On a somewhat related topic: What would the performance/accuracy
tradeoff be like if sander/pmemd used 32-bit floats instead of 64-bit
doubles? How was the decision made to use 64 instead of 32 bits? Are
floats considered "safe" for MD codes? I ask purely out of
curiosity...
Thanks,
--Tom
2010/5/1 Robert Duke <rduke.email.unc.edu>:
> Should be ieee 754, 64 bits, ie. 8 bytes. I think maybe some of the crays
> in the past had a slightly different meaning; these days practically
> everything is intel ia32-or a derivative thereof-based, or itanium
> (disappearing), or ibm power chips. You specify intel below I believe.
> Intel does have an internal 80 bit format when you are in the registers; I
> expect that compiler options determine when this is used, but it can cut
> rounding error further. The thing is, once you store in memory or send dp
> over the net, you are back to 8 bytes. I have not recently done a lot of
> research on this, but that was the state of the world last time I looked...
> It was also true for the various now-essentially-defunct risc chips like
> MIPS and Alpha.
> Regards - Bob Duke
> ----- Original Message ----- From: "sop dadoun" <dadoun.sop.gmail.com>
> To: "AMBER Mailing List" <amber.ambermd.org>
> Sent: Saturday, May 01, 2010 6:14 PM
> Subject: Re: [AMBER] size of real
>
>
>> Hi,
>>
>> Thank you for your answers.
>> In fact, my question is about the width of "double precision" (64, 128 or
>> more bits ?)
>> Note : amber 8 , sander , ramd
>> Station : Intel(R) Pentium(R) 4 CPU 3.20GHz
>>
>> Best regards,
>> Sophie
>>
>>
>> 2010/5/1 Robert Duke <rduke.email.unc.edu>
>>
>>> All of pmemd just uses "double precision" instead of "real". The use of
>>> "double precision" is I think deprecated in the latest fortran standards,
>>> but it is really clear what you are doing, and they may deprecate the
>>> usage,
>>> but it is unlikely to go away, given the wide use. The whole "kind"
>>> system
>>> focuses on the range of values you can represent, which certainly is
>>> mathematically useful. But for a lot of computer-related issues, it is
>>> actually (at least to me) also useful to know how much storage is
>>> required.
>>> The two are related, but not precisely - depends on the exact nature of
>>> the
>>> number representation, especially for floating point, and also I think
>>> that
>>> there is an "at least" sense to "kind", so the compiler maker can just
>>> use
>>> more storage than required to represent a specified range.
>>> Regards - Bob Duke
>>> ----- Original Message ----- From: "Jason Swails"
>>> <jason.swails.gmail.com>
>>> To: "AMBER Mailing List" <amber.ambermd.org>
>>> Sent: Saturday, May 01, 2010 10:25 AM
>>> Subject: Re: [AMBER] size of real
>>>
>>>
>>>
>>> Hello,
>>>>
>>>> On Sat, May 1, 2010 at 9:03 AM, sop dadoun <dadoun.sop.gmail.com> wrote:
>>>>
>>>> Dear amber users,
>>>>>
>>>>> I would like to know the size of real for a fortran program used in
>>>>> amber
>>>>> with Pentium 4 CPU.
>>>>>
>>>>>
>>>> Most amber programs include the file $AMBERHOME/include/dprec.h or .fh,
>>>> depending on what version of amber you're talking about. The precision
>>>> of
>>>> _REAL_ depends on the preprocessor flags that are given to (u)cpp.
>>>> Typically, though, they're double unless you modify something.
>>>>
>>>> All the best,
>>>> Jason
>>>>
>>>> Thanks you for rplying
>>>>
>>>>> Best regards,
>>>>> Sophie
>>>>> _______________________________________________
>>>>> AMBER mailing list
>>>>> AMBER.ambermd.org
>>>>> http://lists.ambermd.org/mailman/listinfo/amber
>>>>>
>>>>>
>>>>
>>>>
>>>> --
>>>> Jason M. Swails
>>>> Quantum Theory Project,
>>>> University of Florida
>>>> Ph.D. Graduate Student
>>>> 352-392-4032
>>>> _______________________________________________
>>>> AMBER mailing list
>>>> AMBER.ambermd.org
>>>> http://lists.ambermd.org/mailman/listinfo/amber
>>>>
>>>>
>>>>
>>>
>>> _______________________________________________
>>> AMBER mailing list
>>> AMBER.ambermd.org
>>> http://lists.ambermd.org/mailman/listinfo/amber
>>>
>> _______________________________________________
>> AMBER mailing list
>> AMBER.ambermd.org
>> http://lists.ambermd.org/mailman/listinfo/amber
>>
>>
>
>
> _______________________________________________
> AMBER mailing list
> AMBER.ambermd.org
> http://lists.ambermd.org/mailman/listinfo/amber
>
_______________________________________________
AMBER mailing list
AMBER.ambermd.org
http://lists.ambermd.org/mailman/listinfo/amber
Received on Sat May 01 2010 - 16:30:03 PDT
