[Beowulf] programming multicore clusters
Joseph Mack NA3T
jmack at wm7d.net
Wed Jun 13 07:29:29 PDT 2007
I've googled the internet and searched the Beowulf archives
for "hybrid" || "multicore" and the only definitive
statement I've found is by Greg Lindahl, 17 Dec 2004
"Most of the folks interested in hybrid models a few years
ago have now given it up".
I assume this was from the era of 2-way SMP nodes.
Multicore CPUs are being projected for 15yrs into the
future (statement by Pat Gelsinger, Intel's CTO, quoted in
I expect the programming model will be a little different
for single image machines like the Altix, than for beowulfs
where each node has its own kernel (and which I assume will
be running dual quadcore mobos).
Still if a flat, one network model is used, all processes
communicate through the off-board networking. Someone with a
quadcore machine, running MPI on a flat network, told me
that their application scales poorly to 4 processors.
Instead if processes on cores within a package were working
on adjacent parts of the compute volume and communicated
through the on-board networking, then for a quadcore
machine, the off-board networking bandwidth requirement
would drop by a factor of 4 and scaling would improve.
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? On a large single image machine, could a
thread be scheduled on another node and have to communicate
over the off-board network? 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)?
I expect everyone here knows all this. How is everyone going
to program the quadcore machines?
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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