Breaking up a computation vs "..." to continue line - Huge Speed Difference !?
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I was profiling an ode solver today and found something very strange. The system has 9 states and the computation of the ode RHS was taking about 100 times longer to compute the 5th state than any other state.
The 5th state equation was longer than the other equations, but not so drastically longer that it should take 100 times longer to compute. It ran over 4 lines in the editor so there were "..." continuations at the end of each line.
I rewrote the equation using multiple assignments instead of one big assignment and got about a 50 times speed up. By this, I mean I changed
value(5, :) = big_expression1 ...
+ big_expression2 ...
+ big_expression3 ...
+ big_expression4;
to
value(5,:) = big_expression1;
value(5,:) = value(5,:) + big_expression2;
value(5,:) = value(5,:) + big_expression3;
value(5,:) = value(5,:) + big_expression4;
Can you think of any reason why the second version should drastically outperform the first?
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Jan
2013년 2월 15일
It depends on the real processing in "big_expression". Therefore your paraphrased code does not allow to reveal the problem.
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José-Luis
2013년 2월 15일
편집: José-Luis
2013년 2월 15일
I don't think it has anything to do with "...". Shot in the dark:
Cache locality? Maybe big_expression(n) and big_expression(n+1) fit together in the cache whereas when you call them all at the same time values need to be moved around in memory.
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per isakson
2013년 2월 15일
Did you find the reason to the big difference in performance?
I've done some experiments with R2012a (64bit) without seeing anything like it.
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