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ParallelDo

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ParallelDo[expr,{imax}]

evaluates expr in parallel imax times.

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ParallelDo[expr,{i,imax}]

evaluates expr in parallel with the variable i successively taking on the values 1 through imax (in steps of 1).

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ParallelDo[expr,{i,imin,imax}]

starts with i=imin.

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ParallelDo[expr,{i,imin,imax,di}]

uses steps di.

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ParallelDo[expr,{i,{i1,i2,…}}]

uses the successive values i1, i2, ….

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ParallelDo[expr,{i,imin,imax},{j,jmin,jmax},…]

evaluates expr looping in parallel over different values of j, etc. for each i.

Details and Options

  • ParallelDo is a parallel version of Do that automatically distributes different evaluations of expr among different kernels and processors.
  • If side effects involve unshared variables, they will in general work differently than in Do.
  • Parallelize[Do[expr,iter, …]] is equivalent to ParallelDo[expr,iter,…].
  • The following options can be given:
  • Method Automaticgranularity of parallelization
    DistributedContexts $DistributedContextscontexts used to distribute symbols to parallel computations
    ProgressReporting $ProgressReportingwhether to report the progress of the computation
  • The Method option specifies the parallelization method to use. Possible settings include:
  • "CoarsestGrained"break the computation into as many pieces as there are available kernels
    "FinestGrained"break the computation into the smallest possible subunits
    "EvaluationsPerKernel"->ebreak the computation into at most e pieces per kernel
    "ItemsPerEvaluation"->mbreak the computation into evaluations of at most m subunits each
    Automaticcompromise between overhead and load balancing
  • Method->"CoarsestGrained" is suitable for computations involving many subunits, all of which take the same amount of time. It minimizes overhead, but does not provide any load balancing.
  • Method->"FinestGrained" is suitable for computations involving few subunits whose evaluations take different amounts of time. It leads to higher overhead, but maximizes load balancing.
  • The DistributedContexts option specifies which symbols appearing in expr have their definitions automatically distributed to all available kernels before the computation.
  • The default value is DistributedContexts:>$DistributedContexts with $DistributedContexts:=$Context, which distributes definitions of all symbols in the current context, but does not distribute definitions of symbols from packages.
  • The ProgressReporting option specifies whether to report the progress of the parallel computation.
  • The default value is ProgressReporting:>$ProgressReporting.

Examples

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Basic Examples  (3)Summary of the most common use cases

ParallelDo works like Do, but in parallel:

In[1]:=1
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https://wolfram.com/xid/0rsz1su7i-c8yhm9
Out[1]=1
In[2]:=2
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https://wolfram.com/xid/0rsz1su7i-dy5k2k
Out[2]=2

No results are returned by ParallelDo:

In[1]:=1
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https://wolfram.com/xid/0rsz1su7i-dj3qx2

Use a shared variable to communicate results found to the master kernel:

In[1]:=1
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https://wolfram.com/xid/0rsz1su7i-bmafci
In[2]:=2
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https://wolfram.com/xid/0rsz1su7i-v8a0u
Out[2]=2

Options  (9)Common values & functionality for each option

Method  (2)

Calculations with vastly differing runtimes should be parallelized as finely as possible:

In[1]:=1
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https://wolfram.com/xid/0rsz1su7i-b82tnj

A large number of simple calculations should be distributed into as few batches as possible:

In[1]:=1
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https://wolfram.com/xid/0rsz1su7i-bq3f3v
Out[1]=1

DistributedContexts  (5)

By default, definitions in the current context are distributed automatically:

In[1]:=1
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https://wolfram.com/xid/0rsz1su7i-g9gh34
In[2]:=2
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https://wolfram.com/xid/0rsz1su7i-clbr3t

Do not distribute any definitions of functions:

In[1]:=1
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https://wolfram.com/xid/0rsz1su7i-6xao0
In[2]:=2
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https://wolfram.com/xid/0rsz1su7i-kx0x4h

Distribute definitions for all symbols in all contexts appearing in a parallel computation:

In[1]:=1
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https://wolfram.com/xid/0rsz1su7i-hu6d1y
In[2]:=2
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https://wolfram.com/xid/0rsz1su7i-s80e6

Distribute only definitions in the given contexts:

In[1]:=1
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https://wolfram.com/xid/0rsz1su7i-yht7o
In[2]:=2
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https://wolfram.com/xid/0rsz1su7i-n7ym7a

Restore the value of the DistributedContexts option to its default:

In[1]:=1
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https://wolfram.com/xid/0rsz1su7i-qkhgjy
Out[1]=1

ProgressReporting  (2)

Do not show a temporary progress report:

In[1]:=1
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https://wolfram.com/xid/0rsz1su7i-bzw7un

Use Method"FinestGrained" for the most accurate progress report:

In[1]:=1
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https://wolfram.com/xid/0rsz1su7i-w8nh1s

Applications  (1)Sample problems that can be solved with this function

Generate a number of animation frames and save them to individual files:

In[1]:=1
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https://wolfram.com/xid/0rsz1su7i-c5o03k

Import every 5^(th) file and display it:

In[2]:=2
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https://wolfram.com/xid/0rsz1su7i-ehj2w1
Out[2]=2
In[3]:=3
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https://wolfram.com/xid/0rsz1su7i-gyy4d
Out[3]=3

Properties & Relations  (2)Properties of the function, and connections to other functions

ParallelDo performs the same iterations as ParallelTable, but does not return the values:

In[1]:=1
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https://wolfram.com/xid/0rsz1su7i-b12pi4
Out[1]=1
In[2]:=2
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https://wolfram.com/xid/0rsz1su7i-hysuqh

Parallelization happens along the outermost (first) index:

In[1]:=1
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https://wolfram.com/xid/0rsz1su7i-fjj6q1

Possible Issues  (2)Common pitfalls and unexpected behavior

A function used that is not known on the parallel kernels has no effect:

In[1]:=1
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https://wolfram.com/xid/0rsz1su7i-em4wbu
In[2]:=2
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https://wolfram.com/xid/0rsz1su7i-jofo5

Define the function on all parallel kernels:

In[3]:=3
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https://wolfram.com/xid/0rsz1su7i-m3hzn

The function is now evaluated on the parallel kernels:

In[4]:=4
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https://wolfram.com/xid/0rsz1su7i-jtpu5w

Definitions of functions in the current context are distributed automatically:

In[5]:=5
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https://wolfram.com/xid/0rsz1su7i-5axng5
In[6]:=6
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https://wolfram.com/xid/0rsz1su7i-3cyctq

Side effects are local to each parallel kernel:

In[1]:=1
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https://wolfram.com/xid/0rsz1su7i-h6cblz
In[2]:=2
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https://wolfram.com/xid/0rsz1su7i-bf2q17
Out[2]=2

Use a shared variable to support global side effects:

In[3]:=3
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https://wolfram.com/xid/0rsz1su7i-g8q98c
In[4]:=4
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https://wolfram.com/xid/0rsz1su7i-f8hnf9
Out[4]=4
Wolfram Research (2008), ParallelDo, Wolfram Language function, https://reference.wolfram.com/language/ref/ParallelDo.html (updated 2021).
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Wolfram Research (2008), ParallelDo, Wolfram Language function, https://reference.wolfram.com/language/ref/ParallelDo.html (updated 2021).

Text

Wolfram Research (2008), ParallelDo, Wolfram Language function, https://reference.wolfram.com/language/ref/ParallelDo.html (updated 2021).

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Wolfram Research (2008), ParallelDo, Wolfram Language function, https://reference.wolfram.com/language/ref/ParallelDo.html (updated 2021).

CMS

Wolfram Language. 2008. "ParallelDo." Wolfram Language & System Documentation Center. Wolfram Research. Last Modified 2021. https://reference.wolfram.com/language/ref/ParallelDo.html.

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Wolfram Language. 2008. "ParallelDo." Wolfram Language & System Documentation Center. Wolfram Research. Last Modified 2021. https://reference.wolfram.com/language/ref/ParallelDo.html.

APA

Wolfram Language. (2008). ParallelDo. Wolfram Language & System Documentation Center. Retrieved from https://reference.wolfram.com/language/ref/ParallelDo.html

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Wolfram Language. (2008). ParallelDo. Wolfram Language & System Documentation Center. Retrieved from https://reference.wolfram.com/language/ref/ParallelDo.html

BibTeX

@misc{reference.wolfram_2025_paralleldo, author="Wolfram Research", title="{ParallelDo}", year="2021", howpublished="\url{https://reference.wolfram.com/language/ref/ParallelDo.html}", note=[Accessed: 24-March-2025 ]}

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@misc{reference.wolfram_2025_paralleldo, author="Wolfram Research", title="{ParallelDo}", year="2021", howpublished="\url{https://reference.wolfram.com/language/ref/ParallelDo.html}", note=[Accessed: 24-March-2025 ]}

BibLaTeX

@online{reference.wolfram_2025_paralleldo, organization={Wolfram Research}, title={ParallelDo}, year={2021}, url={https://reference.wolfram.com/language/ref/ParallelDo.html}, note=[Accessed: 24-March-2025 ]}

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@online{reference.wolfram_2025_paralleldo, organization={Wolfram Research}, title={ParallelDo}, year={2021}, url={https://reference.wolfram.com/language/ref/ParallelDo.html}, note=[Accessed: 24-March-2025 ]}