Massively parallel on single core? Post: 302394358

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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

02-11-2010

Registered User

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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LEARN ABOUT SUNOS

sleep

sleep(3UCB)					     SunOS/BSD Compatibility Library Functions					       sleep(3UCB)

NAME

       sleep - suspend execution for interval

SYNOPSIS

       /usr/ucb/cc [ flag ... ] file ...

       int sleep( seconds);
       unsigned seconds;

DESCRIPTION

       sleep() suspends the current process from execution for the number of seconds specified by the argument.  The actual suspension time may be
       up to 1 second less than that requested, because scheduled wakeups occur at fixed 1-second intervals, and may be an arbitrary amount longer
       because of other activity in the system.

       sleep()	is implemented by setting an interval timer and pausing until it expires.  The previous state of this timer is saved and restored.
       If the sleep time exceeds the time to the expiration of the previous value of the timer, the process sleeps only until the timer would have
       expired, and the signal which occurs with the expiration of the timer is sent one second later.

ATTRIBUTES

       See attributes(5) for descriptions of the following attributes:

       +-----------------------------+-----------------------------+
       |      ATTRIBUTE TYPE	     |	    ATTRIBUTE VALUE	   |
       +-----------------------------+-----------------------------+
       |MT-Level		     |Async-Signal-Safe 	   |
       +-----------------------------+-----------------------------+

SEE ALSO

       alarm(2), getitimer(2), longjmp(3C), siglongjmp(3C), sleep(3C), usleep(3C), attributes(5)

NOTES

       Use of these interfaces should be restricted to only applications written on BSD platforms.  Use of these interfaces with any of the system
       libraries or in multi-thread applications is unsupported.

       SIGALRM should not be blocked or ignored during a call to sleep(). Only a prior call to alarm(2) should generate SIGALRM  for  the  calling
       process	during a call to sleep(). A signal-catching function should not interrupt a call to sleep() to call siglongjmp(3C) or  longjmp(3C)
       to restore an environment saved prior to the sleep() call.

WARNINGS

       sleep() is slightly incompatible with alarm(2).	Programs that do not execute for at least one second  of  clock  time  between	successive
       calls  to  sleep()  indefinitely delay the alarm signal. Use sleep(3C). Each sleep(3C) call postpones the alarm signal that would have been
       sent during the requested sleep period to occur one second later.

SunOS 5.10							    12 Feb 1993 						       sleep(3UCB)