How is the Simulink total block I/O size calculated exactly? It would help me prevent the I/O buffer size error

I have a wind farm simulation model which computes among others wind speeds at millions of grid nodes in a three-dimensional volume. The model mainly consists of c s-functions.

When increasing the number of nodes in a uniform grid - basically a three-dimensional matrix - further and further, at some point I get this message:

Simulink supports total block I/O buffer size up to 2147483647 number of bytes. The total block I/O size is calculated based on data type size, number of elements, and signal type (real/complex) of all signals in the model. Consider reducing signal sizes before compiling or simulating the model.

How is the "total block I/O buffer size" exactly determined? I'd love to see a more precise definiton of "The total block I/O size is calculated based on data type size, number of elements, and signal type (real/complex) of all signals in the model."

Let's assume there are two c s-functions (mex). Will all these things contribute to the "total block I/O buffer size":

• number of inputs and outputs
• dimensions and size of inputs and outputs
• dialog parameters
• internal DWork
• internal variables (int_T, real_T, real32T, vectors, pointers, ...)
• function step size (I have a multi rate model)
• number of computations/functions per time step

As it says "total block I/O buffer size", I would assume only and exclusively dimensions and size of block inputs/outputs within the simulink model are considered. Applying this logic to a simple Simulink model configuration as shown here:

I would guess the "total block I/O buffer size" is the sum of:

• 1000 (from C1)
• 1000 (to c s-function input)
• 200x50x20 (from C2)
• 200x50x20 (to gain)
• 200x50x20 (from gain)
• 200x50x20 (to c s-function)
• 3 times 700x50x20 (from c s-function)
• 3 times 700x50x20 (to "To Workspace" block)

ist that correct? Or is there some redundancy/duplication in this approach? Or did I miss something?

Answer 1

[Sumukh]

Hi v k.

MATLAB documentation does not provide details regarding how exactly the total block I/O size in Simulink is calculated. The limit on the total block I/O size is the maximum size of 32-bit signed integer (2^31-1=2147483647) for 64-bit systems, up to MATLAB R2019a, as given by below documentation:

https://www.mathworks.com/help/releases/R2019a/simulink/slref/simulink-limits.html#:~:text=Maximum%20number%20of%20bytes%20for%20the%20total%20block%20I/O%20buffer%20length%20in%20a%20model

However, the total block I/O size limit has been increased to that of a 64-bit integer size (2^64-1) on 64-bit systems, from MATLAB R2019b, as provided in the release notes below:

https://www.mathworks.com/help/simulink/release-notes.html?rntext=Simulink+limits&startrelease=R2019a&endrelease=R2019b&groupby=release&sortby=descending&searchHighlight=Simulink+limits

I would suggest you upgrade to MATLAB R2019b to increase the total block I/O size limit, as well as try to reduce the no. of Simulink blocks in the model.

I do hope this helps with your issue.

[v k]

Thanks for this interesting and helpful hint. I ran a few tests in R2023a and the issues did not appear. I was not aware of the change of this limitation between R2019a and R2019b, as this table still shows the 2^31-1 limitation in R2024a:

https://de.mathworks.com/help/simulink/slref/simulink-limits.html

So it seems this information was not updated?

Anyway, my model is running now even without changing its structure or number of Simulink blocks. Thanks again!