[gmx-developers] What is the best choice to profile

Theodore Si sjyzhxw at gmail.com
Fri Aug 8 05:23:34 CEST 2014

Hi Mark,

I set these parameters:

CMAKE_CXX_FLAGS=-vt:cxx mpicxx
CMAKE_C_FLAGS=-vt:cc mpicc

When I execute make, I get this:
vtcc: unknown option -- -vt:cxx

How come that vtcc get the flags set to vtcxx ?

I just keep running into strange things like this.

On 8/7/2014 10:56 PM, Mark Abraham wrote:
> Hi,
> First, what are you hoping to learn? Many questions have a known 
> answer, or are known not to have a clear answer ;-)
> It's a compound problem. Profilers that just need to compile with 
> standard profiling flags are fine - just configure with 
> -DCMAKE_BUILD_TYPE=Profile and go for it. Those that need to influence 
> the compilation or linker line are more problematic. Just passing the 
> (full path to) the wrapper compiler should work fine. If you need to 
> pass things to the wrapper compiler, make yourself a script to wrap 
> the wrapper compiler and give that to CMake. Whether the things the 
> tool's wrapper compiler does clashes with things GROMACS is doing 
> varies a lot, and you will need to get involved in the details to see 
> what the origin of any problems are. That's not anybody's fault, per 
> se, but the writer of a wrapper compiler typically hopes the end user 
> is not managing details themselves, but to get high performance you 
> often have to manage details, and some of those details show up on the 
> compiler command lines generated by the GROMACS build system. And 
> naturally you'll be interested in MPI+OpenMP+CUDA, each of which 
> compounds the problem with further wrapper compilers or command-line 
> stuff.
> Once it's working, you have the problem of whether you can get useful 
> data. Instrumenting every function call, or compromising function 
> inlining is guaranteed to be useless because that overhead kills 
> things. The main interesting case to profile is where the MD step 
> iteration time is a few milliseconds, and you can't introduce 
> thousands of increments of tens of nanoseconds and get sensible 
> profiles. So either you have to restrict the instrumentation to 
> high-level functions (which is painful; the output at the end of the 
> GROMACS log file is a coarse version of this averaged over many steps 
> and execution contexts), or use a sampling-based approach.
> Then you need to start collecting data after the run-time performance 
> tuning that mdrun does has already stabilized - at least a few hundred 
> MD steps. Longer if the MD load is imbalanced, which is also the main 
> interesting case to consider for code modifications.
> Mark
> On Wed, Aug 6, 2014 at 10:58 AM, Theodore Si <sjyzhxw at gmail.com 
> <mailto:sjyzhxw at gmail.com>> wrote:
>     what kind of tools work best with gromacs 5.0?
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