[Q-e-developers] Improved Communication in exx.f90

[Taylor Barnes]

Dear All,

   I am a postdoc who is currently working on QE as part of NERSC's NESAP
efforts, with particular interest in improving the efficiency of the exact
exchange parts of the code.  In the coming months I intend to contribute
several changes to the parallelization strategy of exx.f90, but in the
meantime I would like to present one small modification to vexx_k.
   The attached patch reduces the total amount of data communication in
subroutine vexx_k.  This is accomplished by moving several "mp_sum" calls
to a later point in the code, so that quantities that were communicated in
real-space are instead communicated in g-space.  For some calculations,
this can result in a non-negligible reduction in walltime.  The patch can
be applied by running "patch < exx.patch" within PW/src.

Sincerely,
Dr. Taylor Barnes
Postdoctoral Scholar,
Lawrence Berkeley National Laboratory
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