[Q-e-developers] Improvements to the EXX Implementation

[Filippo Spiga]

Hello Taylor,

thanks for sharing all your progresses of your work with us. If you want these contributions to be merge in the v5.4 please remember to provide us a patch or the relevant files by the weekend of April 23-24. 

Best Regards

--
Filippo SPIGA
* Sent from my iPhone, sorry for typos *

> On 4 Apr 2016, at 21:05, Taylor Barnes <tbarnes at lbl.gov> wrote:
> 
> Dear All,
> 
>    I wanted to inform everyone about some improvements that we have been making at LBNL to the implementation of exact exchange in QE.  These improvements have been made as part of NERSC's Exascale Science Applications Program, which is an effort to update codes for execution on next-generation architectures such as NERSC's upcoming Cori Phase II.  The following is a brief overview of these changes, which we are currently in the process of testing and debugging.  Depending on our progress, we intend to submit these changes as an addition to either QE 5.4 or 6.0.
> 
> 1. Parallelization Over Band Pairs
>    We have extended the parallelization of subroutine vexx_k such that both of the loops over bands (i.e., "LOOP_ON_PSI_BANDS" and "IBND_LOOP_K") are parallelized with respect to band groups.  This improves load balancing, and also enables parallelization using larger numbers of band groups than was previously possible
> 
> 2. Improved OMP Support
>    We have added OMP threading to numerous vector operations within exx.f90.  In addition, we have given special priority to enhancing the threaded performance of the FFTs.
> 
> 3. Implementation of Different and Interchangeable Data Layouts for Local and EXX Portions of the Calculation
>    One observation that we have made is that for calculations that utilize many band groups, the local portion of the calculation (i.e., everything outside of exx.f90) often represents a non-negligible (or even dominant) contribution to the total cost of the calculation.  This is largely because the local portion of the calculation is duplicated on each band group.  We have implemented changes to the code that allow the local portion of the code to be parallelized in a manner that is independent of the number of band groups, thus avoiding duplication of work.
>    This is the single most significant modification that we have made, both in terms of increasing the efficiency of QE, as well as the amount of coding work required.  For several test calculations we are finding that this change results in more than a factor of two speedup.
>    In terms of code development, the primary challenge of our approach is that when the EXX part of the calculation is performed (such as when vexx is called), we must change the data structure from the one that is used by the local portion of the code to a different data structure that is used by the EXX portion of the code.  This change of data structure requires a great deal of bookkeeping in order to update arrays like igk, ig_l2g, psi, and hpsi.  As a result, we a still the process of making our updated code compatible with gamma-point only calculations and with calculations that employ multiple k-points.
> 
> Sincerely,
> Dr. Taylor Barnes
> Postdoctoral Scholar,
> Lawrence Berkeley National Laboratory
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