ARSC T3D Users' Newsletter 67, December 29, 1995

Using the -base Flag on mppexec

The man page for mppexec on Denali shows many different options for submitting jobs to the T3D:


  > MPPEXEC(1)         Cray Research, Inc.         UNICOS MAX 1.0
  > 
  > NAME
  >   mppexec - Initiates and services a user application on a
  >   CRAY T3D system
  > 
  > SYNOPSIS
  >   mppexec a.out [-base node] [-bwire c] [-debug] [-nosleep]
  >   [-npes n or n-m] [-pool pool_name] [-time seconds] [-wired]
  >   [-ypesim host:[path]] [user_options] [user_args]
So a user can submit a job in either of two ways:

  mppexec a.out -base 0x010
or

  a.out -base 0x010
they both work the same.

The -base switch is described in the mppexec manpage as:


  > -base node    Causes the partition to be allocated such that
  >               virtual PE0 will be located on the specified
  >               node.  If the PEs on node are currently busy,
  >               the application will sleep waiting for them to
  >               be released.  For example:
  > 
  >                 a.out -base 0x020 -npes 2
  > 
  >               This example will assign the 2 PEs on node
  >               0x020 to the application.  If the PEs on node
  >               0x020 are currently busy, the application will
  >               sleep waiting for them to be released.
From this description we know that we can at least specify which of the 128 PEs on the ARSC T3D will be PE0. Because there are 2 PEs to a single node board on the T3D it is not possible to get only 1 PE. (You can always run on 1PE but the other PE on the node board is necessarily idle.) And by experimenting with the -npes flags you can see that a user is always restricted to a power of 2 PEs.

From the mppview utility on Denali it is possible to assign a base node to each node of 2 PEs on the T3D:


 _________________________________________________________
 \ 0x000  0x002  0x004  0x006  0x008  0x00a  0x00c  0x00e \
  \0x010  0x012  0x014  0x016  0x018  0x01a  0x01c  0x01e  \
   \________________________________________________________\
 _________________________________________________________
 \ 0x100  0x102  0x104  0x106  0x108  0x10a  0x10c  0x10e \
  \0x110  0x112  0x114  0x116  0x118  0x11a  0x11c  0x11e  \
   \________________________________________________________\
 _________________________________________________________
 \ 0x200  0x202  0x204  0x206  0x208  0x20a  0x20c  0x20e \
  \0x210  0x212  0x214  0x216  0x218  0x21a  0x21c  0x21e  \
   \________________________________________________________\
 _________________________________________________________
 \ 0x300  0x302  0x304  0x306  0x308  0x30a  0x30c  0x30  \
  \0x310  0x312  0x314  0x316  0x318  0x31a  0x31c  0x31   \
   \________________________________________________________\
Although this is the mapping that mppview uses and is the mapping used in /usr/spool/mpp/mppsyslog, I found that to get PE0 into the intended position in the display, I had to transpose the first two digits of the node address. So:

  a.out -base 0x130 goes to the lower left  hand corner as 0x310
  a.out -base 0x13e goes to the lower right hand corner as 0x310
  a.out -base 0x12e goes to the 0x21e
This switch only assigns PE0 and does not specify the shape of the partition used by the job. The partitions generally look like a square or a rectangle in the mppview display but it can sometimes wrap around the torus and produce what looks like two disjointed rectangles.

Knowing how the partition is allocated would provide the user with the -base switch, the ability to avoid certain PEs. But in any case, knowing where the partition begins allows the user to avoid particular PEs. For example:


  a.out -base 0x020 -npes 1 
  a.out -base 0x020 -npes 2 
  a.out -base 0x020 -npes 4 
  a.out -base 0x020 -npes 8 
  a.out -base 0x020 -npes 16 
  a.out -base 0x020 -npes 32 
  a.out -base 0x020 -npes 64 
would all avoid the PE 0x01a.

NQS and the T3D

Stuart Paton of the Edinburgh Parallel Computing Centre writes in about the limitations of NQS in scheduling the T3D (Newsletter #65(12/15/95)):

  > Here at EPCC the NQS-T3D problems you mention were very visible
  > in the early period of production these have now been basically
  > eliminated by careful tuning. The system manager, Mike Brown,
  > recently delivered a paper at CUG on how this was achieved. I
  > am not sure whether you have seen this but it should address
  > the problems that you are seeing. I assume that it can be
  > retrieved somehow either from the CUG home page or direct from
  > M.D.Brown@ed.ac.uk.
On the CUG home page, I saw no way of getting the papers from any of the past CUG conferences and also the paper was not available through the EPCC web page. But I know Mike Brown presented a description of EPCC's work with NQS and the T3D at the Alaska CUG. I missed most of that presentation but I do have a copy of slides and we are all waiting for the Proceedings of the Alaska CUG to come out.

Thanks to Dale Clark

Being the end of the year, I'd like to thank Dale Clark, a Research Assistant at ARSC and a graduate student at UAF for making these newsletters into web documents.
Current Editors:
Ed Kornkven ARSC HPC Specialist ph: 907-450-8669
Kate Hedstrom ARSC Oceanographic Specialist ph: 907-450-8678
Arctic Region Supercomputing Center
University of Alaska Fairbanks
PO Box 756020
Fairbanks AK 99775-6020
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