[Pw_forum] A "relax" input runs on CPU (pw.x) but not on CPU-GPU (pw-gpu.x)

[Reza Behjatmanesh-Ardakani]

Dear Axel
This was just a proposal. If I am right, Terachem code can use gaming cards for GPU calculations (I saw some of its authors' papers). 
As you know, the main problem of GTX cards comes back to two important things. One, single precision, and the other lack of ECC.

It is not necessary to write a stand alone code. We can test the QE-GPU with both TESLA and/or GTX and QE (cpu only), and compare the outputs.
I tested it for only one case (rutile 3*3*2 supercell), and saw that the GTX output is similar to the CPU one. 

However, It is needed to test for different cases and different clusters to be sure that the lack of ECC and double precision has no effect on results.

As Filippo said formerly for GTX cards, the output may be not reproducible. However, I think due to the nature of SCF algorithm, the code can be used at least
for VC-RELAX, RELAX, and SCF types of calculations with GTX cards. Of course, it should be tested. Thank you for your interest.

With the Best Regards

   Reza Behjatmanesh-Ardakani
   Associate Professor of Physical Chemistry
   Address:
   Department of Chemistry,
   School of Science,
   Payame Noor University (PNU),
   Ardakan,
   Yazd,
   Iran.
   E-mails: 
          1- reza_b_m_a at yahoo.com (preferred),
          2- behjatmanesh at pnu.ac.ir, 
          3- reza.b.m.a at gmail.com.

--------------------------------------------
On Fri, 6/20/14, Axel Kohlmeyer <akohlmey at gmail.com> wrote:

 Subject: Re: [Pw_forum] A "relax" input runs on CPU (pw.x) but not on CPU-GPU (pw-gpu.x)
 To: "PWSCF Forum" <pw_forum at pwscf.org>
 Date: Friday, June 20, 2014, 2:19 PM
 
 On Fri, Jun 20, 2014 at 4:22 AM, Reza
 Behjatmanesh-Ardakani
 <reza_b_m_a at yahoo.com>
 wrote:
 > Dear Filippo
 >
 > Due to the nature of QE which is iterative, I think
 lack of ECC and even double precision floating point in
 gaming cards (GTX) comparing to tesla cards
 >
 > is not serious problem for QE-GPU. Some authors have
 checked this for AMBER molecular dynamics simulation code.
 See following site:
 
 classical MD is a very different animal than what you do
 with QE.
 errors due to single precision to some properties in
 classical MD are
 huge with all single precision calculations. to compute a
 force from a
 distance will not be much affected, but summing up the force
 can
 already be a problem. "good" classical MD codes usually
 employ a mixed
 precision approach, where only the accuracy insensitive
 parts are done
 in single precision. for very large system, even double
 precision can
 show significant floating point truncation errors. usually
 you are
 dependent on error cancellation, too, i.e. when you study a
 simple
 homogenous system (as it is quite common in those tests).
 
 
 >
 > http://www.hpcwire.com/2014/03/13/ecc-performance-price-worth-gpus
 >
 >
 > and see the following paper:
 >
 >
 >
 www.rosswalker.co.uk/papers/2014_03_ECC_AMBER_Paper_10.1002_cpe.3232.pdf
 >
 >
 >
 > I encourage the users of QE-GPU to test it for QE, and
 report the difference on the site.
 
 it is a waste of time and effort. people have done DFT and
 HF in
 (partial) single precision before and only if you write a
 new code
 from scratch and have an extremely skilled programmer, you
 will
 succeed. have a look at the terachem software out of the
 group of todd
 martinez for example.
 
 axel.
 
 > PS: to be able to test the results for GTX and TESLA,
 it is needed QE-GPU code to be run on GTX :-)
 >
 >
 > With the Best Regards
 >
 >    Reza Behjatmanesh-Ardakani
 >    Associate Professor of Physical Chemistry
 >    Address:
 >    Department of Chemistry,
 >    School of Science,
 >    Payame Noor University (PNU),
 >    Ardakan,
 >    Yazd,
 >    Iran.
 >    E-mails:
 >           1- reza_b_m_a at yahoo.com
 (preferred),
 >           2- behjatmanesh at pnu.ac.ir,
 >           3- reza.b.m.a at gmail.com.
 > _______________________________________________
 > Pw_forum mailing list
 > Pw_forum at pwscf.org
 > http://pwscf.org/mailman/listinfo/pw_forum
 
 
 
 -- 
 Dr. Axel Kohlmeyer  akohlmey at gmail.com 
 http://goo.gl/1wk0
 College of Science & Technology, Temple University,
 Philadelphia PA, USA
 International Centre for Theoretical Physics, Trieste.
 Italy.
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