[Pw_forum] Restarting PWSCF - usability issues

[Paolo Giannozzi]

On Monday 08 March 2004 23:07, Konstantin Kudin wrote:

> [...] Now, one of the issues that came up in the context
> of multi-node jobs is the management of "*.wfc*" files.

it's a known issue, and not a simple one. There is a tradeoff
between speed (maximised If each processor writes to its 
local scratch system, as it happens now), portability of the 
results (maximised if just one file is written, in a format that
can be read independently on the number of processors,
as it happens in Car-Parrinello codes), developers' time 
(minimised if things are kept simple, or if they are kept the 
way they are).

For machines having a parallel file system (i.e. the sp4 in 
Cineca, Bologna), the problem is not that serious: all files 
are in the same place, and the I/O is still very fast (or at 
least, that is what I understood). Trouble arises only if you 
want to restart with a different number of processors . 

For machines without a parallel file system (i.e. the sp3 in
PMI, Princeton) your *wfc files are scattered on different file 
systems in different processors (forget using I/O via NFS!).
Restarting is a mess: you need a copy of all files on all 
processors, since there is no way to tell which physical
processor corresponds to which logical one in MPI.

Car-Parrinello people restart from wavefunctions more often 
than not, so the possibility to restart from a single file with a
different number of processors is essential (and implemented)
for them. PWscf people tend to restart less often, so restarting
is a rougher process. This will be fixed sooner or later (I hope
sooner: all the needed stuff is already there, and maybe it's
even working: see __NEW_PUNCH)

> the program should understand the startingwfc=atomic
> line even when a restart is requested

doesn't it? not good

Paolo

-- 
Paolo Giannozzi             e-mail:  giannozz at nest.sns.it
Scuola Normale Superiore    Phone:   +39/050-509876, Fax:-563513 
Piazza dei Cavalieri 7      I-56126 Pisa, Italy