Provided by: pdl_2.019-5build1_amd64

**NAME**

PDL::ParallelCPU - Parallel Processor MultiThreading Support in PDL (Experimental)

**DESCRIPTION**

PDL has support (currently experimental) for splitting up numerical processing between multiple parallel processor threads (or pthreads) using theset_autopthread_targandset_autopthread_sizefunctions. This can improve processing performance (by greater than 2-4X in most cases) by taking advantage of multi-core and/or multi-processor machines.

**SYNOPSIS**

use PDL; # Set target of 4 parallel pthreads to create, with a lower limit of # 5Meg elements for splitting processing into parallel pthreads. set_autopthread_targ(4); set_autopthread_size(5); $a = zeroes(5000,5000); # Create 25Meg element array $b = $a + 5; # Processing will be split up into multiple pthreads # Get the actual number of pthreads for the last # processing operation. $actualPthreads = get_autopthread_actual();

**Terminology**

The use of the termthreadingcan be confusing with PDL, because it can refer toPDLthreading, as defined in the PDL::Threading docs, or toprocessormulti-threading. To reduce confusion with the existing PDL threading terminology, this document usespthreadingto refer toprocessormulti-threading, which is the use of multiple processor threads to split up numerical processing into parallel operations.

**Functions** **that** **control** **PDL** **PThreads**

This is a brief listing and description of the PDL pthreading functions, see the PDL::Core docs for detailed information. set_autopthread_targ Set the target number of processor-threads (pthreads) for multi-threaded processing. Setting auto_pthread_targ to 0 means that no pthreading will occur. See PDL::Core for details. set_autopthread_size Set the minimum size (in Meg-elements or 2**20 elements) of the largest PDL involved in a function where auto-pthreading will be performed. For small PDLs, it probably isn't worth starting multiple pthreads, so this function is used to define a minimum threshold where auto-pthreading won't be attempted. See PDL::Core for details. get_autopthread_actual Get the actual number of pthreads executed for the last pdl processing function. See PDL::get_autopthread_actual for details.

**Global** **Control** **of** **PDL** **PThreading** **using** **Environment** **Variables**

PDL PThreading can be globally turned on, without modifying existing code by setting environment variablesPDL_AUTOPTHREAD_TARGandPDL_AUTOPTHREAD_SIZEbefore running a PDL script. These environment variables are checked when PDL starts up and calls toset_autopthread_targandset_autopthread_sizefunctions made with the environment variable's values. For example, if the environment varPDL_AUTOPTHREAD_TARGis set to 3, andPDL_AUTOPTHREAD_SIZEis set to 10, then any pdl script will run as if the following lines were at the top of the file: set_autopthread_targ(3); set_autopthread_size(10);

**How** **It** **Works**

The auto-pthreading process works by analyzing threaded array dimensions in PDL operations and splitting up processing based on the thread dimension sizes and desired number of pthreads (i.e. the pthread target or pthread_targ). The offsets and increments that PDL uses to step thru the data in memory are modified for each pthread so each one sees a different set of data when performing processing.Example$a = sequence(20,4,3); # Small 3-D Array, size 20,4,3 # Setup auto-pthreading: set_autopthread_targ(2); # Target of 2 pthreads set_autopthread_size(0); # Zero so that the small PDLs in this example will be pthreaded # This will be split up into 2 pthreads $c = maximum($a); For the above example, themaximumfunction has a signature of "(a(n); [o]c())", which means that the first dimension of $a (size 20) is aCoredimension of themaximumfunction. The other dimensions of $a (size 4,3) arethreadeddimensions (i.e. will be threaded-over in themaximumfunction. The auto-pthreading algorithm examines the threaded dims of size (4,3) and picks the 4 dimension, since it is evenly divisible by the autopthread_targ of 2. The processing of the maximum function is then split into two pthreads on the size-4 dimension, with dim indexes 0,2 processed by one pthread and dim indexes 1,3 processed by the other pthread.

**Limitations**

MusthavePOSIXThreadsEnabledAuto-PThreading only works if your PDL installation was compiled with POSIX threads enabled. This is normally the case if you are running on linux, or other unix variants.Non-ThreadsafeCodeNot all the libraries that PDL intefaces to are thread-safe, i.e. they aren't written to operate in a multi-threaded environment without crashing or causing side-effects. Some examples in the PDL core is thefftfunction and thepnmoutfunctions. To operate properly with these types of functions, the PPCode flagNoPthreadhas been introduced to indicate a function asnotbeing pthread-safe. See PDL::PP docs for details.SizeofPDLDimensionsandPThreadTargetDue to the way a PDL is split-up for operation using multiple pthreads, the size of a dimension must be evenly divisible by the pthread target. For example, if a PDL has threaded dimension sizes of (4,3,3) and theauto_pthread_targhas been set to 2, then the first threaded dimension (size 4) will be picked to be split up into two pthreads of size 2 and 2. However, if the threaded dimension sizes are (3,3,3) and theauto_pthread_targis still 2, then pthreading won't occur, because no threaded dimensions are divisible by 2. The algorithm that picks the actual number of pthreads has some smarts (but could probably be improved) to adjust down from theauto_pthread_targto get a number of pthreads that can evenly divide one of the threaded dimensions. For example, if a PDL has threaded dimension sizes of (9,2,2) and theauto_pthread_targis 4, the algorithm will see that no dimension is divisible by 4, then adjust down the target to 3, resulting in splitting up the first threaded dimension (size 9) into 3 pthreads.Speedimprovementmightbelessthanyouexpect.If you have a 8 core machine and callauto_pthread_targwith 8 to generate 8 parallel pthreads, you probably won't get a 8X improvement in speed, due to memory bandwidth issues. Even though you have 8 separate CPUs crunching away on data, you will have (for most common machine architectures) common RAM that now becomes your bottleneck. For simple calculations (e.g simple additions) you can run into a performance limit at about 4 pthreads. For more complex calculations the limit will be higher.

**COPYRIGHT**

Copyright 2011 John Cerney. You can distribute and/or modify this document under the same terms as the current Perl license. See: http://dev.perl.org/licenses/