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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max_thread_proc

max_thread_proc(5)						File Formats Manual						max_thread_proc(5)

NAME

       max_thread_proc - defines the maximum number of concurrent threads allowed per process.

VALUES

   Failsafe
   Default
   Allowed values
   Recommended values
DESCRIPTION

       is  a  dynamic  tunable	that  limits  the  maximum number of threads allowed per process on the system.  When tuning the maximum number of
       threads allowed per process will be the new value of No process will be able to create new threads such that the total count of its threads
       exceeds

   Who is Expected to Change This Tunable?
       System administrators that run applications on their systems requiring a high number of threads per process.

   Restrictions on Changing
       The effect of tuning on existing processes is undefined.  However, existing threads will not be effected.

   When Should the Value of This Tunable Be Raised?
       The  value  of  should be raised if there is a need to run an application that requires a higher number of threads per process than what is
       currently set to accommodate.  Also, if thread creation fails with the value of then it could indicate that the has been reached.  However,
       reaching  is  not the only reason why a thread creation could fail with may be returned if the system tunable has been reached or memory on
       the system is exhausted.

   What Are the Side Effects of Raising the Value?
       A group of processes may be able to exhaust the system wide limit of threads on the system.

   When Should the Value of This Tunable Be Lowered?
       If you feel that it is necessary to limit the number of threads allowed per process.

   What Are the Side Effects of Lowering the Value?
       Some applications which need a high number of threads may behave differently or fail to operate.

   What Other Tunable Values Should Be Changed at the Same Time?
       You may want to look at the value of

WARNINGS

       All HP-UX kernel tunable parameters are release specific.  This parameter may be removed or have its meaning changed in future releases	of
       HP-UX.

       Installation  of  optional  kernel  software, from HP or other vendors, may cause changes to tunable parameter values.  After installation,
       some tunable parameters may no longer be at the default or recommended values.  For information about the effects of installation  on  tun-
       able  values,  consult  the documentation for the kernel software being installed.  For information about optional kernel software that was
       factory installed on your system, see at

AUTHOR

       was developed by HP.

SEE ALSO

       nkthread(5).

							     Tunable Kernel Parameters						max_thread_proc(5)