On Wed, 13 Jun 2007, Greg Lindahl wrote:
Still if a flat, one network model is used, all processes
communicate through the off-board networking.
No, the typical MPI implementation does not use off-board networking
for messages to local ranks. You use the same MPI calls, but the
underlying implementation uses shared memory when possible.
My apparently erroneous assumption was that in a beowulf of
quadcore processors, each processor would be assigned a
random rank, in which case adjacent processors in the
quadcore package would not be working on adjacent parts of
the compute space.
What's the mechanism for assigning a processor a particular
rank? (a url, or pointer to the MPI docs is fine). How does
MPI know that one process is running on the same mobo and to
use shared memory and that another process is running
off-board? I take it there's a map somewhere other than the
machines.LINUX file?
Someone with a quadcore machine, running MPI on a flat
network, told me that their application scales poorly to
4 processors.
Which could be because he's out of memory bandwith, or
network bandwidth, or message rate. There are a lot of
postential reasons.
OK
In a quadcore machine, if 4 OMP/threads processes are
started on each quadcore package, could they be
rescheduled at the end of their timeslice, on different
cores arriving at a cold cache?
Most MPI and OpenMP implementations lock processes to
cores for this very reason.
am off googling
In a single image machine (with a single address space)
how does the OS know to malloc memory from the on-board
memory, rather than some arbitary location (on another
board)?
Generally the default is to always malloc memory local to
the process. Linux grew this feature when it started being
used on NUMA machines like the Altix and the Opteron.
ditto
I expect everyone here knows all this. How is everyone
going to program the quadcore machines?
Using MPI?
I see. It's a bit clearer now.
You can go read up on new approaches like UPC, Co-Array
Fortran, Global Arrays, Titanium, Chapel/X-10/Fortress,
etc, but MPI is going to be the market leader for a long
time.
Thanks Joe
--
Joseph Mack NA3T EME(B,D), FM05lw North Carolina
jmack (at) wm7d (dot) net - azimuthal equidistant map
generator at http://www.wm7d.net/azproj.shtml
Homepage http://www.austintek.com/ It's GNU/Linux!
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