loop is in a sub-vi with a higher priority
Droplet Measurement Technologies, 2545 Central Avenue, Boulder, CO 80301, ph: , fax:

6. SP2 4.0 Data Acq. Loop
Registers a Daq “software” event which then triggers acquisition when the hardware buffer is full
Typical buffer is 1,000,000 points, 2 bytes/point, 4 channels = 8 MB
Need an efficient data transfer mechanism to send data to Main scan loop
Droplet Measurement Technologies, 2545 Central Avenue, Boulder, CO 80301, ph: , fax:

7. SP2 4.0 – Data Transfer Functional Global Global Queue Lossy Queue
Historically recommended
Global
Historically frowned upon, especially for large data sets
Queue
Recommended by Steve Solga
Lossy Queue
Only want most recent data anyway
Droplet Measurement Technologies, 2545 Central Avenue, Boulder, CO 80301, ph: , fax:

8. SP2 4.0 – Data Transfer Results Ratios for time required by Data Loop to post data
Functional Global
Global x1.9
Queue x 2.2
Lossy Queue (with reg x 2.1
queue in unused case)
Lossy Queue x 8.2
(larger numbers are faster)
Droplet Measurement Technologies, 2545 Central Avenue, Boulder, CO 80301, ph: , fax:

9. Data Transfer – Conclusions
Something is wrong with the Functional Global
Globals aren't THAT bad
Queues are pretty good for large data sets
Lossy Queues are GREAT!
IF you don't include a regular Enqueue anywhere on the block diagram!
Droplet Measurement Technologies, 2545 Central Avenue, Boulder, CO 80301, ph: , fax:

10. Data Transfer – Main Conclusion
Question all your assumptions
Droplet Measurement Technologies, 2545 Central Avenue, Boulder, CO 80301, ph: , fax:

11. SP2 4.0 – Additional Performance Point
Don't bundle large data sets into a multi- purpose cluster in a state machine
Bundling and unbundling incur large memory/performance hits
Droplet Measurement Technologies, 2545 Central Avenue, Boulder, CO 80301, ph: , fax:

12. Another Performance Bug
Depending on how an array is indexed and the resulting array is put on a shift register, the subsequent steps of the VI slow down a factor of 10
See example and notes posted on ni.com
search for “odd memory performance bug”
Droplet Measurement Technologies, 2545 Central Avenue, Boulder, CO 80301, ph: , fax: