Re: Correctness of Simulation Results

From: Rahul (rahulmehta87_at_gmail.com)
Date: Tue Jun 10 2008 - 10:15:56 CDT

Thank you for your replies. Does anyone have an explanation for the three
observations I have mentioned?

On Mon, Jun 9, 2008 at 10:33 PM, Axel Kohlmeyer <akohlmey_at_cmm.chem.upenn.edu>
wrote:

> On Mon, 9 Jun 2008, Rahul wrote:
>
> RM> Hi all,
>
> hi rahul,
>
>
> RM> I am a college student currently working on a research project aimed at
> RM> porting NAMD simulations to the NVIDIA Tesla. I have basic questions
> RM> regarding the importance of the accuracy of MD results, and the
> motivation
> RM> for these questions arises from the following observations.
> RM>
> RM> 1. I have run NAMD for the same input coordinates and other
> simulation
> RM> parameters on different machines. Even when NAMD is compiled without
> any
> RM> optimizations, output coordinates are significantly different for
> different
> RM> machines. This leads to the basic question: How important is the
> correctness
> RM> of the trajectories as far as molecular dynamics simulation is
> concerned,
> RM> and depending on this, how does one determine which result is
> correct, given
> RM> that results are different on different machines?
>
> a classical MD is practically the numerical solution to a
> system of coupled partial differential equations and as such
> not lyapunov stable, i.e. the tinest errors will accumulate
> to exponential divergence of the trajectories. since you are
> using floating point math (and particularly in single precision)
> numerical errors (e.g. through summing up differently due to
> different load balancing) accumulate fast.
>
> the resulting trajectories should still be statistical mechanical
> meaningful and for that you have to use parameters related to that.
> the easiest property to follow is energy conservation (without
> using a thermostat!). there are other properties that should be
> preserved as well (average volume in variable cell, average pressure,
> individual forces for a given, identical conformation, the distribution
> of forces over time, radial distribution functions, velocity
> auto-correlation functions etc.).
>
>
> RM> 2. Running the publicly available binaries of CUDA-accelerated NAMD
> gives
> RM> significantly different results from those obtained by running the
> RM> non-accelerated versions. For reference, I carried out tests on the
> ApoA1
> RM> benchmark, freely available on the NAMD website. Does this mean that
> the
> RM> CUDA accelerated version of NAMD gives inaccurate results? Or
> alternatively,
> RM> are results that important?
> RM> 3. I am trying to look for opportunities for CUDAfication within the
> RM> ComputeNonbondedUtil class, and just for experience, I am currently
> trying
> RM> to port the first j loop within the function 'calc_pair' to the GPU.
> This
> RM> loop is the one that follows the first call to
> 'pairlist_from_pairlist', and
> RM> is defined under the NORMAL flag. So I have created a kernel
> function and
> RM> enclosed the loop within it. I am ensuring that all required
> parameters are
> RM> being correctly passed and any changes in variables having a bigger
> scope
> RM> are reflected as the control comes out of the function. The odd
> thing is
> RM> that the output is equivalent to the reference output (i.e. the
> output I get
> RM> on the same machine using pre-compiled binaries) when this kernel
> function
> RM> is run on the host. However, when it is run on the GPU device, the
> output
> RM> becomes different. Analysis using gdb shows that during the first
> run of the
> RM> kernel function itself, the values pointed by 'f_1' are different
> from what
> RM> they are in the reference (unmodified source) compilation. The
> difference is
> RM> very small though, of the order of 0.0000000000001 %. I am not sure
> whether
> RM> the overall difference in output is due to this very reason, or due
> to other
> RM> factors also. But all other variables in the scope of the kernel are
> RM> absolutely acurate in their values.
> RM>
> RM> Does anyone have an idea about the level of accuracy required in MD
> RM> simulations, and the reason for the above discrepancies?
>
> hope this helps. you may want to have a look into a text book
> on MD and its errors, too.
>
> axel.
>
> RM>
> RM> Thank you.
> RM>
> RM> Rahul
> RM>
>
> --
> =======================================================================
> Axel Kohlmeyer akohlmey_at_cmm.chem.upenn.edu http://www.cmm.upenn.edu
> Center for Molecular Modeling -- University of Pennsylvania
> Department of Chemistry, 231 S.34th Street, Philadelphia, PA 19104-6323
> tel: 1-215-898-1582, fax: 1-215-573-6233, office-tel: 1-215-898-5425
> =======================================================================
> If you make something idiot-proof, the universe creates a better idiot.
>

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