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58 lines
3.1 KiB
TeX
58 lines
3.1 KiB
TeX
\newpage
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\section{Affinity Query Functions}
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\label{sec: affinity_query}
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\index{affinity query!omp_get_num_places routine@\kcode{omp_get_num_places} routine}
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\index{routines!omp_get_num_places@\kcode{omp_get_num_places}}
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\index{omp_get_num_places routine@\kcode{omp_get_num_places} routine}
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\index{affinity query!omp_get_place_num routine@\kcode{omp_get_place_num} routine}
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\index{routines!omp_get_place_num@\kcode{omp_get_place_num}}
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\index{omp_get_place_num routine@\kcode{omp_get_place_num} routine}
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\index{affinity query!omp_get_place_num_procs routine@\kcode{omp_get_place_num_procs} routine}
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\index{routines!omp_get_place_num_procs@\kcode{omp_get_place_num_procs}}
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\index{omp_get_place_num_procs routine@\kcode{omp_get_place_num_procs} routine}
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\index{affinity!spread policy@\kcode{spread} policy}
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\index{spread policy@\kcode{spread} policy}
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\index{environment variables!OMP_PLACES@\kcode{OMP_PLACES}}
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\index{OMP_PLACES@\kcode{OMP_PLACES}}
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In the example below a team of threads is generated on each socket of
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the system, using nested parallelism. Several query functions are used
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to gather information to support the creation of the teams and to obtain
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socket and thread numbers.
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For proper execution of the code, the user must create a place partition, such that
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each place is a listing of the core numbers for a socket. For example,
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in a 2 socket system with 8 cores in each socket, and sequential numbering
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in the socket for the core numbers, the \kcode{OMP_PLACES} variable would be set
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to "\{0:8\},\{8:8\}", using the place syntax \{\splc{lower_bound:length:stride}\},
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and the default stride of 1.
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The code determines the number of sockets (\ucode{n_sockets})
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using the \kcode{omp_get_num_places()} query function.
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In this example each place is constructed with a list of
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each socket's core numbers, hence the number of places is equal
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to the number of sockets.
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The outer parallel region forms a team of threads, and each thread
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executes on a socket (place) because the \kcode{proc_bind} clause uses
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\kcode{spread} in the outer \kcode{parallel} construct.
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Next, in the \ucode{socket_init} function, an inner parallel region creates a team
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of threads equal to the number of elements (core numbers) from the place
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of the parent thread. Because the outer \kcode{parallel} construct uses
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a \kcode{spread} affinity policy, each of its threads inherits a sub-partition of
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the original partition. Hence, the \kcode{omp_get_place_num_procs} query function
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returns the number of elements (here procs = cores) in the sub-partition of the thread.
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After each parent thread creates its nested parallel region on the section,
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the socket number and thread number are reported.
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Note: Portable tools like hwloc (Portable HardWare LOCality package), which support
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many common operating systems, can be used to determine the configuration of a system.
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On some systems there are utilities, files or user guides that provide configuration
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information. For instance, the socket number and proc\_id's for a socket
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can be found in the /proc/cpuinfo text file on Linux systems.
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\cexample[4.5]{affinity_query}{1}
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\ffreeexample[4.5]{affinity_query}{1}
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