[gmx-developers] Double precision on GPU's

[Berk Hess]

Hi,

The energy conservation check can still be done with a thermostat, as 
GROMACS keeps track of the amount of energy the thermostat adds to or 
removes from the system.

I don't understand what you mean exactly with "energy balance". Either 
you are interested in energy dissipation between different parts of the 
system, in which case you often can not use a thermostat, or you are not 
and then you can, and probably should, use a thermostat (and still keep 
track of energy conservation).

Cheers,

Berk

On 2019-07-23 21:19 , James wrote:
> Hi Berk,
>
> Thank you for the information. I prefer not to use thermostats, at 
> least when trying to get quantitative values, because (and I may be 
> misunderstanding thermostats, but since changing the temperature 
> changes the energy, I assume this is true) then I cannot use "energy 
> in = energy out" as a sanity check. I'm more interested in energy 
> balance than in maintaining a given temperature.
>
> If I were to be able to get the double precision GPU stuff coded, 
> would the team be willing to maintain it? Or, since I don't know the 
> code, perhaps a better question is: Does someone who is already 
> familiar with the relevant code have time to do this as a side project 
> (with compensation)? If anyone is interested, please feel free to 
> contact me off list.
>
> Thanks,
> James
>
> =======================================
> Hi,
>
> IIRC all Nvidia Tesla cards have always had double precision, at half
> the throughput of single precision. But there are very few cases where
> double precision is needed. Energy drift in single precision is never an
> issue, unless you really can not use a thermostat.
>
> But having said that, making the GPU code, either CUDA or OpenCL work in
> double precision is probably not much effort. But making it work
> efficiently requires optimizing several algorithmic parameters and maybe
> changing the arrangement of some data in the different GPU memory levels.
>
> Cheers,
>
> Berk
>
> On 7/22/19 10:10 PM, James wrote:
> >/Hi, />//>/My apologies if this question has been previously discussed. I just />/joined the list and all I know is that from reading the docs and />/release comments, writing code for double precision on GPU's is not a />/priority. />//>/However, I believe all recent upper-end Nvidia cards have native />/double precision (which was not true several generations ago). So, you />/don't have to have a real "scientific computing" GPU to take advantage />/of this -- most people probably already have the hardware. Still, I />/understand that most people do not need/want to run double precision. />/But, some do (and you have to if you are concerned with conservation />/of energy -- the energy drift in single precision is substantial). />//>/So, I would like to ask what the level of effort to do this is />/believed to be? Would it require a lot of new code, or would it be />/porting the single precision code to double precision? />//>/Sincerely, />/James />
>
>

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