backport #3962

benches/src/util.js

@@ -19,6 +19,12 @@ export function afterFrameAsync() {
 	return promise;
 }
 
+const majorTask = () =>
+	new Promise(resolve => {
+		window.addEventListener('message', resolve, { once: true });
+		window.postMessage('major task delay', '*');
+	});
+
 let count = 0;
 const channel = new MessageChannel();
 const callbacks = new Map();
@@ -64,10 +70,20 @@ export async function mutateAndLayout(mutation, times = 1) {
 	await forceLayout();
 }
 
-export function measureMemory() {
+export async function measureMemory() {
 	if ('gc' in window && 'memory' in performance) {
 		// Report results in MBs
+		performance.mark('gc-start');
 		window.gc();
+		performance.measure('gc', 'gc-start');
+
+		// window.gc synchronously triggers one Major GC. However that MajorGC
+		// asynchronously triggers additional MajorGCs until the
+		// usedJSHeapSizeBefore and usedJSHeapSizeAfter are the same. Here, we'll
+		// wait a moment for some (hopefully all) additional GCs to finish before
+		// measuring the memory.
+		await majorTask();
+		performance.mark('measure-memory');
 		window.usedJSHeapSize = performance.memory.usedJSHeapSize / 1e6;
 	} else {
 		window.usedJSHeapSize = 0;