[Q-e-developers] nb less than the number of proc

[xiaochuan Ge]

Dear all,

I wish this is the right place to ask this question.

Since, I don't know exactly when, but definitely this year, when one try to
do pw calculation with many cores, the code will stop and leave this error
message:

 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
     Error in routine  blk2cyc_redist (1):
      nb less than the number of proc
 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

The error usually happens when the number of cores is larger than the
number of bands. While USING OLD VERSION OF THE CODE WON"T HAVE THIS
PROBLEM.

If one looks into the code the meaning of nb seems misleading. Because in
the code, the trigger of this error is due to this command:

   IF( desc%n < nproc ) &
      CALL errore( ' cyc2blk_redist ', ' nb less than the number of proc ',
1 )

While the definition of nb at this part was defined as:
      nb = desc%nrcx    !  leading dimension of the local matrix block

On the other hand this error happens when the number of processor is
 larger than the Number of Bands.

So what NB actually means here? After all, I really don't understand why
the code should stop when I am using 32 cores to calculate a system with 6
bands. The plane wave parallelization should not care about how many bands
there are in total.

Could anyone give any comments, thank you very much. I don't attach an
input here, because one could easily reproduce this error by calculating
any small molecule with many cores on a parallel machine.

===================
Ge Xiaochuan(Giovanni)
4th year PHD Student
Condensed Matter
SISSA,Italy
===================
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