Dear Ed,
It is not usual to find small discrepancies in floating point
calculations. A simple test to demonstrate this is to calculate
sum 1.0/n n=1->1000000 and n=1000000->1. You might find
that the results differ. For 4-byte reals will tend to differ in the
6th significant digit and 8-byte reals in the 15th. Compilers are a
lot more complicated so I am not surprised the results differ slightly.
In general there is a trade-off between speed and precision, the
-ffast-math option is obviously faster but results will be less precise.
Or memory and precision, double vs single precision. The speed up
in refmac is impressive but there are also the libraries to consider.
Adam
> Thierry,
> I ran both versions on the same input file and numerical results are
> essentially the same. After 10 cycles of refinement the r.m.s.d.
> between two models produced with different versions is <0.0004A.
> Basically, about 5% of coordinates differ in the last digit (i.e. by
> 0.001A). Frankly, I expected results to be exactly identical, but the
> difference is too small to be of concern, imho.
>
> Cheers,
>
> Ed.
On 03/04/2013 10:19 AM, Fischmann, Thierry wrote:
> Ed,
>
> Are the numerical results the same ? Not likely that there is a problem. But
> if you haven't done it already it is worth checking by running the tests
> provided with the suite. Aggressive optimization can be a source of bugs.
>
> Best regards
> Thierry
>
> -----Original Message-----
> From: CCP4 bulletin board [mailto:[email protected]] On Behalf Of Ed
> Pozharski
> Sent: Monday, March 04, 2013 8:55 AM
> To: [email protected]
> Subject: Re: [ccp4bb] compiling refmac5 on Ubuntu 12.04
>
> On Mon, 2013-03-04 at 11:37 +0000, Marcin Wojdyr wrote:
>> Running times were, correspondingly, 32.2s, 35.1s and 18.7s.
>>
> Numbers are almost too impressive to believe :)
>
> How does it compare with ifort (which I thought should be the fastest
> option on intel processors and thus unavailable (not free) for most DIY
> compilation given licensing issues)?
>
> --
> Oh, suddenly throwing a giraffe into a volcano to make water is crazy?
> Julian, King of Lemurs
