Developer Reference

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blacs_get

Gets values that BLACS use for internal defaults.

Syntax

call blacs_get
(
icontxt
,
what
,
val
)
Input Parameters
icontxt
INTEGER
. On values of
what
that are tied to a particular context, this parameter is the integer handle indicating the context. Otherwise, ignored.
what
INTEGER
. Indicates what BLACS internal(s) should be returned in
val
. Present options are:
  • what
    = 0 : Handle indicating default system context.
  • what
    = 1 : The BLACS message ID range.
  • what
    = 2 : The BLACS debug level the library was compiled with.
  • what
    = 10 : Handle indicating the system context used to define the BLACS context whose handle is
    icontxt
    .
  • what
    = 11 : Number of rings multiring broadcast topology is presently using.
  • what
    = 12 : Number of branches general tree broadcast topology is presently using.
  • what
    = 13 : Number of rings multiring combine topology is presently using.
  • what
    = 14 : Number of branches general tree combine topology is presently using.
  • what
    = 15 : Whether topologies are forced to be repeatable or not. A non-zero return value indicates that topologies are being forced to be repeatable. See Repeatability and Coherence for more information about repeatability.
  • what
    = 16 : Whether topologies are forced to be heterogenous coherent or not. A non-zero return value indicates that topologies are being forced to be heterogenous coherent. See Repeatability and Coherence for more information about coherence.
Output Parameters
val
INTEGER
. The value of the BLACS internal.
Description
This routine gets the values that the BLACS are using for internal defaults. Some values are tied to a BLACS context, and some are more general. The most common use is in retrieving a default system context for input into
blacs_gridinit
or
blacs_gridmap
.
Some systems, such as MPI*, supply their own version of context. For those users who mix system code with BLACS code, a BLACS context should be formed in reference to a system context. Thus, the grid creation routines take a system context as input. If you wish to have strictly portable code, you may use
blacs_get
to retrieve a default system context that will include all available processes. This value is not tied to a BLACS context, so the parameter
icontxt
is unused.
blacs_get
returns information on three quantities that are tied to an individual BLACS context, which is passed in as
icontxt
. The information that may be retrieved is:
  • The handle of the system context upon which this BLACS context was defined
  • The number of rings for
    TOP = 'M'
    (multiring broadcast/combine)
  • The number of branches for
    TOP = 'T'
    (general tree broadcast/general tree gather).
  • Whether topologies are being forced to be repeatable or heterogenous coherent.

Product and Performance Information

1

Performance varies by use, configuration and other factors. Learn more at www.Intel.com/PerformanceIndex.