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

2012 12 18
2 个回答
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0 个投票

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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Matt Fig
Matt Fig 2012-12-18

0 个投票

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
gkk gkk 2012-12-18
编辑：gkk gkk 2012-12-18
If I set a breakpoint in the middle of my larger program it takes an hour to reach it... not very useful for developing. I suspect something isn't quite right here. Slower sure, but not THAT much slower. There must be something wrong somewhere. What about if you run the sample program above on your system, do you see the same performance or is it much faster with the breakpoint set?
Matt Fig
Matt Fig 2012-12-18
Ah yes, you didn't mention that in the original post! Have you tried to use:
dbstop if error
instead of setting break points? That is faster.
gkk gkk
gkk gkk 2012-12-18
编辑：gkk gkk 2012-12-18
Actually, stop on warning produces same results (> 9 seconds). But, stop on error as you say is fast (0.04 seconds, same as with no breakpoint). Is that what you'd expect?
Matt Fig
Matt Fig 2012-12-18
编辑：Matt Fig 2012-12-18
Based on experience, yes. I don't know why, but this is what I have found in the past. If I want to force a break at a point and it is taking too long I can call
dbstop if error
then make the code stop where I want by calling
error('Here I am on line 366')
gkk gkk
gkk gkk 2012-12-18
True, but then you can't continue the program. Is that right? At least I haven't figured out how.
Matt Fig
Matt Fig 2012-12-18
编辑：Matt Fig 2012-12-18
To continue use TRY-ERROR-CATCH and
dbstop if caught error
then DBSTEP, DBCONT, etc work.
gkk gkk
gkk gkk 2012-12-18
编辑：gkk gkk 2012-12-18
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
Matt Fig 2012-12-18
编辑：Matt Fig 2012-12-18
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
gkk gkk 2012-12-18
Thanks so much Matt, that really helps.

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Teja Muppirala
Teja Muppirala 2012-12-18

2 个投票

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;

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