Thank you, Szilárd. Do you know where (say, in a piece of code or some documentation) I can be sure about that? Cheers,
Pedro. Em 17 de abril de 2012 15:43, Szilárd Páll <[email protected]>escreveu: > 2012/4/17 Pedro Alexandre de Araújo Gomes Lapido Loureiro < > [email protected]>: > > Hi, > > > > I've come across a discussion about the "single X double precision" > issue in > > a NAMD users list e-mail > > (http://www.ks.uiuc.edu/Research/namd/mailing_list/namd-l/9166.html). > > I would like to know what do you think about these 3 specific points: > > > > 1) "there are some places where single precision (FFT, force/energy > > computation), where single precision can be applied, but in other > > places (summations) where even double precision may result in > > artefacts, due to limited numerical accuracy for large enough > > systems. there are some applications that show little sensitivity > > to single/double precision issues (homogeneous bulk lennard-jones > > systems), but others are notoriously difficult (systems with > > significant potential drops in one or two dimensions, e.g. metal > > surface slabs or lipid bilayers) since you don't have that much > > error cancellation anymore." > > > > 2) "if you store coordinates in single precision, you have to take > > extra precautions for handling very large systems, since you > > would store coordinates with different relative precision, > > depending on how close you are to the origin. e.g., with domain > > decomposition, you can define per domain offsets, but that > > would work only well in case of a large enough number of domains. " > > > > 3) "the best way to study the impact of single vs. double would be > > by running tests with the gromacs code. gromacs _can_ be compiled > > in single or double precision. for a proper comparison, you'll have > > to turn off the assembly innerloops though, since they may use single > > precision even in double precision mode." > > > > Specifically this last point startled me. In fact, is this true? > > AFAIK no, the assembly (SSE intrinsic in 4.6) kernels have both single > and double versions. If double precision is requested that's always > respected in the code. > > Additionally, single precision in GROMACS is in fact not *pure* single > precision, in some parts of the code where needed double precision is > used (e.g. constraints). Therefore, technically what we call single > corresponds to what some MD packages call mixed precision. > > -- > Szilárd > > > > Cheers, > > > > Pedro. > > > > > > -- > > gmx-users mailing list [email protected] > > http://lists.gromacs.org/mailman/listinfo/gmx-users > > Please search the archive at > > http://www.gromacs.org/Support/Mailing_Lists/Search before posting! > > Please don't post (un)subscribe requests to the list. Use the > > www interface or send it to [email protected]. > > Can't post? Read http://www.gromacs.org/Support/Mailing_Lists > -- > gmx-users mailing list [email protected] > http://lists.gromacs.org/mailman/listinfo/gmx-users > Please search the archive at > http://www.gromacs.org/Support/Mailing_Lists/Search before posting! > Please don't post (un)subscribe requests to the list. Use the > www interface or send it to [email protected]. > Can't post? Read http://www.gromacs.org/Support/Mailing_Lists >
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