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59 lines
2.9 KiB
TeX
59 lines
2.9 KiB
TeX
%\pagebreak
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\section{Internal Control Variables (ICVs)}
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\label{sec:icv}
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\index{internal control variables}
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According to the \docref{Internal Control Variables} section of the OpenMP 4.0 specification, an OpenMP implementation must act as if there are ICVs that control
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the behavior of the program. This example illustrates two ICVs, \plc{nthreads-var}
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and \plc{max-active-levels-var}. The \plc{nthreads-var} ICV controls the
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number of threads requested for encountered parallel regions; there is one copy
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of this ICV per task. The \plc{max-active-levels-var} ICV controls the maximum
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number of nested active parallel regions; there is one copy of this ICV for the
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whole program.
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In the following example, the \plc{nest-var}, \plc{max-active-levels-var},
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\plc{dyn-var}, and \plc{nthreads-var} ICVs are modified through calls to
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the runtime library routines \kcode{omp_set_nested},\\ \kcode{omp_set_max_active_levels}, \kcode{omp_set_dynamic},
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and \kcode{omp_set_num_threads} respectively. These ICVs
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affect the operation of \kcode{parallel} regions. Each implicit task generated
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by a \kcode{parallel} region has its own copy of the \plc{nest-var}, \plc{dyn-var},
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and \plc{nthreads-var} ICVs.
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In the following example, the new value of \plc{nthreads-var} applies only to
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the implicit tasks that execute the call to \kcode{omp_set_num_threads}. There
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is one copy of the \plc{max-active-levels-var} ICV for the whole program and
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its value is the same for all tasks. This example assumes that nested parallelism
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is supported.
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The outer \kcode{parallel} region creates a team of two threads; each of the threads
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will execute one of the two implicit tasks generated by the outer \kcode{parallel}
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region.
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Each implicit task generated by the outer \kcode{parallel} region calls \kcode{omp_set_num_threads(\ucode{3})},
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assigning the value 3 to its respective copy of \plc{nthreads-var}. Then each
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implicit task encounters an inner \kcode{parallel} region that creates a team
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of three threads; each of the threads will execute one of the three implicit tasks
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generated by that inner \kcode{parallel} region.
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Since the outer \kcode{parallel} region is executed by 2 threads, and the inner
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by 3, there will be a total of 6 implicit tasks generated by the two inner \kcode{parallel}
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regions.
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Each implicit task generated by an inner \kcode{parallel} region will execute
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the call to\\ \kcode{omp_set_num_threads(\ucode{4})}, assigning the value 4 to its respective
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copy of \plc{nthreads-var}.
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The print statement in the outer \kcode{parallel} region is executed by only one
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of the threads in the team. So it will be executed only once.
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The print statement in an inner \kcode{parallel} region is also executed by only
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one of the threads in the team. Since we have a total of two inner \kcode{parallel}
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regions, the print statement will be executed twice -- once per inner \kcode{parallel}
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region.
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\pagebreak
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\cexample{icv}{1}
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\fexample{icv}{1}
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