xPC Target in Freerun mode: extra long sample time

Question

Simone Marcheschi 2014-3-14

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https://ww2.mathworks.cn/matlabcentral/answers/121570-xpc-target-in-freerun-mode-extra-long-sample-time

回答： James Brown 2014-8-11

I run a quasi-empty model in xPC Target Freerun mode.

To approximately evaluate how much the sample time is constant, I insert in the model an S-function which outputs the system time:

static void mdlOutputs(SimStruct *S, int_T tid) {

#ifndef MATLAB_MEX_FILE

    real_T *y;
    y=ssGetOutputPortSignal(S,0);
    y[0] = xpcGetElapsedTime( (xpcTime *)NULL );

#endif

}

I have logged the output signal of this S-function and I have plotted the difference (by diff matlab function) for visualizing the approximate sample time: I found that the sample time is very little and quite constant for almost all the samples (its value is 1.2e-6 sec) except for some regulars extra long sample (9.9e-3 sec) which occur every 100 samples.

I write to ask why this happens and how it is possible to solve this problem.

Thank in advance you for your help

Best regards

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Answer 1

Suneesh 2014-3-14

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There is a shipped block called 'Task Execution Time' that does exactly what you did.
The sporadic spikes are usually due to:

Cache misses when loading states into memory at the beginning of model execution
Cache misses when background tasks move lots of data
System Management Interrupts (SMI)

Please see:

http://www.mathworks.com/matlabcentral/answers/91970-what-is-the-definition-of-task-execution-time-%28tet%29-and-what-does-it-measure-in-xpc-target

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Answer 2

Rajiv Ghosh-Roy 2014-3-17

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In addition to the other replies, Freerun mode simply runs the model as fast as it can and does not attempt to get real-time (== matches wall-clock) performance. In this mode, every so often, xPC Target will stop running the model and service other things such as network communication, screen updates, etc. This is why it takes a differing amount of time.

Freerun mode is not intended to have any determinism as to intervals between running each sample. For this, you should use Real-Time mode.