adding breakpoint to program slows it down a factor of 250x

Asked by gkk gkk on 18 Dec 2012

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I observed this in a larger program, then created a simple test case to ask this question.

The following program takes 0.038 seconds to execute without any breakpoints.

    tic;
    for  ii=1:16000000
        xyz=1; 
    end
    toc;  
    z=1;

But if I set a breakpoint at the line of code for z=1, it takes 9.3 seconds to execute.

Is this normal? Is my system corrupted somehow? Any feedback much appreciated.

Rebooting the computer doesn't make a difference. I have 32G of RAM available so that shouldn't be the problem. Not sure what other knobs are available to try to get better performance.

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MATLAB

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Answer by Matt Fig on 18 Dec 2012

Accepted Answer

It makes sense to me that MATLAB would be slower when you are debugging. So what is the big deal? You won't be in debug mode once the program is ready for use, will you?

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gkk gkk on 18 Dec 2012

Thanks, any chance you can run the for loop above and report timing with and without a breakpoint? It'll help me understand whether there's anything unusual going on in my system.

Matt Fig on 18 Dec 2012

I did. Without, .037 seconds. With a break point on the last line, 4.43 seconds. Like this:

function [r] = check_debug()
% Run in normal and with breakpoint on r=1.  Observe time difference.    
tic;
for  ii=1:16000000
    xyz=1;
end
toc;
try
    error('Here I am on line 10')
catch
end
r=1;

I run from the command line and get the following output. As you can see, the code is fast this way and I can still step through.

>> dbstop if caught error
>> check_debug
Elapsed time is 0.037994 seconds.
Caught-error breakpoint was hit in check_debug at line 10. The error was:
Error using check_debug (line 10)
Here I am on line 10
10      error('Here I am on line 10')
K>> whos
  Name      Size            Bytes  Class     Attributes
    ii        1x1                 8  double              
    xyz       1x1                 8  double              
K>> dbstep
14  r=1;
K>> dbstep
End of function check_debug.
K>> dbcont
ans =
       1

gkk gkk on 18 Dec 2012

Thanks so much Matt, that really helps.

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Answer by Teja Muppirala on 18 Dec 2012

Might I suggest using the KEYBOARD command? It's a bit simpler than setting up dbstops and trys and catches, and works just as fast.

    tic;
    for  ii=1:16000000
        xyz=1; 
    end
    toc;  
    keyboard;
    z=1;

2 Comments

Matt Fig on 18 Dec 2012

Indeed!

gkk gkk on 18 Dec 2012

Great! And there's no hit from performance using keyboard.

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