Hi All,
I'm testing the performance of Parallel, CMS, and G1. I found that their
available heap sizes are different under the same -Xmx configuration.
For example, I set the -Xmx to 6.5GB (6656m) and
use Runtime.getRuntime().maxMemory() to obtain the real heap size.
The output is as follows.
Running with: [-Xmx6656m, -XX:+UseParallelGC, -XX:+UseParallelOldGC]
Runtime.getRuntime().maxMemory(): 6,058,496K, 5916.50MB, 5.78GB.
memoryStore: 3.29GB.
Running with: [-Xmx6656m, -XX:+UseConcMarkSweepGC]
Runtime.getRuntime().maxMemory(): 6,747,648K, 6589.50MB, 6.44GB.
memoryStore: 3.69GB.
Running with: [-Xmx6656m, -XX:+UseG1GC]
Runtime.getRuntime().maxMemory(): 6,815,744K, 6656.00MB, 6.50GB.
memoryStore: 3.72GB.
We can see that the available heap sizes are [Parallel: 5.78GB < CMS:
6.44GB < G1: 6.50GB].
My JDK version is
java version "1.8.0_121"
Java(TM) SE Runtime Environment (build 1.8.0_121-b13)
Java HotSpot(TM) 64-Bit Server VM (build 25.121-b13, mixed mode)
Any suggestions will be appreciated.
The test script is as follows.
#!/bin/bash
XMX=6656m
java -Xmx$XMX -XX:+UseParallelGC -XX:+UseParallelOldGC HeapSizeDifferences
java -Xmx$XMX -XX:+UseConcMarkSweepGC HeapSizeDifferences
java -Xmx$XMX -XX:+UseG1GC HeapSizeDifferences
The testing code is as follows.
import java.lang.management.ManagementFactory;
import java.util.ArrayList;
import java.util.Collection;
import java.util.List;
public class HeapSizeDifferences {
static Collection<Object> objects = new ArrayList<Object>();
static long lastMaxMemory = 0;
public static void main(String[] args) {
try {
List<String> inputArguments =
ManagementFactory.getRuntimeMXBean().getInputArguments();
System.out.println("Running with: " + inputArguments);
while (true) {
printMaxMemory();
consumeSpace();
}
} catch (OutOfMemoryError e) {
freeSpace();
printMaxMemory();
}
}
static void printMaxMemory() {
long currentMaxMemory = Runtime.getRuntime().maxMemory();
if (currentMaxMemory != lastMaxMemory) {
lastMaxMemory = currentMaxMemory;
System.out.format("Runtime.getRuntime().maxMemory(): %,dK, %.2fMB,
%.2fGB.%n",
currentMaxMemory / 1024, (float) currentMaxMemory /
1024 / 1024,
(float) currentMaxMemory / 1024 / 1024 / 1024);
double memoryStore = (currentMaxMemory - 300 * 1024 * 1024) * 0.6;
System.out.format("memoryStore: %.2fGB.%n", memoryStore / 1024 / 1024
/ 1024);
}
}
static void consumeSpace() {
objects.add(new int[1_000_000]);
}
static void freeSpace() {
objects.clear();
}
}
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