long execution time for simple lines like end and vector value addition
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Oded Scharf
on 1 May 2018
Commented: Walter Roberson
on 7 May 2018
I wanted to make my code more time efficient so looked in the profiler and saw that the most time consuming lines are 'end' and another quite simple line. it is a long loop but other lines in it runs much faster. Does anyone know why they take so long or have an idea how ti improve this loop?
Thanks!
1 Comment
Jan
on 7 May 2018
Under Matlab R2016b/Win10_64/i7 I get
sec calls
0.08 2399800 Efoot(n)=Efoot(n-1)+dt*(qin(n-1)+qm_foot-qsk(n-1));
0.08 2399800 Esk(n)=Esk(n-1)+dt*(qsk(n-1)+qm_sk-qout(n-1));
In your example the input data are 4 times bigger, but ths code needs 240 times longer. This is surprising. The main work is done in findpeaks.
Which Matlab version are you using?
Accepted Answer
Walter Roberson
on 7 May 2018
>> 19.30/10799100
ans =
1.78718596920114e-06
The iterations are not taking long; you are just doing a lot of them.
You are doing 2 additions, 4 subtractions, and 1 multiplication on the line that is taking the most time.
You could potentially reduce the time by assigning n-1 to a variable, but I would not be surprised if the Just In Time compiler is effectively already doing that internally.
2 Comments
Walter Roberson
on 7 May 2018
When MATLAB detects a code sequence that can be handled by the high performance libraries it calls into them. The code pattern might cross multiple lines so the required time cannot always be allocated to one line. MATLAB allocates it to the end statement instead.
More Answers (2)
Jan
on 1 May 2018
A very important part of the code is missing: Is Efoot and Esk pre-allocated properly? Otherwise the arrays grow iteratively, which is very expensive.
Analysing the run time behavior of a code, which is shown as screenshot and partially only, is a very bold task. I cannot guess e.g., if "qin" is an array or a function. Please post the code as text, such that it can be used by copy&paste, and add some meaningful test data - either by attaching them as MAT file or by some rand() commands with relevant sizes.
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