lonestar/tutorial_examples/GraphTraversalPushOperator.cpp

/*
 * This file belongs to the Galois project, a C++ library for exploiting
 * parallelism. The code is being released under the terms of the 3-Clause BSD
 * License (a copy is located in LICENSE.txt at the top-level directory).
 *
 * Copyright (C) 2018, The University of Texas at Austin. All rights reserved.
 * UNIVERSITY EXPRESSLY DISCLAIMS ANY AND ALL WARRANTIES CONCERNING THIS
 * SOFTWARE AND DOCUMENTATION, INCLUDING ANY WARRANTIES OF MERCHANTABILITY,
 * FITNESS FOR ANY PARTICULAR PURPOSE, NON-INFRINGEMENT AND WARRANTIES OF
 * PERFORMANCE, AND ANY WARRANTY THAT MIGHT OTHERWISE ARISE FROM COURSE OF
 * DEALING OR USAGE OF TRADE.  NO WARRANTY IS EITHER EXPRESS OR IMPLIED WITH
 * RESPECT TO THE USE OF THE SOFTWARE OR DOCUMENTATION. Under no circumstances
 * shall University be liable for incidental, special, indirect, direct or
 * consequential damages or loss of profits, interruption of business, or
 * related expenses which may arise from use of Software or Documentation,
 * including but not limited to those resulting from defects in Software and/or
 * Documentation, or loss or inaccuracy of data of any kind.
 */

// This example shows
// 0. reading in a graph from a file
// 1. serial iteration over nodes
// 2. for_each iteration over nodes
// 3. access to node and edge data
// 4. usage of galois::StatTimer
// 5. how to change # of threads
// 6. push-style operator using atomic intrinsics in do_all
// 7. push-style operator using atomic intrinsics in for_each w/o conflict
// detection
#include "galois/Galois.h"
#include "galois/graphs/LCGraph.h"
#include "galois/Timer.h"
#include <iostream>

using Graph = galois::graphs::LC_CSR_Graph<int, int>;
using GNode = Graph::GraphNode;

void initialize(Graph& g) {
  galois::do_all(galois::iterate(g.begin(), g.end()), // range
                 [&](GNode n) { g.getData(n) = 0; }   // operator
  );
};

int main(int argc, char* argv[]) {
  galois::SharedMemSys G;

  if (argc < 3) {
    std::cerr << "Usage: " << argv[0] << " filename num_threads" << std::endl;
    return 1;
  }

  Graph g;
  galois::graphs::readGraph(g, argv[1]); // argv[1] is the file name for graph
  galois::setActiveThreads(std::atoi(argv[2])); // argv[2] is # of threads

  //******************************************************
  // serial traversal over a graph
  // sum over nodes and edges in C++11 syntax
  galois::StatTimer T("sum_serial");
  T.start();
  for (auto n : g) {
    auto& sum = g.getData(n);
    sum       = 0;
    for (auto e : g.edges(n)) {
      sum += g.getEdgeData(e);
    }
  }
  T.stop();

  //******************************************************
  // parallel traversal over a graph using galois::for_each
  // 1. push operator is specified using lambda expression
  // 2. for_each is named "sum_in_for_each_with_push_operator" to show stat
  // after this program finishes
  initialize(g);
  galois::for_each(
      galois::iterate(g.begin(), g.end()), // range
      [&](GNode n, auto&) {                // operator
        for (auto e : g.edges(n)) {        // cautious point
          auto dst = g.getEdgeDst(e);
          g.getData(dst) += g.getEdgeData(e);
        }
      },
      galois::loopname("sum_in_for_each_with_push_operator") // options
  );

  // define lambda expression as a varible for reuse
  auto sumEdgeWeightsAtomically = [&](GNode n) {
    for (auto e : g.edges(n)) {
      auto dst        = g.getEdgeDst(e);
      auto& dstData   = g.getData(dst);
      auto edgeWeight = g.getEdgeData(e);
      __sync_fetch_and_add(&dstData, edgeWeight);
    }
  };

  //******************************************************
  // parallel traversal over a graph using galois::do_all w/o work stealing
  // 1. push operator uses atomic intrinsic
  // 2. do_all is named "sum_in_do_all_with_push_atomic" to show stat after this
  // program finishes
  initialize(g);
  galois::do_all(galois::iterate(g.begin(), g.end()), // range
                 sumEdgeWeightsAtomically             // operator
                 ,
                 galois::loopname("sum_in_do_all_with_push_atomic") // options
  );

  //******************************************************
  // parallel traversal over a graph using galois::for_each
  // 1. push operator uses atomic intrinsic
  // 2. for_each is named "sum_in_do_for_each_with_push_atomic" to show stat
  // after this program finishes
  initialize(g);
  galois::for_each(
      galois::iterate(g.begin(), g.end()),                 // range
      [&](GNode n, auto&) { sumEdgeWeightsAtomically(n); } // operator
      ,
      galois::loopname("sum_in_for_each_with_push_atomic") // options
      ,
      galois::no_pushes(), galois::no_conflicts());

  return 0;
}