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 HPUX

aio_proc_threads

aio_proc_threads(5)						File Formats Manual					       aio_proc_threads(5)

NAME

       aio_proc_threads - maximum number of process threads allowed in AIO pool

VALUES

   Failsafe
   Default
   Allowed values
   Recommended values
DESCRIPTION

       The  implementation of POSIX AIO on HP-UX uses kernel threads to perform I/Os to filesystems that do not directly support true asynchronous
       I/O.  (This distinction is transparent to the user.)  The kernel threads are organized into worker-thread pools (called AIO  thread  pools)
       created	on  a  per-process basis.  Since a thread pool mechanism for I/Os introduces a variety of trade-offs concerning utilization of CPU
       time vs. I/O resources, four dynamic  tunables  are  available  to  customize  the  behavior  of  this  thread  pool:  aio_proc_threads(5),
       aio_proc_thread_pct(5),	aio_req_per_thread(5),	and  aio_monitor_run_sec(5).   Please see individual manpages for details on each of these
       tunables.

       The tunable specifies, on a per-process basis, the maximum number of process threads that can be used by the POSIX  AIO	system	as  kernel
       threads for issuing I/Os.

       This  tunable interacts with in the following way: the maximum number of threads used for AIO will be the smaller of the two values defined
       by the two tunables, i.e.:

       This allows the number of AIO threads to vary dynamically with but to always be bound by an absolute limit of

   Who Is Expected to Change This Tunable?
       System administrators that run applications requiring heavy usage of POSIX AIO to filesystems.

   Restrictions on Changing
       This tunable is dynamic.  Changes to to this tunable take effect immediately for new processes started after the change.  They also  impact
       existing processes, but the speed with which the changes propagate to running processes is determined by the tunable

   When Should the Value of This Tunable Be Raised?
       should be raised for applications that do not use very many threads for their own work, but desire high performance from the POSIX AIO sub-
       system.

   What Are the Side Effects of Raising the Value of This Tunable?
       Some applications that use POSIX AIO but also require a large number of threads may find that they are unable to create new threads.  (This
       could happen if the POSIX AIO thread pool ends up using too many of a process' allowable threads.)

       In addition, using a larger number of kernel threads might lead to increased CPU utilization.

   When Should the Value of This Tunable Be Lowered?
       should  be  lowered  when  POSIX AIO performance is acceptable but applications using POSIX AIO are seeing errors when trying to create new
       threads for other work.

   What Are the Side Effects of Lowering the Value of This Tunable?
       By ultimately reducing the number of threads available to handle POSIX AIO requests, overall I/O throughput  of	the  POSIX  AIO  subsystem
       could be reduced.

   What Other Tunables Should Be Changed at the Same Time as This One?
       interacts  with	this  tunable  by  setting  a  limit on the number of AIO threads based on a percentage of the maximum number of allowable
       process threads.  This allows the AIO thread pools to respond dynamically to changes in

       defines the desired relationship between the number of POSIX AIO kernel threads and the number of I/Os to be serviced.

       defines how often (in seconds) the AIO thread mechanism will monitor itself for adherence to the constraints defined by the tunables above.

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

       kctune(1M), sam(1M), gettune(2), settune(2), aio_proc_thread_pct(5), aio_req_per_thread(5), aio_monitor_run_sec(5).

							     Tunable Kernel Parameters					       aio_proc_threads(5)