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Full Discussion: Massively parallel on single core?
Special Forums UNIX and Linux Applications High Performance Computing Massively parallel on single core? Post 302394358 by Andre_Merzky on Thursday 11th of February 2010 07:48:20 AM
Old 02-11-2010
Question Massively parallel on single core?
Hia all,

I am not sure how many people actually follow the HPC forum on unix.com, but you may be interested in discussing the following (academic) problem:

Assume you want to run a *very* large number (say 100.000) of very lightweight synchronous operations. As an example, assume that you want to run 100.000 instances of

Code:
sleep (3600); // thats one hour sleep

The trivial (aka braindead) approach would be

Code:
for ( int i = 0; i < 100000; i++ )
{
  ::sleep (3600);
}

Takes about 15 years to finish ;-)

One could start 1000 threads, and run a sleep in each of them. That reduces the runtime to 100 hours - still 4 days, and the system is totally idle all the time.

So, using more threads? Won't work, as the max-threads-per-process limit will be hit at some point.

So, spawn 100 processes which spawn 1000 threads each?
The max-threads-per-process limit is, on Linux, close to the max-threads-per-system limit, so that won't work. On other Unixes that is different, but I don't think you get 100.000 threads on a normal single CPU system. Do you?

So, what would your approach be?

I am not looking for a sleep replacement: so saying that I should set alarm or something similar is of not much use. Sleep is obviously only an example here - replace it with an extremely lightweight job, like running a very time consuming synchronous remote operation.

I am looking forward to the ideas you guys can come up with! :-)

Cheers, Andre.
Last edited by Andre_Merzky; 02-11-2010 at 08:48 AM.. Reason: layout...
 

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       "LAM Frequently Asked Questions"
	      at http://www.lam-mpi.org/faq/

       "MPI Primer / Developing with LAM", Ohio Supercomputer Center

       "MPI: A Message-Passing Interface Standard", Message-Passing Interface Forum, version 1.1
	      at http://www.mpi-forum.org/

       "MPI-2: Extensions to the Message Passing Interface", Message Passing Interface Forum, version 2.0
	      at http://www.mpi-forum.org/

   MPI Quick Tutorials
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