[Wrf-users] How wrf processes nests

[John Michalakes]

Hi Dave,
WRF only has the capability to run domains sequentially over all the
processors.  There is no concurrent ("simultaneous") mode, at least not for
ARW.  The HWRF/NMM code in release 3.8 does have the ability to run domains
on the same nesting level concurrently each on a subset of the processors,
but this capability hasn't been extended to the ARW yet.
John
 
From: wrf-users-bounces at ucar.edu [mailto:wrf-users-bounces at ucar.edu] On
Behalf Of David Smith
Sent: Thursday, June 9, 2016 10:34 AM
To: Gustafson, William I <William.Gustafson at pnnl.gov>; wrf-users
<wrf-users at ucar.edu>
Subject: Re: [Wrf-users] How wrf processes nests
 
   Thanks Bill, but let me pursue this just a bit to be sure I'm not missing
something.  I understand how the coarse and fine nests are processed.  I am
asking how the two equal, but separate, 0.333 km fine nests are handled when
their "time" comes.  Are you saying that there is no (significant)
difference whether wrf runs each of the fine nests sequentially (each
getting all the allotted processors) or simultaneously (sharing the total
number of processors)?  
  Consider a simplistic example.  For a sequential run of the two fine
nests, if I make the x and y dimensions of each fine nest divisible by 10
and 5 respectively, then I know wrf can create 50 subgrids for an efficient
run on 50 processors.  I would want each of the fine grids to have this same
structure.  
   But if they will be running simultaneously, then wouldn't I want the
total number of subgrids for the two nests combined to be 50?  (My two fine
nests are not the same size.) Of course, I am assuming that wrf makes some
reasonable choice for the sub grids - maybe I need to better understand how
it does that as well.
 Thanks!
 
Dave
 
Dr. David J Smith
College of Science and Mathematics
University of the Virgin Islands
St. Thomas,  VI  00802
340 693-1257
 <mailto:dsmith at uvi.edu> dsmith at uvi.edu
 
From: "Gustafson, William I" <William.Gustafson at pnnl.gov
<mailto:William.Gustafson at pnnl.gov> >
Date: Wed, 8 Jun 2016 20:25:35 +0000
To: Dave Smith <dsmith at uvi.edu <mailto:dsmith at uvi.edu> >, wrf-users
<wrf-users at ucar.edu <mailto:wrf-users at ucar.edu> >
Subject: Re: [Wrf-users] How wrf processes nests
 
Dave,
 
The grids all share the same processors and are kept in sync at the end of
each coarse-domain timestep. So, the outer nest runs on the processors
first, then that nest waits while the finer nest is calculated. Then, the
fine nest waits while the coarse nest calculates, and on-and-on it goes. So,
within a timestep, each nest is run sequentially. However, if one sees
thinks of this in terms of the processor usage, they are simultaneous.
 
-Bill
___________________________________________________________________
 
William I. Gustafson Jr., Ph.D.
Scientist
ATMOSPHERIC SCIENCES AND GLOBAL CHANGE DIVISION
 
Pacific Northwest National Laboratory
P.O. 999, MSIN K9-30
Richland, WA  99352
Tel: 509-372-6110
 <mailto:William.Gustafson at pnnl.gov> William.Gustafson at pnnl.gov
 <http://www.pnnl.gov/atmospheric/staff/staff_info.asp?staff_num=5716>
http://www.pnnl.gov/atmospheric/staff/staff_info.asp?staff_num=5716
http://www.researcherid.com/rid/A-7732-2008
 
 
 
From: <wrf-users-bounces at ucar.edu <mailto:wrf-users-bounces at ucar.edu> > on
behalf of David Smith <dsmith at uvi.edu <mailto:dsmith at uvi.edu> >
Date: Wednesday, June 8, 2016 at 12:55 PM
To: wrf-users <wrf-users at ucar.edu <mailto:wrf-users at ucar.edu> >
Subject: [Wrf-users] How wrf processes nests
 
Hi,
 
  I am running wrf using mpirun on a 10 node cluster with 12 processors each
- 120 total.  I have two fine nests (0.333 km) within a 1.0 km coarser nest.
I am trying to make a judicious choice for the dimensions of the fine nests,
to minimize runtime.  (Making one fine nest would double the area.)  Will
wrf run the two fine grids sequentially or simultaneously?  I would think
this info would be in the users guide, but I'm not finding it.  Thanks!
 
Dave Smith
 
Dr. David J Smith
College of Science and Mathematics
University of the Virgin Islands
St. Thomas,  VI  00802
340 693-1257
dsmith at uvi.edu <mailto:dsmith at uvi.edu> 
 
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