C++ code running slow after a few seconds

Question

I have C++ code which solves a partial differential equation numerically. Everything works fine, but after the first 10 or so iterations, the code starts to run about 3 times as slow.

Some possibly relevant information:

I am using the package Eigen for a lot of calculations
I am storing a large amount of data in an array (about 100000 double precision values every iteration)

I don't know very much about how computers work or about coding in c++, and was wondering if anybody might be able to point me in the right direction so I can possibly fix this problem.

I can also post the code if it would be helpful, but it's very long.

Is the hard drive light on steady when it slows down? Perhaps you are exhausting your ram and hitting the swap/ pagefile. — drescherjm, Jan 12 '13 at 21:36
If speed changes suddenly, I suspect denorms. Is it possible that you have many variables very close to `0`? — zch, Jan 12 '13 at 21:38
COuld also be that the processor gets too hot and starts throttling? — Mats Petersson, Jan 12 '13 at 21:41
@zch - Thanks, that seems like it could be possible, many of the variables are close to zero, and when I use a coarser mesh, I don't see any sudden speed changes. — user1968603, Jan 12 '13 at 21:53
I don't think it's memory, as a hundred thousand double precision values is not very large amount of data these days, when computers have gigabytes of memory, and one gigabyte fits over a hundred *million* doubles... — hyde, Jan 12 '13 at 22:27
The problem is on line 42, column 10. I could be wrong, but the only way to tell is if YOU POST SOME CODE. — Thomas Matthews, Jan 13 '13 at 01:45

score 2 · Answer 1 · answered Jan 12 '13 at 21:46

Like Oli said, it could be anything.

It could be the algorithm. Do you know if that code is not supposed to slow down after certain amount of iteration? Maybe the nature of your problem is such that the program has to slow down.

It could be the computer running out of memory. Try running the same program on a computer with more memory, does it do better? Then you have to do memory profiling and identify any objects (like large arrays) in memory that are lying around but are not being used. And fix the code so that it doesn't happen. Look up "memory leak" and understand how to fix it.

It could be that there is no memory leak, but as the iterations pile up, the process takes more memory to the point that it needs to constantly swap memory pages back and forth from the hard drive, this behavior is common and is consistent with a sudden dip in speed. Try to rewrite the program so that the memory footprint is not expanding like that. Try to manage the data so that unused data is either deleted or written to a file.

Or something else...

Hope this helps.

Thank you for the response! The memory footprint idea seems plausible, I will try and see if that helps. — user1968603, Jan 12 '13 at 21:57

C++ code running slow after a few seconds

1 Answers1