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

[Theodore Si]

Hi Mark,

I run cmake two times.
The first time I use these options:
cmake .. -DCMAKE_C_FLAGS="-L /usr/local/cuda/lib64-vt:cc mpicc" 
-DCMAKE_CXX_FLAGS="-L /usr/local/cuda/lib64 -vt:cxx mpicxx" 
-DCMAKE_INSTALL_PREFIX=/home/theo/myprg/gmx 
-DCUDA_TOOLKIT_ROOT_DIR=/usr/local/cuda 
-DCMAKE_C_COMPILER=/home/theo/myprg/vt/bin/vtcc-DCMAKE_CXX_COMPILER=/home/theo/myprg/vt/bin/vtcxx 
-DCUDA_NVCC_EXECUTABLE=/home/theo/myprg/vt/bin/vtcxx-DCUDA_NVCC_EXECUTABLE=/home/theo/myprg/vt/bin/vtnvcc

  and the second time just cmake .. without any option.

If I build GMX with the first approach, when I run mdrun, I get a 
segmentation fault. If I build GMX with the second one, everything is fine.
So I compared the output of the two cmake commands, I found this:
  17 -- Detecting best SIMD instructions for this CPU
  18 -- Run output: Segmentation fault
  19 -- Detected best SIMD instructions for this CPU - None

  17 -- Detecting best SIMD instructions for this CPU
  18 -- Detected best SIMD instructions for this CPU - AVX_256

I believe that you know where I am going.
So do you know how to fix this? Looking forward to your reply, thanks!


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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