c++,algorithm,parallel-processing,c++14 , Algorithm for [inclusive/exclusive]_scan in parallel <algorithm> proposal N3554


Algorithm for [inclusive/exclusive]_scan in parallel proposal N3554

Question:

Tag: c++,algorithm,parallel-processing,c++14

Proposal N3554 (A Parallel Algorithms Library) for C++14, proposes (among other things), what seem to be parallel versions of the current std::partial_sum, e.g.:

template<
    class ExecutionPolicy,
    class InputIterator,
    class OutputIterator,
    class BinaryOperation>
OutputIterator inclusive_scan(
    ExecutionPolicy &&exec,
    InputIterator first,
    InputIterator last,
    OutputIterator result,
    BinaryOperation binary_op);

With the explanation

Effects: For each iterator i in [result,result + (last - first)), performs *i = prefix_sum, where prefix_sum is the result of the corresponding sum init + *iter_0 + *iter_1 + *iter_2 + ... or binary_op(init, binary_op(*iter_0, binary_op(*iter_1, binary_op(*iter_2, ...))) for every iterator iter_j in the range [first,first + (i - result) - 1) ... The order of operands of the sum is unspecified.


How could this operation be made parallel? It seems like, almost by definition, each output prefix_sum must be calculated for the immediate next one to be calculated - essentially leading to a serial operation.


Edit Many thanks to Aasmund Eldhuset for his answer. Personally, I found "Prefix Sums and Their Applications" by Guy E. Blelloch to be very useful.


Answer:

Parallel prefix sum is a classical distributed programming algorithm, which elegantly uses a reduction followed by a distribution (as illustrated in the article). The key observation is that you can compute parts of the partial sums before you know the leading terms.


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