Blog Post: Understanding Garbage Collector Performance

blog/modules/ROOT/attachments/GC-bench.sc

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+val liveSets = Seq(400, 800, 1600, 3200, 6400)
+val heapSizes = Seq(800, 1600, 3200, 6400, 12800)
+val lists = for (liveSet <- liveSets) yield {
+  for (heapSize <- heapSizes) yield {
+    if (liveSet >= heapSize) ("", "")
+    else {
+      println(s"Benchmarking liveSet=$liveSet heapSize=$heapSize")
+      val javaBin = os.Path(sys.env("JAVA_HOME")) / "bin"/"java"
+      val res = os.proc(javaBin, s"-Xmx${heapSize}m", "-XX:+UseZGC", "GC.java", liveSet, 10000, 5)
+        .call(check = false)
+
+      res.out.lines().collectFirst { case s"longest-gc: $n, throughput: $m" => (n, m) }
+        .getOrElse(sys.error(res.err.text()))
+    }
+  }
+}
+
+def renderTable(lists: Seq[Seq[String]]) = {
+  def printRow(x: Seq[String]) = println(x.map(s => s"| $s ").mkString)
+  printRow(Seq("live-set\\heap-size") ++ heapSizes.map(_ + " mb"))
+  for ((liveSet, list) <- liveSets.zip(lists)) printRow(Seq(liveSet + " mb") ++ list)
+}
+
+renderTable(lists.map(_.map(_._1)))
+renderTable(lists.map(_.map(_._2)))

blog/modules/ROOT/attachments/GC.java

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+public class GC {
+    public static void main(String[] args) throws Exception{
+        final long liveSetByteSize = Integer.parseInt(args[0]) * 1000000L;
+        final int benchMillis = Integer.parseInt(args[1]);
+        final int benchCount = Integer.parseInt(args[2]);
+        // 0-490 array entries per object, * 4-bytes per entry,
+        // + 20 byte array header = average 1000 bytes per entry
+        final int maxObjectSize = 490;
+        final int averageObjectSize = (maxObjectSize / 2) * 4 + 20;
+
+        final int liveSetSize = (int)(liveSetByteSize / averageObjectSize);
+
+        long maxPauseTotal = 0;
+        long throughputTotal = 0;
+
+        for(int i = 0; i < benchCount + 1; i++) {
+            int chunkSize = 256;
+            Object[] liveSet = new Object[liveSetSize];
+            for(int j = 0; j < liveSetSize; j++) liveSet[j] = new int[j % maxObjectSize];
+            System.gc();
+            long maxPause = 0;
+            long startTime = System.currentTimeMillis();
+
+            long loopCount = 0;
+            java.util.Random random = new java.util.Random(1337);
+            int liveSetIndex = 0;
+
+            while (startTime + benchMillis > System.currentTimeMillis()) {
+                if (loopCount % liveSetSize == 0) Thread.sleep(1);
+                long loopStartTime = System.currentTimeMillis();
+                liveSetIndex = random.nextInt(liveSetSize);
+                liveSet[liveSetIndex] = new int[liveSetIndex % maxObjectSize];
+                long loopTime = System.currentTimeMillis() - loopStartTime;
+                if (loopTime > maxPause) maxPause = loopTime;
+                loopCount++;
+            }
+            if (i != 0) {
+                long benchEndTime = System.currentTimeMillis();
+                long bytesPerLoop = maxObjectSize / 2 * 4 + 20;
+                throughputTotal += (long) (1.0 * loopCount * bytesPerLoop / 1000000 / (benchEndTime - startTime) * averageObjectSize);
+                maxPauseTotal += maxPause;
+            }
+
+            System.out.println(liveSet[random.nextInt(liveSet.length)]);
+        }
+
+        long maxPause = maxPauseTotal / benchCount;
+        long throughput = throughputTotal / benchCount;
+
+        System.out.println("longest-gc: " + maxPause + " ms, throughput: " + throughput + " mb/s");
+    }
+}

blog/modules/ROOT/nav.adoc

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+* xref:6-garbage-collector-perf.adoc[]
 * xref:5-executable-jars.adoc[]
 * xref:4-flaky-tests.adoc[]
 * xref:3-selective-testing.adoc[]