PopularContents: Make a single copy of the graph per thread

Copying it 1k or 10k for each thread is wasteful, even though they are
lightweight copies.

This brings the runtime down from 133 to ~100 hours.

java/src/main/java/org/softwareheritage/graph/utils/PopularContents.java

@@ -37,6 +37,10 @@ import org.slf4j.LoggerFactory;
 
 public class PopularContents {
     private SwhBidirectionalGraph graph;
+    /*
+     * A copy of the graph for each thread to reuse between calls to processChunk
+     */
+    private ThreadLocal<SwhBidirectionalGraph> threadGraph;
     private int NUM_THREADS = 96;
 
     final static Logger logger = LoggerFactory.getLogger(PopularContents.class);
@@ -52,6 +56,7 @@ public class PopularContents {
         long popularityThreshold = Long.parseLong(args[2]);
 
         PopularContents popular_contents = new PopularContents();
+        popular_contents.threadGraph = new ThreadLocal<SwhBidirectionalGraph>();
 
         popular_contents.loadGraph(graphPath);
 
@@ -70,7 +75,7 @@ public class PopularContents {
         System.out.format("SWHID,length,filename,occurrences\n");
 
         long totalNodes = graph.numNodes();
-        long numChunks = NUM_THREADS * 10000;
+        long numChunks = NUM_THREADS * 1000;
 
         ProgressLogger pl = new ProgressLogger(logger);
         pl.itemsName = "nodes";
@@ -94,9 +99,13 @@ public class PopularContents {
 
     private void processChunk(long numChunks, long chunkId, int maxResults, long popularityThreshold,
             ProgressLogger pl) {
+        if (threadGraph.get() == null) {
+            threadGraph.set(this.graph.copy());
+        }
+        SwhBidirectionalGraph graph = threadGraph.get();
         long totalNodes = graph.numNodes();
         HashMap<Long, Long> names = new HashMap<>();
-        SwhUnidirectionalGraph backwardGraph = graph.getBackwardGraph().copy();
+        SwhUnidirectionalGraph backwardGraph = graph.getBackwardGraph();
 
         long chunkSize = totalNodes / numChunks;
         long chunkStart = chunkSize * chunkId;