Calculating cpu usage when running matlab program.

Hi there,

Would this code be correct in giving me the cpu usage?

close all; clear; 

tstart = tic;
initime = cputime;

prompt = ('Please enter time step');
result = input(prompt);
tstep = result;

%define known variables
g = 9.81; c = 12.5; m = 70; 

%define analytic time vector
t = 0 : tstep : 10;

%get analytic velocity using equation from lectures
v_analytic = (g*m/c)*(1-exp(-c*t/m));

%set up plot and plot analytic velocity 
figure(1); hold on; grid on; box on; 
plot(t,v_analytic,'b*-')
title('parachutist');
xlabel('time (s)');
ylabel('velocity (m/s)');

%do numerical calculations here using a 'for' loop
v1 = 0;
N = length(t);

v(1)=0;
   for i = 2 : N;

      v(i) = v(i-1) + (g-c*v(i-1)/m)*(tstep);

     end

%numerical results 
plot(t,v,'ro-')
legend('analytical','numerical')

%do error calculations at 5s here
Eabs = abs(v_analytic(5) - v(5));
Erel = Eabs/v_analytic(5);

fintime = cputime;
telapsed = toc(tstart);

display(tstep, 'tsize');
display(Eabs,'Absolute error');
display(Erel,'Relative error');

display((fintime -initime)/telapsed,'CPU usage (%):');

I think this is correct. I want to establish the cpu usage under different input conditions (stepsizes) as if the stepsize is smaller then more calculations are made by the program.

[Walter Roberson]

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Answer 1

[Walter Roberson]

It is recommended that you use timeit() . If you have an older MATLAB that does not have that built in, you can find it in the File Exchange.

[jonathan douglas-smith]

like this?

%set up plot and plot analytic velocity 
figure(1); hold on; grid on; box on; 
plot(t,v_analytic,'b*-')
title('parachutist');
xlabel('time (s)');
ylabel('velocity (m/s)');

%do numerical calculations here using a 'for' loop
v1 = 0;
N = length(t);

v(1)=0;

 initime = cputime;

     for i = 2 : N;

      v(i) = v(i-1) + (g-c*v(i-1)/m)*(tstep);

     end
   fintime = cputime; telapsed = timeit(v(i));

%numerical results 
plot(t,v,'ro-')
legend('analytical','numerical')

%do error calculations at 5s here
Eabs = abs(v_analytic(5) - v(5));
Erel = Eabs/v_analytic(5);

display(tstep, 'tsize');
display(Eabs,'Absolute error');
display(Erel,'Relative error');

display((fintime -initime)/telapsed,'CPU usage (%):');

[jonathan douglas-smith]

This way I am using the cputime function to record cputime before and after the for loop and then using the timeit() function to record the real time used by the program to complete the for loop?

Then I calculate the cpu usage using: fintime -initime)/telapsed.

Does this seem right?

[Walter Roberson]

There is an example at http://www.mathworks.com/help/matlab/ref/timeit.html . You put the code to be timed in a function or function handle and timeit() the handle.