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50 lines
2.3 KiB
TeX
50 lines
2.3 KiB
TeX
%\pagebreak
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\section{\kcode{taskloop} Construct}
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\label{sec:taskloop}
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\index{constructs!taskloop@\kcode{taskloop}}
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\index{taskloop construct@\kcode{taskloop} construct}
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\index{taskloop construct@\kcode{taskloop} construct!grainsize clause@\kcode{grainsize} clause}
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\index{taskloop construct@\kcode{taskloop} construct!nogroup clause@\kcode{nogroup} clause}
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\index{clauses!grainsize@\kcode{grainsize}}
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\index{grainsize clause@\kcode{grainsize} clause}
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\index{clauses!nogroup@\kcode{nogroup}}
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\index{nogroup clause@\kcode{nogroup} clause}
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The following example illustrates how to execute a long running task concurrently with tasks created
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with a \kcode{taskloop} directive for a loop having unbalanced amounts of work for its iterations.
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The \kcode{grainsize} clause specifies that each task is to execute at least \ucode{500} iterations of the loop.
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The \kcode{nogroup} clause removes the implicit taskgroup of the \kcode{taskloop} construct;
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the explicit \kcode{taskgroup} construct in the example ensures that the function is not exited
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before the long-running task and the loops have finished execution.
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\cexample[4.5]{taskloop}{1}
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\ffreeexample[4.5]{taskloop}{1}
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%\clearpage
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Because a \kcode{taskloop} construct encloses a loop, it is often incorrectly
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perceived as a worksharing construct (when it is directly nested in
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a \kcode{parallel} region).
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While a worksharing construct distributes the loop iterations across all threads in a team,
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the entire loop of a \kcode{taskloop} construct is executed by every thread of the team.
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In the example below the first taskloop occurs closely nested within
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a \kcode{parallel} region and the entire loop is executed by each of the \ucode{T} threads;
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hence the reduction sum is executed \ucode{T}*\ucode{N} times.
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The loop of the second taskloop is within a \kcode{single} region and is executed
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by a single thread so that only \ucode{N} reduction sums occur. (The other
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\ucode{N}-1 threads of the \kcode{parallel} region will participate in executing the
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tasks. This is the common use case for the \kcode{taskloop} construct.)
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In the example, the code thus prints \pout{x1 = 16384} (\ucode{T}*\ucode{N}) and
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\pout{x2 = 1024} (\ucode{N}).
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\cexample[4.5]{taskloop}{2}
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\ffreeexample[4.5]{taskloop}{2}
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