Discrepancies between single and double precision sum over time

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I attempted to isolate and better understand an issue happening in a more complex model. It came down to an internal "clock" we have to count elapsed time. For a sample time of 0.05, the clock implementation would just be a sum coupled with a delay. However, we noticed a considerable cumulative error between single and double precision operations. The order of magnitude of the differences seen in the scope below seems to be way higher than the ones due to the difference in precision between single and double. There's a 1s drift after merely 2100 seconds.
Something else that I am confused about is why the difference seems to shift direction (t≈1000s and t≈4100s). Any insights would be appreciated.

Accepted Answer

Jan
Jan on 16 Dec 2021
This is the expected behaviour. Remember that the sum is an instable numerical operation.
d = zeros(1,1e7);
s = zeros(1, 1e7, 'single');
for k = 2:1e7
d(k) = d(k-1) + 0.05;
s(k) = s(k-1) + single(0.05);
end
plot(d - s)
The rounding error accumulate. Single precision means about 7 valid digits. So the magnitude of the rounding effects is in the expected range.
  5 Comments
Stephen23
Stephen23 on 17 Dec 2021
Edited: Stephen23 on 17 Dec 2021
"The part I don't get is why the rounding error does not accumulate monotonically?"
Why should it?
The error (difference between the decimal values that you probably expect vs the actual binary values) is not constant, but depends on both addends... one of which is continuously changing. So the binary amount that you are actually adding changes, because the values that you are adding change.
If there was a simple monotonic linear relationship then all binary floating point error could be compensated for using a trivial offset after any calcuation. But that is definitely not the case.

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