Why this specific buffer Allocation?

Win Vista, LabVIEW 8.6.1, PXI 8196, LV RT 8.6.1

I have a problem with a PXI program used to control a motor/dynamometer by PID loops on a cycle prescribed thresholds (objectives specific speed/torque to meet).

The normal use of the processor (as the output of the PXI monitor shows) is normally 2%.  My loop is 100,000 Hz, and I have been diligent in spreading the bypass, so it runs very efficiently.

A 'normal' test was for 20 minutes, so we have never noticed a problem.  However, my client has recently published a test of 8 hours, and he noticed that the CPU would creep upward and upward during the test, to where it was 75% after 8 hours.

I ran my own test of 2 hours and observed the same thing - the CPU usage crept up to 20%.

In the hunt for things, I noticed it is correlated with the size of an array of values of the PXI controller: larger than array, the CPU usage more.  During the test, the set values are located above the host right in front of the time, so the table grows slowly as the test goes along.

When the test is finished, the table does not, and nor does the CPU usage.  I can go right through to start a new test and the CPU usage drops when I send an order to clear the table.  Therefore, I am sure that the size of the array is the problem.

A two hour test contains 120 x 60 x 10 = 72000 set points (the host set points are 10 Hz, 100 Hz setpoints are interpolated from the controller) to 24 bytes, which is 1.728 MB.  NOT a RAM huge hit (available from 2 gigabytes).

I couldn't imagine why the existence of this large table would be time-consuming CPU, but the evidence is with myself: the bigger picture, the most used CPU.

In the search for an answer, I started the buffer ALLOCATIONS view and found a place where LV makes a COPY of this buffer, and I don't understand why.

But that would explain things: copy a table from 1.7 MByte to 100 Hz will certainly put a load on the processor.

So here's the code: in the TRANSIENT state table is used to select and interpolate the next set.

When the test is finished, we go to the State of SLOWDOWN, if the table is not yet used:

After that, we are in idle state, where even once, the big Board is not used.

Then, before I have to change the architecture of things to eliminate former set points (I have many problems with accounting), can someone explain WHY the buffer allocation shown is happening?

Note that I show the case of '0', the other is 'default' which manages other 9 out of 10 cases.  But there is there again, a buffer allocation when the table is not yet connected to the inside.

More importantly, avoid this allowance?

It's hard to say without poking at him but I saw the similar models result in a copy of the additional buffer.

(1) (less likely) the wire branch off to feed the size of the table is confusing LV and he thinks he needs another copy. I saw this in all LV 7.1 and can not ask with LV 8.6.1.

(2) the sub - VI "to choose and interpolate the next set" has NO control on the connector of the icon on the "root" of the diagram and or data are not connected through. To understand what I'm upgrading to please see the "clear as mud" thread , where it was called Grg McKaskle out of retirement to explain how it can work Subvi in the buffer of the appellant (that's really what you want in the code you showed us).

Reader Digest Version:

When LV 'sees' data be passed to and from a Subvi and there is no operation that will change the content of the data, the data will be passed by reference so sub - VI will do its job in the buffer of appellants.

This image of code shows the difference.

Ben

PS "Buffer Allocation" in the hand catches my attention.

Tags: NI Software

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