From 6d5cdbafca1e5bad685ed51504996c42b4ff44fa Mon Sep 17 00:00:00 2001
From: Andreas Huber <bitbucket@lucares.de>
Date: Fri, 30 Jul 2021 19:45:44 +0200
Subject: [PATCH] FastTime a faster alternative to System.currentTimeMillis

FastTime is 100 times faster (according to my primitive
benchmark) than System.currentTimeMillis. It is less accurate.
---
 .../main/java/org/lucares/utils/FastTime.java | 81 +++++++++++++++++++
 .../lucares/performance/db/PerformanceDb.java |  9 ++-
 2 files changed, 86 insertions(+), 4 deletions(-)
 create mode 100644 pdb-utils/src/main/java/org/lucares/utils/FastTime.java

diff --git a/pdb-utils/src/main/java/org/lucares/utils/FastTime.java b/pdb-utils/src/main/java/org/lucares/utils/FastTime.java
new file mode 100644
index 0000000..cf3b6d3
--- /dev/null
+++ b/pdb-utils/src/main/java/org/lucares/utils/FastTime.java
@@ -0,0 +1,81 @@
+package org.lucares.utils;
+
+import java.time.Duration;
+import java.util.concurrent.TimeUnit;
+
+/**
+ * Faster than {@link System#currentTimeMillis()}, but with a lower resolution.
+ */
+public class FastTime {
+    private static final class TimeUpdaterThread extends Thread {
+
+        private static final long UPDATE_INTERVAL = Duration.ofMillis(50).toMillis();
+
+        public TimeUpdaterThread() {
+            setDaemon(true);
+            setName("fast-time");
+        }
+
+        @Override
+        public void run() {
+            while (true) {
+                try {
+                    TimeUnit.MILLISECONDS.sleep(UPDATE_INTERVAL);
+                    currentTimeMillis = System.currentTimeMillis();
+                } catch (final InterruptedException e) {
+                    // interrupted -> abort
+                }
+            }
+        }
+    }
+
+    private volatile static long currentTimeMillis = System.currentTimeMillis();
+
+    static {
+        new TimeUpdaterThread().start();
+    }
+
+    public static long currentTimeMillis() {
+        return currentTimeMillis;
+    }
+
+    /**
+     * <pre>
+     * ---------------------------------------- 
+     * Benchmark:
+     * fastTime   took: 0.024366 ms -> sum = 162766570282900000
+     * timeMillis took: 2.587725 ms -> sum = 162766570283110644
+     * timeMillis / fastTime = 106.20229007633587
+     * </pre>
+     */
+
+    public static void main(final String[] args) {
+        for (int i = 0; i < 100; i++) {
+            final double f = fastTime();
+            final double t = timeMillis();
+            System.out.println("t / f = " + (t / f));
+        }
+    }
+
+    private static double fastTime() {
+        final long start = System.nanoTime();
+        long sum = 0;
+        for (int j = 0; j < 100000; j++) {
+            sum += FastTime.currentTimeMillis();
+        }
+        final double duration = (System.nanoTime() - start) / 1_000_000.0;
+        System.out.println("fastTime   took: " + duration + " ms -> sum = " + sum);
+        return duration;
+    }
+
+    private static double timeMillis() {
+        final long start = System.nanoTime();
+        long sum = 0;
+        for (int j = 0; j < 100000; j++) {
+            sum += System.currentTimeMillis();
+        }
+        final double duration = (System.nanoTime() - start) / 1_000_000.0;
+        System.out.println("timeMillis took: " + duration + " ms -> sum = " + sum);
+        return duration;
+    }
+}
diff --git a/performanceDb/src/main/java/org/lucares/performance/db/PerformanceDb.java b/performanceDb/src/main/java/org/lucares/performance/db/PerformanceDb.java
index e8aab8f..82a05cf 100644
--- a/performanceDb/src/main/java/org/lucares/performance/db/PerformanceDb.java
+++ b/performanceDb/src/main/java/org/lucares/performance/db/PerformanceDb.java
@@ -31,6 +31,7 @@ import org.lucares.pdb.datastore.WriteException;
 import org.lucares.pdb.datastore.internal.DataStore;
 import org.lucares.pdb.datastore.internal.PartitionDiskStore;
 import org.lucares.pdb.datastore.lang.SyntaxException;
+import org.lucares.utils.FastTime;
 import org.slf4j.Logger;
 import org.slf4j.LoggerFactory;
 
@@ -94,7 +95,7 @@ public class PerformanceDb implements AutoCloseable {
         long insertionsSinceLastSync = 0;
 
         try {
-            long lastSync = System.currentTimeMillis();
+            long lastSync = FastTime.currentTimeMillis();
             long nextSync = lastSync + timeBetweenSyncs.toMillis();
 
             while (true) {
@@ -116,15 +117,15 @@ public class PerformanceDb implements AutoCloseable {
                         count++;
                         insertionsSinceLastSync++;
 
-                        if (nextSync <= System.currentTimeMillis()) {
-                            final long end = System.currentTimeMillis();
+                        if (nextSync <= FastTime.currentTimeMillis()) {
+                            final long end = FastTime.currentTimeMillis();
                             final long duration = end - lastSync;
                             final long entriesPerSecond = (long) (insertionsSinceLastSync / (duration / 1000.0));
 
                             METRICS_LOGGER.debug(String.format(Locale.US, "inserting %,11d/s ; total: %,d; last: %s",
                                     entriesPerSecond, count, entry));
 
-                            lastSync = System.currentTimeMillis();
+                            lastSync = FastTime.currentTimeMillis();
                             nextSync = lastSync + timeBetweenSyncs.toMillis();
                             insertionsSinceLastSync = 0;
                         }