From b0cd4ea0fb1cd713c759989687697fe2b534777b Mon Sep 17 00:00:00 2001
From: Luke Seelenbinder <me@lukeseelenbinder.com>
Date: Thu, 28 Mar 2024 19:17:53 +0200
Subject: [PATCH] Use a queue to split graph builder work (instead of splitting
 the iterator).

---
 src/mjolnir/graphbuilder.cc | 50 +++++++++++++++++++++----------------
 1 file changed, 29 insertions(+), 21 deletions(-)

diff --git a/src/mjolnir/graphbuilder.cc b/src/mjolnir/graphbuilder.cc
index 72ab336101..ef23c33527 100644
--- a/src/mjolnir/graphbuilder.cc
+++ b/src/mjolnir/graphbuilder.cc
@@ -390,8 +390,8 @@ void BuildTileSet(const std::string& ways_file,
                   const std::string& complex_restriction_to_file,
                   const std::string& tile_dir,
                   const OSMData& osmdata,
-                  std::map<GraphId, size_t>::const_iterator tile_start,
-                  std::map<GraphId, size_t>::const_iterator tile_end,
+                  std::queue<std::pair<GraphId, size_t>>& tiles,
+                  std::mutex& tiles_lock,
                   const uint32_t tile_creation_date,
                   const boost::property_tree::ptree& pt,
                   std::promise<DataQuality>& result) {
@@ -453,10 +453,21 @@ void BuildTileSet(const std::string& ways_file,
   std::map<std::pair<uint8_t, uint8_t>, uint32_t> langMap;
   ////////////////////////////////////////////////////////////////////////////
   // Iterate over tiles
-  for (; tile_start != tile_end; ++tile_start) {
+  while (true) {
+    tiles_lock.lock();
+    if (tiles.empty()) {
+      // all work done
+      tiles_lock.unlock();
+      break;
+    }
+
+    auto tile = tiles.front();
+    tiles.pop();
+    tiles_lock.unlock();
+
     try {
       // What actually writes the tile
-      GraphId tile_id = tile_start->first.Tile_Base();
+      GraphId tile_id = tile.first.Tile_Base();
       GraphTileBuilder graphtile(tile_dir, tile_id, false);
 
       // Information about tile creation
@@ -496,12 +507,12 @@ void BuildTileSet(const std::string& ways_file,
 
       ////////////////////////////////////////////////////////////////////////
       // Iterate over nodes in the tile
-      auto node_itr = nodes[tile_start->second];
+      auto node_itr = nodes[tile.second];
       // to avoid realloc we guess how many edges there might be in a given tile
       geo_attribute_cache.clear();
-      geo_attribute_cache.reserve(5 * (std::next(tile_start) == tile_end
-                                           ? nodes.end() - node_itr
-                                           : std::next(tile_start)->second - tile_start->second));
+      // geo_attribute_cache.reserve(5 * (std::next(tile_start) == tile_end
+      //                                      ? nodes.end() - node_itr
+      //                                      : std::next(tile_start)->second - tile_start->second));
 
       while (node_itr != nodes.end() && (*node_itr).graph_id.Tile_Base() == tile_id) {
         // amalgamate all the node duplicates into one and the edges that connect to it
@@ -509,7 +520,7 @@ void BuildTileSet(const std::string& ways_file,
         auto bundle = collect_node_edges(node_itr, nodes, edges);
 
         // Make sure node has edges
-        if (bundle.node_edges.size() == 0) {
+        if (bundle.node_edges.empty()) {
           LOG_ERROR("Node has no edges - skip");
           continue;
         }
@@ -1251,7 +1262,7 @@ void BuildTileSet(const std::string& ways_file,
     catch (std::exception& e) {
       // ..gets sent back to the main thread
       result.set_exception(std::current_exception());
-      LOG_ERROR((boost::format("Failed tile %1%: %2%") % tile_start->first % e.what()).str());
+      LOG_ERROR((boost::format("Failed tile %1%: %2%") % tile.first % e.what()).str());
       return;
     }
   }
@@ -1295,25 +1306,22 @@ void BuildLocalTiles(const unsigned int thread_count,
   std::vector<std::promise<DataQuality>> results(threads.size());
 
   // Divvy up the work
-  size_t floor = tiles.size() / threads.size();
-  size_t at_ceiling = tiles.size() - (threads.size() * floor);
-  std::map<GraphId, size_t>::const_iterator tile_start, tile_end = tiles.begin();
+  std::queue<std::pair<GraphId, size_t>> tile_queue;
+  std::mutex tile_lock;
+  for (auto id : tiles) {
+    tile_queue.push(id);
+  }
 
   // Atomically pass around stats info
   for (size_t i = 0; i < threads.size(); ++i) {
-    // Figure out how many this thread will work on (either ceiling or floor)
-    size_t tile_count = (i < at_ceiling ? floor + 1 : floor);
-    // Where the range begins
-    tile_start = tile_end;
-    // Where the range ends
-    std::advance(tile_end, tile_count);
     // Make the thread
     threads[i].reset(new std::thread(BuildTileSet, std::cref(ways_file), std::cref(way_nodes_file),
                                      std::cref(nodes_file), std::cref(edges_file),
                                      std::cref(complex_from_restriction_file),
                                      std::cref(complex_to_restriction_file), std::cref(tile_dir),
-                                     std::cref(osmdata), tile_start, tile_end, tile_creation_date,
-                                     std::cref(pt.get_child("mjolnir")), std::ref(results[i])));
+                                     std::cref(osmdata), std::ref(tile_queue), std::ref(tile_lock),
+                                     tile_creation_date, std::cref(pt.get_child("mjolnir")),
+                                     std::ref(results[i])));
   }
 
   // Join all the threads to wait for them to finish up their work