MPI_REDUCE()

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MPI_REDUCE()

• Philip Madron
• Eric Remington

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Basic Overview

• MPI_Reduce() simply applies an MPI operation to
  select local memory values on each process, with
  a combined result placed in a memory location on
  the target process.
• For example

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Basic Overview

• Consider a system of 3 processes, which wants to
  sum the values of its local variable int to_sum
  and place it is result at process 0.
• An abstraction of what occurs with a MPI_Reduce()
  statement is as follows

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Basic Overview

• All processes reach the MPI_Reduce() function
  call.
• Every process, including the target, sends their
  local value to be an operand in computing the
  desired result.
• For the example (with the subscript indicating
  process number) here is what we in effect
  achieve

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Basic Overview

• Here are operations that can be performed with
  MPI_Reduce()
• MPI_MAX maximum MPI_MIN minimum MPI_SUM sum
  MPI_PROD product MPI_LAND logical and MPI_BAND
  bit-wise and MPI_LOR logical or MPI_BOR
  bit-wise or MPI_LXOR logical xor MPI_BXOR
  bit-wise xor MPI_MAXLOC max value and location
  MPI_MINLOC min value and location

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Basic Overview

• Notice there are no operations such as one that
  performs a subtraction. Consider the following

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Basic Overview
Proc 0
Proc 1
Proc 2
Sum op

• For the example, all processes are sending their
  data values to be summed, then placed in Proc 0.
• As we are dealing with a parallel system, the
  scheduling of the processes is unknown, and the
  time at which they arrive to be summed is
  unknownbut as addition is commutative this is
  not important.

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Basic Overview

• If we had a subtract function it would not be
  possible to always correctly obtain the correct
  result, as subtraction is not commutative. A
  programmer would have to develop their own
  operation to account for the random process
  execution in the parallel environment.
• All operations provided for MPI_Reduce() are
  commutative, and its implementation gives
  insight into how parallel program execute.
• MPI_Reduce() can be used to perform a large
  variety of operations, where the programmer wants
  to apply an operation to a set of numbers with
  only copy of the operations result on a single
  process.