core(lantern): Remove min task duration from lantern graphs

lighthouse-core/computed/page-dependency-graph.js

@@ -16,8 +16,9 @@ const NetworkRecords = require('./network-records.js');
 
 /** @typedef {import('../lib/dependency-graph/base-node.js').Node} Node */
 
-// Tasks smaller than 10 ms have minimal impact on simulation
-const MINIMUM_TASK_DURATION_OF_INTEREST = 10;
+// Shorter tasks have negligible impact on simulation results.
+const SIGNIFICANT_DUR_THRESHOLD_MS = 10;
+
 // TODO: video files tend to be enormous and throw off all graph traversals, move this ignore
 //    into estimation logic when we use the dependency graph for other purposes.
 const IGNORED_MIME_TYPES_REGEX = /^video/;
@@ -111,24 +112,18 @@ class PageDependencyGraph {
 
     TracingProcessor.assertHasToplevelEvents(traceOfTab.mainThreadEvents);
 
-    const minimumEvtDur = MINIMUM_TASK_DURATION_OF_INTEREST * 1000;
     while (i < traceOfTab.mainThreadEvents.length) {
       const evt = traceOfTab.mainThreadEvents[i];
+      i++;
 
       // Skip all trace events that aren't schedulable tasks with sizable duration
-      if (
-        !TracingProcessor.isScheduleableTask(evt) ||
-        !evt.dur ||
-        evt.dur < minimumEvtDur
-      ) {
-        i++;
+      if (!TracingProcessor.isScheduleableTask(evt) || !evt.dur) {
         continue;
       }
 
       // Capture all events that occurred within the task
       /** @type {Array<LH.TraceEvent>} */
       const children = [];
-      i++; // Start examining events after this one
       for (
         const endTime = evt.ts + evt.dur;
         i < traceOfTab.mainThreadEvents.length && traceOfTab.mainThreadEvents[i].ts < endTime;
@@ -308,6 +303,39 @@ class PageDependencyGraph {
         node.addDependency(rootNode);
       }
     }
+
+    // Second pass to prune the graph of short tasks.
+    for (const node of cpuNodes) {
+      if (node.event.dur >= SIGNIFICANT_DUR_THRESHOLD_MS * 1000) {
+        // Don't prune this node. The task is long so it will impact simulation.
+        continue;
+      }
+      // Prune the node if it isn't highly connected to minimize graph size. Rewiring the graph
+      // here replaces O(M + N) edges with (M * N) edges, which is fine if either  M or N is at
+      // most 1.
+      if (node.getNumberOfDependencies() === 1 || node.getNumberOfDependents() <= 1) {
+        PageDependencyGraph._pruneNode(node);
+      }
+    }
+  }
+
+  /**
+   * Removes the given node from the graph, but retains all paths between its dependencies and
+   * dependents.
+   * @param {Node} node
+   */
+  static _pruneNode(node) {
+    const dependencies = node.getDependencies();
+    const dependents = node.getDependents();
+    for (const dependency of dependencies) {
+      node.removeDependency(dependency);
+      for (const dependent of dependents) {
+        dependency.addDependent(dependent);
+      }
+    }
+    for (const dependent of dependents) {
+      node.removeDependent(dependent);
+    }
   }
 
   /**

lighthouse-core/lib/dependency-graph/base-node.js

@@ -85,6 +85,12 @@ class BaseNode {
     return this._dependents.slice();
   }
 
+  /**
+   * @return {number}
+   */
+  getNumberOfDependents() {
+    return this._dependents.length;
+  }
   /**
    * @return {Node[]}
    */

lighthouse-core/test/computed/page-dependency-graph-test.js

@@ -272,6 +272,7 @@ describe('PageDependencyGraph computed artifact:', () => {
 
       const getDependencyIds = node => node.getDependencies().map(node => node.id);
 
+      assert.equal(nodes.length, 7);
       assert.deepEqual(getDependencyIds(nodes[0]), []);
       assert.deepEqual(getDependencyIds(nodes[1]), [1, '1.200000']);
       assert.deepEqual(getDependencyIds(nodes[2]), [1]);
@@ -281,6 +282,91 @@ describe('PageDependencyGraph computed artifact:', () => {
       assert.deepEqual(getDependencyIds(nodes[6]), [4]);
     });
 
+    it('should prune short tasks', () => {
+      const request0 = createRequest(0, '0', 0);
+      const request1 = createRequest(1, '1', 100, null, NetworkRequest.TYPES.Script);
+      const request2 = createRequest(2, '2', 200, null, NetworkRequest.TYPES.XHR);
+      const request3 = createRequest(3, '3', 300, null, NetworkRequest.TYPES.Script);
+      const request4 = createRequest(4, '4', 400, null, NetworkRequest.TYPES.XHR);
+      const networkRecords = [request0, request1, request2, request3, request4];
+
+      // Long task, should be kept in the output.
+      addTaskEvents(120, 50, [
+        {name: 'EvaluateScript', data: {url: '1'}},
+        {name: 'ResourceSendRequest', data: {requestId: 2}},
+        {name: 'XHRReadyStateChange', data: {readyState: 4, url: '2'}},
+      ]);
+
+      // Short task, should be pruned, but the 4->5 relationship should be retained
+      addTaskEvents(350, 5, [
+        {name: 'EvaluateScript', data: {url: '3'}},
+        {name: 'ResourceSendRequest', data: {requestId: 4}},
+        {name: 'XHRReadyStateChange', data: {readyState: 4, url: '4'}},
+      ]);
+
+      const graph = PageDependencyGraph.createGraph(traceOfTab, networkRecords);
+      const nodes = [];
+      graph.traverse(node => nodes.push(node));
+
+      const getDependencyIds = node => node.getDependencies().map(node => node.id);
+
+      assert.equal(nodes.length, 6);
+
+      assert.deepEqual(getDependencyIds(nodes[0]), []);
+      assert.deepEqual(getDependencyIds(nodes[1]), [0]);
+      assert.deepEqual(getDependencyIds(nodes[2]), [0, '1.120000']);
+      assert.deepEqual(getDependencyIds(nodes[3]), [0]);
+      assert.deepEqual(getDependencyIds(nodes[4]), [0, 3]);
+
+      assert.equal('1.120000', nodes[5].id);
+      assert.deepEqual(getDependencyIds(nodes[5]), [1]);
+    });
+
+    it('should not prune highly-connected short tasks', () => {
+      const request0 = createRequest(0, '0', 0);
+      const request1 = {
+        ...createRequest(1, '1', 100, null, NetworkRequest.TYPES.Document),
+        documentURL: '1',
+        frameId: 'frame1',
+      };
+      const request2 = {
+        ...createRequest(1, '2', 200, null, NetworkRequest.TYPES.Script),
+        documentURL: '1',
+        frameId: 'frame1',
+      };
+      const request3 = createRequest(3, '3', 300, null, NetworkRequest.TYPES.XHR);
+      const request4 = createRequest(4, '4', 400, null, NetworkRequest.TYPES.XHR);
+      const networkRecords = [request0, request1, request2, request3, request4];
+
+      // Short task, evaluates script (2) and sends two XHRs.
+      addTaskEvents(220, 5, [
+        {name: 'EvaluateScript', data: {url: '2', frame: 'frame1'}},
+
+        {name: 'ResourceSendRequest', data: {requestId: 3}},
+        {name: 'XHRReadyStateChange', data: {readyState: 4, url: '3'}},
+
+        {name: 'ResourceSendRequest', data: {requestId: 4}},
+        {name: 'XHRReadyStateChange', data: {readyState: 4, url: '4'}},
+      ]);
+
+      const graph = PageDependencyGraph.createGraph(traceOfTab, networkRecords);
+      const nodes = [];
+      graph.traverse(node => nodes.push(node));
+
+      const getDependencyIds = node => node.getDependencies().map(node => node.id);
+
+      assert.equal(nodes.length, 6);
+
+      assert.deepEqual(getDependencyIds(nodes[0]), []);
+      assert.deepEqual(getDependencyIds(nodes[1]), [0]);
+      assert.deepEqual(getDependencyIds(nodes[2]), [0]);
+      assert.deepEqual(getDependencyIds(nodes[3]), [0, '1.220000']);
+      assert.deepEqual(getDependencyIds(nodes[4]), [0, '1.220000']);
+
+      assert.equal('1.220000', nodes[5].id);
+      assert.deepEqual(getDependencyIds(nodes[5]), [1, '1:duplicate']);
+    });
+
     it('should set isMainDocument on first document request', () => {
       const request1 = createRequest(1, '1', 0, null, NetworkRequest.TYPES.Other);
       const request2 = createRequest(2, '2', 5, null, NetworkRequest.TYPES.Document);

package.json

@@ -117,7 +117,7 @@
     "idb-keyval": "2.2.0",
     "intl-messageformat-parser": "^1.8.1",
     "isomorphic-fetch": "^2.2.1",
-    "jest": "^24.3.0",
+    "jest": "^24.9.0",
     "jsdom": "^12.2.0",
     "npm-run-posix-or-windows": "^2.0.2",
     "nyc": "^13.3.0",
@@ -134,6 +134,7 @@
     "configstore": "^3.1.1",
     "cssstyle": "1.2.1",
     "details-element-polyfill": "^2.4.0",
+    "global": "^4.4.0",
     "http-link-header": "^0.8.0",
     "inquirer": "^3.3.0",
     "intl": "^1.2.5",