Re: [eigen] Performance tuning

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On Wed, Dec 28, 2011 at 6:50 PM, Tim Holy <holy@xxxxxxxxx> wrote:

I'm writing to ask about people's favorite techniques for tuning the
performance of their code, and in particular whether there exists a HOWTO (or
a desire to write one).

Here's one example: for the kinds of problems I'm studying, I've found that
one of the most helpful steps is to avoid the creation of temporaries. Because
temporaries are usually created "silently," I am sometimes surprised by which
lines are triggering temporaries. Once I discovered that this was an issue,
here's how I went about solving the problem at first:
1. Compile the program with debugging symbols and optimization
2. Profile my code in valgrind
3. See how many times posix_memalign gets called.
4. If it's too many, comment out a suspicious-looking block of code.
5. Go back to step 1 until the problem is localized, then fix it.
But after growing tired of this process, I finally realizing that a better way
to discover the problematic line(s) is to set a breakpoint in
Core/Util/Memory.h::aligned_realloc. And, of course, maybe there's even a
better way I haven't yet stumbled across, like setting some compiler flag to
throw an exception upon the creation of a temporary?

The EIGEN_RUNTIME_NO_MALLOC macro might help here. It is especially useful for unit tests. Minimal usage example:

#define EIGEN_RUNTIME_NO_MALLOC // Define this symbol to enable runtime tests for allocations
#include <Eigen/Dense>

int main(int argc, char** argv)
  // It's OK to allocate here
  Eigen::MatrixXd A = Eigen::MatrixXd::Random(20, 20);

  // It's NOT OK to allocate here
  // An assertion will be triggered if an Eigen-related heap allocation takes place

  // It's OK to allocate again



But there are other issues I don't know how to solve. For example, in
valgrind, I frequently wish that the cost of inlined code could be "folded in"
to the cost computed for the individual lines of my source code. Does anyone
know of a way of doing that? Or is there a different tool that can do this?

A natural gathering place for useful tips might be's_Corner#Profiling_tips
but currently that section is fairly short.


Adolfo Rodríguez Tsouroukdissian
Robotics engineer

c/ Pujades 77-79, 4º4ª
08005 Barcelona, Spain.
Tel. +34.93.414.53.47

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