[Wien] Parallelize Spin Up and Down

[Peter Blaha]

If you get your scf cycle speeded up by a factor of 2  such a parallelization 
is of course usefull. (Do you really get a speedup of 2 ? usually 2 parallel
jobs "interfere", i.e. they run slower although the get "100%". Check the
start and stop time for the both cases (Maybe your IBM p630 has a fast
enough memory bus).

However, it helps ONLY

- on a shared memory machine with ENOUGH memory to run 2 lapw1 simultaneously
- in a spin polarized case with ONLY 1 -kpoint

I'm not aware of many cases where these constrains hold (maybe for single
atom-supercells).

>  I didn't get a chance to test running spin-up and down for almost one month
> because I have
> been switching among different machines. Finally, I can settle down in an
> IBM p630 with
> 4 processors. I modify runsp_lapw and run it. It runs without problems. And
> the up and dn
> tasks don't interfere each other's speed (i.e., 100% parallel efficiency). I
> wonder if this means that parallelizing up and dn is useful.
>  Chiung-Yuan
>  On 8/18/05, Peter Blaha <pblaha at theochem.tuwien.ac.at> wrote:
> >
> > It is not implemented, since I'm not convinced that it is a usefull
> > option.
> >
> > a) Usually even big magnetic systems require more than 1 k-point (maybe
> > not?)
> > b) On most dual processor machines I know you will not gain as much as
> > expected, because the memory bus is slow and the run-time will increase if
> > two lapw1 are running simultaneously.
> > Instead, I tend to use a parallel goto-lib which gives a very nice
> > speedup (at least for dual Xeons).
> >
> > Anyway, for a shared memory machine I think it is trivial to
> > implement. Test the following:
> >
> > Change in runsp_lapw the sections lapw1: lapw1c: lapw2: lapw2c:
> > and add a "background" & character and a "wait" line
> >
> > lapw1:
> > ....
> > if ( "$so" == "-so" ) then
> > total_exec lapw1 $it0 -up $para $nohns & <==change
> > else
> > total_exec lapw1 $it0 -up $para $nohns $orb & <==change
> > endif
> > if ($icycle >= $in1new ) then
> > write_in1_lapw -dn -ql $qlimit >> $dayfile
> > if($status == 0 ) cp $file.in1new $file.in1
> > endif
> > if ( "$so" == "-so" ) then
> > total_exec lapw1 $it0 -dn $para $nohns & <==change
> > else
> > total_exec lapw1 $it0 -dn $para $nohns $orb & <==change
> > endif
> > wait <==add
> > ...
> >
> > lapw2:
> > if ( "$so" == "-so" ) goto lapw2c
> > testinput $file.in2 error_input
> > total_exec lapw2 -up $para & <==change
> > total_exec lapw2 -dn $para & <==change
> > wait <==add
> >
> > And similar in lapw1c and lapw2c sections.
> >
> > In addition I'd expect that you need to comment the testerror statements
> > of the total_exec aliases.
> > I think that should do it. Let me know if you are successfull and it is
> > usefull.
> >
> > > I am running a spin-polarized job with only one k-point on a
> > > 2-processor machine.
> > > If I don't want to do fine grained parallelization, can I distribute
> > > spin-up and down tasks into the 2 processors so that they can run
> > > simultaneously (for lapw1 and
> > > lapw2)?
> > >
> > > Thanks for your attention,
> > > Chiung-Yuan
> 


                                      P.Blaha
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Peter BLAHA, Inst.f. Materials Chemistry, TU Vienna, A-1060 Vienna
Phone: +43-1-58801-15671             FAX: +43-1-58801-15698
Email: blaha at theochem.tuwien.ac.at    WWW: http://info.tuwien.ac.at/theochem/
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