GC off-heap overhead

MapGenerator.java

package map;

import java.io.PrintStream;
import java.util.Base64;
import java.util.concurrent.ThreadLocalRandom;

public class MapGenerator {

    public static void main(String[] args) throws Exception {
        try (PrintStream out = new PrintStream("in.txt")) {
            for (int i = 0; i < 10_000_000; i++) {
                int length = ThreadLocalRandom.current().nextInt(1, 9);
                byte[] b = new byte[length];
                ThreadLocalRandom.current().nextBytes(b);
                String key = Base64.getEncoder().encodeToString(b);
                long value = ThreadLocalRandom.current().nextLong(1000000);
                out.println(key + ": " + value);
            }
        }
    }
}

MapReader.java

package map;

import java.io.BufferedReader;
import java.io.FileReader;
import java.io.IOException;
import java.util.HashMap;
import java.util.Map;

public class MapReader {

    public static void main(String[] args) throws Exception {
        while (true) {
            long startTime = System.nanoTime();
            Map<String, Long> map = readMap("in.txt");
            long time = System.nanoTime() - startTime;

            System.out.printf("Read %d elements in %.3f seconds\n", map.size(), time / 1e9);
        }
    }

    private static Map<String, Long> readMap(String fileName) throws IOException {
        Map<String, Long> map = new HashMap<>();

        try (BufferedReader br = new BufferedReader(new FileReader(fileName))) {
            for (String line; (line = br.readLine()) != null; ) {
                String[] parts = line.split(":");
                String key = parts[0].trim();
                String value = parts[1].trim();
                map.put(key, Long.parseLong(value));
            }
        }

        return map;
    }
}

Memory-G1.txt

# /usr/java/jdk-11.0.1/bin/java -XX:+UseG1GC -Xmx4g -Xms4g -Xlog:gc -XX:NativeMemoryTracking=summary map.MapReader

Native Memory Tracking:

Total: reserved=5787348KB, committed=4473360KB
-                 Java Heap (reserved=4194304KB, committed=4194304KB)
                            (mmap: reserved=4194304KB, committed=4194304KB) 
 
-                     Class (reserved=1056905KB, committed=5769KB)
                            (classes #702)
                            (  instance classes #608, array classes #94)
                            (malloc=137KB #910) 
                            (mmap: reserved=1056768KB, committed=5632KB) 
                            (  Metadata:   )
                            (    reserved=8192KB, committed=5120KB)
                            (    used=4922KB)
                            (    free=198KB)
                            (    waste=0KB =0.00%)
                            (  Class space:)
                            (    reserved=1048576KB, committed=512KB)
                            (    used=434KB)
                            (    free=78KB)
                            (    waste=0KB =0.00%)
 
-                    Thread (reserved=23698KB, committed=986KB)
                            (thread #23)
                            (stack: reserved=23592KB, committed=880KB)
                            (malloc=78KB #130) 
                            (arena=28KB #44)
 
-                      Code (reserved=247753KB, committed=7613KB)
                            (malloc=65KB #633) 
                            (mmap: reserved=247688KB, committed=7548KB) 
 
-                        GC (reserved=258849KB, committed=258849KB)
                            (malloc=70157KB #168287) 
                            (mmap: reserved=188692KB, committed=188692KB) 
 
-                  Compiler (reserved=148KB, committed=148KB)
                            (malloc=18KB #64) 
                            (arena=130KB #5)
 
-                  Internal (reserved=639KB, committed=639KB)
                            (malloc=599KB #1457) 
                            (mmap: reserved=40KB, committed=40KB) 
 
-                    Symbol (reserved=2028KB, committed=2028KB)
                            (malloc=1252KB #2752) 
                            (arena=775KB #1)
 
-    Native Memory Tracking (reserved=2758KB, committed=2758KB)
                            (malloc=7KB #91) 
                            (tracking overhead=2751KB)
 
-               Arena Chunk (reserved=186KB, committed=186KB)
                            (malloc=186KB) 
 
-                   Logging (reserved=5KB, committed=5KB)
                            (malloc=5KB #184) 
 
-                 Arguments (reserved=17KB, committed=17KB)
                            (malloc=17KB #469) 
 
-                    Module (reserved=58KB, committed=58KB)
                            (malloc=58KB #1025) 

Memory-ZGC.txt

# /usr/java/jdk-11.0.1/bin/java -XX:+UnlockExperimentalVMOptions -XX:+UseZGC -Xmx4g -Xms4g -Xlog:gc -XX:NativeMemoryTracking=summary map.MapReader

Native Memory Tracking:

Total: reserved=17180607344KB, committed=4630564KB
-                 Java Heap (reserved=17179869184KB, committed=4155392KB)
                            (mmap: reserved=17179869184KB, committed=4155392KB) 
 
-                     Class (reserved=8327KB, committed=5767KB)
                            (classes #702)
                            (  instance classes #608, array classes #94)
                            (malloc=135KB #915) 
                            (mmap: reserved=8192KB, committed=5632KB) 
                            (  Metadata:   )
                            (    reserved=8192KB, committed=5632KB)
                            (    used=5400KB)
                            (    free=232KB)
                            (    waste=0KB =0.00%)
 
-                    Thread (reserved=21600KB, committed=1312KB)
                            (thread #21)
                            (stack: reserved=21544KB, committed=1256KB)
                            (malloc=30KB #118) 
                            (arena=26KB #40)
 
-                      Code (reserved=247763KB, committed=7623KB)
                            (malloc=75KB #719) 
                            (mmap: reserved=247688KB, committed=7548KB) 
 
-                        GC (reserved=457497KB, committed=457497KB)
                            (malloc=441113KB #7152) 
                            (mmap: reserved=16384KB, committed=16384KB) 
 
-                  Compiler (reserved=148KB, committed=148KB)
                            (malloc=18KB #65) 
                            (arena=130KB #5)
 
-                  Internal (reserved=293KB, committed=293KB)
                            (malloc=253KB #955) 
                            (mmap: reserved=40KB, committed=40KB) 
 
-                    Symbol (reserved=2028KB, committed=2028KB)
                            (malloc=1252KB #2756) 
                            (arena=775KB #1)
 
-    Native Memory Tracking (reserved=238KB, committed=238KB)
                            (malloc=10KB #144) 
                            (tracking overhead=228KB)
 
-               Arena Chunk (reserved=186KB, committed=186KB)
                            (malloc=186KB) 
 
-                   Logging (reserved=5KB, committed=5KB)
                            (malloc=5KB #184) 
 
-                 Arguments (reserved=17KB, committed=17KB)
                            (malloc=17KB #466) 
 
-                    Module (reserved=58KB, committed=58KB)
                            (malloc=58KB #1025)