I_MPI_MSG Family

These environment variables are deprecated and supported mostly for backward compatibility. Use the I_MPI_ADJUST environment variable family whenever possible.

I_MPI_FAST_COLLECTIVES

Control the default library behavior during selection of the most appropriate collective algorithm.

Syntax

I_MPI_FAST_COLLECTIVES=<arg>

Arguments

<arg>

Binary indicator

enable | yes | on | 1

Fast collective algorithms are used. This is the default value

disable | no | off | 0

Slower and safer collective algorithms are used

Description

The IntelĀ® MPI Library uses advanced collective algorithms designed for better application performance by default. The implementation makes the following assumptions:

Set the I_MPI_FAST_COLLECTIVES environment variable to disable if you need to obtain results that do not depend on the physical process layout or other factors.

Note

Some optimizations controlled by this environment variable are of an experimental nature. In case of failure, turn off the collective optimizations and repeat the run.

I_MPI_BCAST_NUM_PROCS

Control MPI_Bcast algorithm thresholds.

Syntax

I_MPI_BCAST_NUM_PROCS=<nproc>

Arguments

<nproc>

Define the number of processes threshold for choosing the MPI_Bcast algorithm

> 0

The default value is 8

I_MPI_BCAST_MSG

Control MPI_Bcast algorithm thresholds.

Syntax

I_MPI_BCAST_MSG=<nbytes1,nbytes2>

Arguments

<nbytes1,nbytes2>

Define the message size threshold range (in bytes) for choosing the MPI_Bcast algorithm

> 0

nbytes2 >= nbytes1

The default pair of values is 12288,524288

Description

Set these environment variables to control the selection of the three possible MPI_Bcast algorithms according to the following scheme (See Table 3.4-1 for algorithm descriptions):

The first algorithm is selected if the message size is less than <nbytes1>, or the number of processes in the operation is less than <nproc>.

The second algorithm is selected if the message size is greater than or equal to <nbytes1> and less than <nbytes2>, and the number of processes in the operation is a power of two.

If none of the above conditions is satisfied, the third algorithm is selected.

I_MPI_ALLTOALL_NUM_PROCS

Control MPI_Alltoall algorithm thresholds.

Syntax

I_MPI_ALLTOALL_NUM_PROCS=<nproc>

Arguments

<nproc>

Define the number of processes threshold for choosing the MPI_Alltoall algorithm

> 0

The default value is 8

I_MPI_ALLTOALL_MSG

Control MPI_Alltoall algorithm thresholds.

Syntax

I_MPI_ALLTOALL_MSG=<nbytes1,nbytes2>

Arguments

<nbytes1,nbytes2>

Defines the message size threshold range (in bytes) for choosing the MPI_Alltoall algorithm

> 0

nbytes2 >= nbytes1

The default pair of values is 256,32768

Description

Set these environment variables to control the selection of the three possible MPI_Alltoall algorithms according to the following scheme (See Table 3.4-1 for algorithm descriptions):

The first algorithm is selected if the message size is greater than or equal to <nbytes1>, and the number of processes in the operation is not less than <nproc>.

The second algorithm is selected if the message size is greater than <nbytes1> and less than or equal to <nbytes2>, or if the message size is less than <nbytes2> and the number of processes in the operation is less than <nproc>.

If none of the above conditions is satisfied, the third algorithm is selected.

I_MPI_ALLGATHER_MSG

Control MPI_Allgather algorithm thresholds.

Syntax

I_MPI_ALLGATHER_MSG=<nbytes1,nbytes2>

Arguments

<nbytes1,nbytes2>

Define the message size threshold range (in bytes) for choosing the MPI_Allgather algorithm

> 0

nbytes2 >= nbytes1

The default pair of values is 81920,524288

Description

Set this environment variable to control the selection of the three possible MPI_Allgather algorithms according to the following scheme (See Table 3.4-1 for algorithm descriptions):

The first algorithm is selected if the message size is less than <nbytes2> and the number of processes in the operation is a power of two.

The second algorithm is selected if the message size is less than <nbytes1> and number of processes in the operation is not a power of two.

If none of the above conditions is satisfied, the third algorithm is selected.

I_MPI_ALLREDUCE_MSG

Control MPI_Allreduce algorithm thresholds.

Syntax

I_MPI_ALLREDUCE_MSG=<nbytes>

Arguments

<nbytes>

Define the message size threshold (in bytes) for choosing the MPI_Allreduce algorithm

> 0

The default value is 2048

Description

Set this environment variable to control the selection of the two possible MPI_Allreduce algorithms according to the following scheme (See Table 3.4-1 for algorithm descriptions):

The first algorithm is selected if the message size is less than or equal <nbytes>, or the reduction operation is user-defined, or the count argument is less than the nearest power of two less than or equal to the number of processes.

If the above condition is not satisfied, the second algorithm is selected.

I_MPI_REDSCAT_MSG

Control the MPI_Reduce_scatter algorithm thresholds.

Syntax

I_MPI_REDSCAT_MSG=<nbytes1,nbytes2>

Arguments

<nbytes>

Define the message size threshold range (in bytes) for choosing the MPI_Reduce_scatter algorithm

> 0

The default pair of values is 512,524288

Description

Set this environment variable to control the selection of the three possible MPI_Reduce_scatter algorithms according to the following scheme (See Table 3.4-1 for algorithm descriptions):

The first algorithm is selected if the reduction operation is commutative and the message size is less than <nbytes2>.

The second algorithm is selected if the reduction operation is commutative and the message size is greater than or equal to <nbytes2>, or if the reduction operation is not commutative and the message size is greater than or equal to <nbytes1>.

If none of the above conditions is satisfied, the third algorithm is selected.

I_MPI_SCATTER_MSG

Control MPI_Scatter algorithm thresholds.

Syntax

I_MPI_SCATTER_MSG=<nbytes>

Arguments

<nbytes>

Define the buffer size threshold range (in bytes) for choosing the MPI_Scatter algorithm

> 0

The default value is 2048

Description

Set this environment variable to control the selection of the two possible MPI_Scatter algorithms according to the following scheme (See Table 3.4-1 for algorithm descriptions):

The first algorithm is selected on the intercommunicators if the message size is greater than <nbytes>.

If the above condition is not satisfied, the second algorithm is selected.

I_MPI_GATHER_MSG

Control MPI_Gather algorithm thresholds.

Syntax

I_MPI_GATHER_MSG=<nbytes>

Arguments

<nbytes>

Define the buffer size threshold range (in bytes) for choosing the MPI_Gather algorithm

> 0

The default value is 2048

Description

Set this environment variable to control the selection of the two possible MPI_Gather algorithms according to the following scheme (See Table 3.4-1 for algorithm descriptions):

The first algorithm is selected on the intercommunicators if the message size is greater than <nbytes>.

If the above condition is not satisfied, the second algorithm is selected.