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KMP_AFFINITY
The KMP_AFFINITY environment variable uses the following general syntax:
Syntax
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KMP_AFFINITY=[<modifier>,...]<type>[,<permute>][,<offset>]
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For example, to list a machine topology map, specify KMP_AFFINITY=verbose,none to use a modifier of verbose and a type of none.
The following table describes the supported specific arguments.
| Argument |
Default
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Description
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modifier
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noverbose
respect
granularity=core
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Optional. String consisting of keyword and specifier.
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type
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none
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Required string. Indicates the thread affinity to use.
-
compact
-
disabled
-
explicit
-
none
-
scatter
-
logical (deprecated; instead use compact, but omit any permute value)
-
physical (deprecated; instead use scatter, possibly with an offset value)
The logical and physical types are deprecated but supported for backward compatibility.
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permute
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0
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Optional. Positive integer value. Not valid with type values of explicit, none, or disabled. |
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offset
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0
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Optional. Positive integer value. Not valid with type values of explicit, none, or disabled.
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Affinity Types
Type is the only required argument.
type = none (default)
Does not bind OpenMP threads to particular thread contexts; however, if the operating system supports affinity, the compiler still uses the OpenMP thread affinity interface to determine machine topology. Specify KMP_AFFINITY=verbose,none to list a machine topology map.
type = compact
Specifying compact assigns the OpenMP thread <n>+1 to a free thread context as close as possible to the thread context where the <n> OpenMP thread was placed. For example, in a topology map, the nearer a node is to the root, the more significance the node has when sorting the threads.
type = disabled
Specifying disabled completely disables the thread affinity interfaces. This forces the OpenMP run-time library to behave as if the affinity interface was not supported by the operating system. This includes the low-level API interfaces such as kmp_set_affinity and kmp_get_affinity, which have no effect and will return a nonzero error code.
type = explicit
Specifying explicit assigns OpenMP threads to a list of OS proc IDs that have been explicitly specified by using the proclist= modifier, which is required for this affinity type.
type = scatter
Specifying scatter distributes the threads as evenly as possible across the entire system. scatter is the opposite of compact; so the leaves of the node are most significant when sorting through the machine topology map.
Deprecated Types: logical and physical
Types logical and physical are deprecated and may become unsupported in a future release. Both are supported for backward compatibility.
For logical and physical affinity types, a single trailing integer is interpreted as an offset specifier instead of a permute specifier. In contrast, with compact and scatter types, a single trailing integer is interpreted as a permute specifier.
Specifying logical assigns OpenMP threads to consecutive logical processors, which are also called hardware thread contexts. The type is equivalent to compact, except that the permute specifier is not allowed. Thus, KMP_AFFINITY=logical,n is equivalent to KMP_AFFINITY=compact,0,n (this equivalence is true regardless of the whether or not a granularity=fine modifier is present).
Permute and offset combinations
For both compact and scatter, permute and offset are allowed; however, if you specify only one integer, the compiler interprets the value as a permute specifier. Both permute and offset default to 0.
The permute specifier controls which levels are most significant when sorting the machine topology map. A value for permute forces the mappings to make the specified number of most significant levels of the sort the least significant, and it inverts the order of significance. The root node of the tree is not considered a separate level for the sort operations.
The offset specifier indicates the starting position for thread assignment.
Modifier Values for Affinity Types
Modifiers are optional arguments that precede type. If you do not specify a modifier, the noverbose, respect, and granularity=core modifiers are used automatically.
Modifiers are interpreted in order from left to right, and can negate each other. For example, specifying KMP_AFFINITY=verbose,noverbose,scatter is therefore equivalent to setting KMP_AFFINITY=noverbose,scatter, or just KMP_AFFINITY=scatter.
modifier = noverbose (default)
Does not print verbose messages.
modifier = verbose
Prints messages concerning the supported affinity. The messages include information about the number of packages, number of cores in each package, number of thread contexts for each core, and OpenMP thread bindings to physical thread contexts.
Information about binding OpenMP threads to physical thread contexts is indirectly shown in the form of the mappings between hardware thread contexts and the operating system (OS) processor (proc) IDs. The affinity mask for each OpenMP thread is printed as a set of OS processor IDs.
KMP_LIBRARY
KMP_LIBRARY = { throughput | turnaround | serial },
Selects the OpenMP run-time library execution mode.
The options for the variable value are throughput, turnaround, and serial.
Execution modes
The compiler with OpenMP enables you to run an application under different
execution modes that can be specified at run time. The libraries support
the serial, turnaround, and throughput modes.
Serial
The serial mode forces parallel applications to run on a single processor.
Turnaround
In a dedicated (batch or single user) parallel environment where all
processors are exclusively allocated to the program for its entire run,
it is most important to effectively utilize all of the processors all
of the time. The turnaround mode is designed to keep active all of the
processors involved in the parallel computation in order to minimize the
execution time of a single job. In this mode, the worker threads actively
wait for more parallel work, without yielding to other threads.
Avoid over-allocating system resources. This
occurs if either too many threads have been specified, or if too few processors
are available at run time. If system resources are over-allocated, this
mode will cause poor performance. The throughput mode should be used instead
if this occurs.
Throughput
In a multi-user environment where the load on the parallel machine is
not constant or where the job stream is not predictable, it may be better
to design and tune for throughput. This minimizes the total time to run
multiple jobs simultaneously. In this mode, the worker threads will yield
to other threads while waiting for more parallel work.
The throughput mode is designed to make the program aware of its environment
(that is, the system load) and to adjust its resource usage to produce
efficient execution in a dynamic environment. This mode is the default.
![[user]](http://www.spec.org/auto/omp2012/flags/user.png)
![[suite]](http://www.spec.org/auto/omp2012/flags/suite.png)
![[benchmark]](http://www.spec.org/auto/omp2012/flags/benchmark.png)