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62 lines
3.1 KiB
TeX
62 lines
3.1 KiB
TeX
%\pagebreak
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\section{Cancellation Constructs}
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\label{sec:cancellation}
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\index{cancellation!cancel construct@\kcode{cancel} construct}
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\index{constructs!cancel@\kcode{cancel}}
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\index{cancel construct@\kcode{cancel} construct}
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The examples in this section show how the \kcode{cancel} directive can be used to terminate
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an OpenMP region. Cancellation of the binding region is activated only if the \plc{cancel-var} ICV
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is true, in which case the \kcode{cancel} construct (except \kcode{taskgroup}) causes the encountering
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\kcode{task} to continue execution at the end of the binding. If the \plc{cancel-var} ICV is false, the
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\kcode{cancel} construct is ignored.
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\index{cancellation!for parallel region@for \kcode{parallel} region}
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\index{cancellation!for worksharing region}
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In the following example although the \kcode{cancel} construct terminates the OpenMP
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worksharing region, programmers must still track the exception through the pointer
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\ucode{ex} and issue a cancellation for the \kcode{parallel} region if an exception has
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been raised. The primary thread checks the exception pointer to make sure that the
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exception is properly handled in the sequential part. If cancellation of the \kcode{parallel}
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region has been requested, some threads might have executed \ucode{phase_1()}.
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However, it is guaranteed that none of the threads executed \ucode{phase_2()}.
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\cppexample[4.0]{cancellation}{1}
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\index{cancellation!cancellation point construct@\kcode{cancellation point} construct}
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\index{constructs!cancellation point@\kcode{cancellation point}}
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\index{cancellation point construct@\kcode{cancellation point} construct}
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The following example illustrates the use of the \kcode{cancel} construct in error
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handling. If there is an error condition from the \bcode{allocate} statement,
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the cancellation is activated. The encountering thread sets the shared variable
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\ucode{err} and other threads of the binding thread set proceed to the end of
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the worksharing construct after the cancellation has been activated.
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\ffreeexample[4.0]{cancellation}{1}
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\index{cancellation!for taskgroup region@for \kcode{taskgroup} region}
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The following example shows how to cancel a parallel search on a binary tree as
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soon as the search value has been detected. The code creates a task to descend
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into the child nodes of the current tree node. If the search value has been found,
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the code remembers the tree node with the found value through an \kcode{atomic}
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write to the result variable (\ucode{found}) and then cancels execution of all search tasks. The
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function \ucode{search_tree_parallel} groups all search tasks into a single
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task group to control the effect of the \kcode{cancel taskgroup} directive. The
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\ucode{level} argument is used to create undeferred tasks after the first ten
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levels of the tree.
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\cexample[5.1]{cancellation}{2}
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The following is the equivalent parallel search example in Fortran.
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The code uses the \kcode{atomic write} directive for atomically
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updating pointer variables -- a feature defined in OpenMP 6.0.
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For earlier versions of OpenMP, the \kcode{critical} directive could
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be used instead.
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\ffreeexample[6.0]{cancellation}{2}
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