1 Internal Control Variables (ICVs)

Specific ICVs and their initial values are given in the table below. Please notice that OMPi can utilize multiple and different threading libraries, each of which may have its own initial / default values for some of these ICVs; library-specific ICVs are marked by an asterisk (*). Values in the table below refer to the default threading library (pthreads).

2 Dynamic Adjustment of Threads

When the dynamic adjustment of threads is disabled and the threading library cannot provide the requested number of threads, the application is aborted with an advice to enable dynamic adjustment either programmatically or using the corresponding environmental variable.

3 Loop Directive

• If the runtime schedule has been selected and the run-sched-var ICV is set to auto, then the iterations are distributed using a static schedule.
• For a collapsed loop, the variable used to compute the iteration count is of type int.

4 Sections Construct

Scheduling of the structured section blocks among threads is competitive; threads are assigned section blocks on a first-come first-serve basis.

5 Single Construct

The selection of a thread to execute a structured single block is competitive; the first thread to ask for it, gets to execute it.

6 Processor

A ‘processor’ is whatever the OS system calls report.

7 Device-specific behaviors

Devices are the host and any additional attached compute units. The host is always the device with id 0. In this version, OMPi can support the Adapteva Epiphany accelerator as a device.

7.1 The is_device_ptr clause

OMPi only supports pointers returned by the omp_target_alloc() call and pointers specified through the use_device_ptr clause.

8 Runtime Routines

• omp_set_num_threads() : if the argument is not a positive integer, then it is assumed to be equal to 1.
• omp_set_schedule() : there are no additional loop schedules defined.
• omp_set_max_active_levels() : if the argument is not a positive integer, then the routine simply returns. If it is called from within a explicit parallel region, the binding thread set is all threads.
• omp_get_max_active_levels() : if it is called from within a explicit parallel region, the binding thread set is all threads.
• omp_get_place_proc_ids() : the numerical identifiers returned correspond to the unique ID of each ‘processor’. Their order matches the one in which they are found in the place list of the execution environment.
• omp_get_initial_device() : returns 0, which is the id of the host device.
• omp_set_default_device() : if the value is a negative integer, or it is greater than the number of available devices minus one, the host becomes the default device.
• omp_init_lock_with_hint() : the hint is currently ignored; the call is equivalent to omp_init_lock().
• omp_init_nest_lock_with_hint() : the hint is currently ignored; the call is equivalent to omp_init_nest_lock().
• omp_target_memcpy_rect() : there is no limit on the number of dimensions.

9 Environmental Variables

• OMP_SCHEDULE : if the value of the variable does not conform to the specified format, it is considered to be equal to auto.
• OMP_NUM_THREADS : if any list value is not a positive integer, the default number of threads is used (which is threading-library specific; see the table above). It the requested number of threads exceeds the capabilities of the threading library (which cannot happen when using the default threading library) the program is aborted with an informative message.
• OMP_PROC_BIND : if the value of the variable is not true, false or a comma-seperated list of master, close or spread, then it is considered to be false. If an initial thread cannot be bound to the first place of the place list, binding for it and it’s child threads is disabled.
• OMP_PLACES : if defined using an explicit list of places described by non-negative numbers, these numbers correspond to the unique ID of each ‘processor’. If the exclusion operator ! is encountered while parsing the e.v., all previous occurrences of the place following the operator will be removed from the place list of the execution environment. No action is taken in case a numeric value cannot be mapped to a ‘processor’ (unavailable or not).

  The only abstract names recognized are threads, cores and sockets. If the Portable Hardware Locality (HWLOC) software package is available topology detection is conducted in order for the abstract names to be mapped into ‘processor’ IDs. In case HWLOC is not available, the default place list is used which consists of as many places as the number of ‘processor’s available in the underlying system and each place contains a single ‘processor’ ID.

  If the place list is created by appending the number n to an abstract name, then a) if n is greater than the available resources, all the resources are used and b) if n is smaller than the available resources, the first n resources are used.

• OMP_DYNAMIC, OMP_NESTED : if the value of the variable is neither true nor false, then it is considered to be false.
• OMP_STACKSIZE : if the value of the variable does not conform to the specified format, it is considered to be equal to 256KB.
• OMP_WAIT_POLICY : if the value of the variable is neither active nor passive, it is ignored. The details of the wait policy behavior are threading-library specific. For the default threading library, the wait policy is always active irrespectively of the value of this environmental variable.
• OMP_MAX_ACTIVE_LEVELS : if the value is not a positive integer, it is ignored. The default behavior, for the case where the requested number of levels is larger than what can be supported, is threading-library specific (does not concern the default threading library). Conflicting value of OMP_NESTED: if OMP_NESTED is false, no nested parallelism level is created, even if OMP_MAX_ACTIVE_LEVELS > 1.
• OMP_THREAD_LIMIT : if the value is not a positive integer, it is ignored. The default behavior, for the case where the requested number of threads is larger than what can be supported, is threading-library specific (does not concern the default threading library).
• OMP_DEFAULT_DEVICE : if the value is a negative integer, or it is greater than the number of available devices minus one, the host becomes the default device.