tic - toc behavior

Hi all,

I'm analyzing how "tic toc" functions work in MATLAB. In the following example I use the loop:

    tic
    while toc<0.05
        toc
    end

This code simply shows the different lap values until 0.05s. When I checked the toc values MATLAB showed in the command Window I realized the first "toc" was at aprox. 12 ms, while for the rest of "tocs", the difference between them was aprox. 0.01 ms.

So my doubt is: why the first iteration takes so much time, while the others are executed much faster?

Thanks in advance.

[José-Luis]

Well, the first iteration includes whatever happens when you call tic (impossible to say since it's built-in) plus one iteration of the while loop plus whatever happens inside toc.

All subsequent iterations have one less step (no call to tic).

[Lask]

Thanks for your answer. However, I doubt the "tic" function execution takes almost 100 times the execution time of a while loop comparison.

[José-Luis]

I haven't looked at the internals but there might be an overhead when initializing a while loop as well.

[José-Luis]

tic
while false
end
toc
tic
toc

Also, using timeit() instead should give you more reliable results.

[Stephen23]

"However, I doubt the "tic" function execution takes almost 100 times the execution time of a while loop comparison."

tic has to access the computer time somehow, create and store a variable somewhere and/or check if the current call has an output. These could quite conceivably take longer than a simple comparison.

@José-Luis: can you show times for that please!

[José-Luis]

Elapsed time is 0.000004 seconds.
Elapsed time is 0.000001 seconds.

After a few runs, pretty steady results. Four times longer when just going through the motions in the while loop.

[KSSV]

YOu should check with this:

tic
t1 = 0 ;
count = 0 ;
t2 = zeros(1,[]) ;
while t1<0.05
    count = count+1 ;
    t1 = toc ;
    t2(count) = t1 ;
end

[Stephen23]

@KSSV: your code expands t2 inside the loop. How will that help measure the timing accuarately?

https://www.mathworks.com/help/matlab/matlab_prog/preallocating-arrays.html

Answer 1

[Cam Salzberger]

As José was saying, there is more happening the first iteration through the loop than all subsequent iterations.

MATLAB uses just-in-time compilation (JIT), which means that it will only compile the code as it goes through it. So the first iteration through the loop, it's compiling the loop as well as running. Subsequent iterations don't have to do the first step, so they are generally faster. If you want to try to get consistent loops through each time, you could use two loops, and only keep the time readings for the inner loop on the second time through:

tic
t = zeros(100000,1);
for outerIdx = 1:2
    k = 1;
    while toc<0.05
        t(k) = toc;
        k = k+1;
    end
end
t(t==0) = [];
dt = diff(t);
fprintf('%.8f\n', dt(1:20))

Additionally, the function definition can be cached. So if you modify code, run it twice and only count the second run. Or better yet, run it many times and take an average of all but the first run.

Even then, it depends on what else is happening on your system and in MATLAB, as that can affect the resources allocated to it.

There are also more accurate tools than tic and toc for timing functions. timeit is one, but the profiler is generally a more useful tool.

-Cam