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49 lines
2.6 KiB
TeX
49 lines
2.6 KiB
TeX
\pagebreak
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\chapter{Asynchronous Execution of a \code{target} Region Using Tasks}
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\label{chap:async_target}
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The following example shows how the \code{task} and \code{target} constructs
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are used to execute multiple \code{target} regions asynchronously. The task that
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encounters the \code{task} construct generates an explicit task that contains
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a \code{target} region. The thread executing the explicit task encounters a task
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scheduling point while waiting for the execution of the \code{target} region
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to complete, allowing the thread to switch back to the execution of the encountering
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task or one of the previously generated explicit tasks.
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\cexample{async_target}{1c}
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The Fortran version has an interface block that contains the \code{declare} \code{target}.
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An identical statement exists in the function declaration (not shown here).
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\fexample{async_target}{1f}
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The following example shows how the \code{task} and \code{target} constructs
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are used to execute multiple \code{target} regions asynchronously. The task dependence
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ensures that the storage is allocated and initialized on the device before it is
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accessed.
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\cexample{async_target}{2c}
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The Fortran example below is similar to the C version above. Instead of pointers, though, it uses
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the convenience of Fortran allocatable arrays on the device. An allocatable array has the
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same behavior in a \code{map} clause as a C pointer, in this case.
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If there is no shape specified for an allocatable array in a \code{map} clause, only the array descriptor
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(also called a dope vector) is mapped. That is, device space is created for the descriptor, and it
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is initially populated with host values. In this case, the \plc{v1} and \plc{v2} arrays will be in a
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non-associated state on the device. When space for \plc{v1} and \plc{v2} is allocated on the device
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the addresses to the space will be included in their descriptors.
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At the end of the device region, the descriptor (of an unshaped specification of an allocatable
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array in a \code{map} clause) is returned with the raw device address of the allocated space.
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The content of the array is not returned. In the example the data in arrays \plc{v1} and \plc{v2}
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are not returned. In the second target directive, the \plc{v1} and \plc{v2} descriptors are
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re-created on the device with the descriptive information; and references to the
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vectors point to the correct local storage, of the space that was not freed in the first target
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directive. At the end of the \code{target} region the data in array \plc{p} is copied back
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to the host, since \plc{p} is not an allocatable array.
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\fexample{async_target}{2f}
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