IntelĀ® MPI Library Reference Manual for Linux* OS
These environment variables are deprecated and supported mostly for backward compatibility. Use the I_MPI_ADJUST environment variable family whenever possible.
Control the default library behavior during selection of the most appropriate collective algorithm.
I_MPI_FAST_COLLECTIVES=<arg>
<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 |
The IntelĀ® MPI Library uses advanced collective algorithms designed for better application performance by default. The implementation makes the following assumptions:
It is safe to utilize the flexibility of the MPI standard regarding the order of execution of the collective operations to take advantage of the process layout and other opportunities.
There is enough memory available for allocating additional internal buffers.
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.
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.
Control MPI_Bcast algorithm thresholds.
I_MPI_BCAST_NUM_PROCS=<nproc>
<nproc> |
Define the number of processes threshold for choosing the MPI_Bcast algorithm |
> 0 |
The default value is 8 |
Control MPI_Bcast algorithm thresholds.
I_MPI_BCAST_MSG=<nbytes1,nbytes2>
<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 |
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.
Control MPI_Alltoall algorithm thresholds.
I_MPI_ALLTOALL_NUM_PROCS=<nproc>
<nproc> |
Define the number of processes threshold for choosing the MPI_Alltoall algorithm |
> 0 |
The default value is 8 |
Control MPI_Alltoall algorithm thresholds.
I_MPI_ALLTOALL_MSG=<nbytes1,nbytes2>
<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 |
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.
Control MPI_Allgather algorithm thresholds.
I_MPI_ALLGATHER_MSG=<nbytes1,nbytes2>
<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 |
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.
Control MPI_Allreduce algorithm thresholds.
I_MPI_ALLREDUCE_MSG=<nbytes>
<nbytes> |
Define the message size threshold (in bytes) for choosing the MPI_Allreduce algorithm |
> 0 |
The default value is 2048 |
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.
Control the MPI_Reduce_scatter algorithm thresholds.
I_MPI_REDSCAT_MSG=<nbytes1,nbytes2>
<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 |
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.
Control MPI_Scatter algorithm thresholds.
<nbytes> |
Define the buffer size threshold range (in bytes) for choosing the MPI_Scatter algorithm |
> 0 |
The default value is 2048 |
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.
Control MPI_Gather algorithm thresholds.
I_MPI_GATHER_MSG=<nbytes>
<nbytes> |
Define the buffer size threshold range (in bytes) for choosing the MPI_Gather algorithm |
> 0 |
The default value is 2048 |
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.