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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vxiod

vxiod(1M)																 vxiod(1M)

NAME

       vxiod - start, stop, and report on Veritas Volume Manager I/O threads

SYNOPSIS

       vxiod

       vxiod [-f ] [set count]

DESCRIPTION

       The  vxiod utility starts, stops, or reports on Veritas Volume Manager (VxVM) I/O kernel threads.  An I/O thread provides a process context
       for performing I/O in VxVM.

       When the vxio module is loaded, 16 I/O threads are created, plus 2 threads per additional CPU for a system with more than 8 CPUs, up  to  a
       maximum	of  64	threads.  At least one I/O thread must be running while the vxio module is loaded, and the number of I/O threads cannot be
       forced to zero.

       When invoked with no arguments, vxiod prints the current number of I/O threads to the standard output.

       The number of threads that is required for handling I/O requests depends on the system load and usage.  If volume recovery seems to proceed
       more slowly at times, it may be possible to improve its performance by increasing the number of I/O threads up to a maximum of 64.

KEYWORDS

       set	 When  invoked with the set keyword, vxiod creates the number of I/O threads specified by count.  If more volume I/O threads exist
		 than are specified by count, the excess processes terminate.  If more than the maximum number(64) are specified,  the  requested
		 number is silently truncated to that maximum.

OPTIONS

       -f	 This option has no effect from release 5.0 onward.  The number of I/O threads cannot be reduced to zero.

EXIT CODES

       The  vxiod  utility  prints a diagnostic on the standard error, and exits if an error is encountered. If an I/O request occurs within a I/O
       thread, the state of that I/O request is not reflected in the exit status for vxiod.  Otherwise, vxiod returns a non-zero  exit	status	on
       error.

       Usage  errors  result in an exit status of 1 and a usage message.  If the requested number of threads cannot be created, the exit status is
       2, and the number of threads that were successfully started is reported.  If any other error occurs, the exit status is 3.

FILES

       /dev/vx/iod		     The device used to report on and start volume I/O threads.

NOTES

       Veritas Volume Manager I/O threads cannot be killed directly through the use of signals.

       Depending on the operating system, VxVM I/O threads may not appear in the list of processes that is output by the ps command.   The  number
       of I/O threads that is currently running can be determined by running vxiod.

SEE ALSO

       fork(2), ps(1), vxconfigd(1M), vxdctl(1M), vxintro(1M), vxio(7), vxiod(7)

VxVM 5.0.31.1							    24 Mar 2008 							 vxiod(1M)