What GC parameters is a JVM running with?

Question

I'm still investigating issues I have with GC tuning (see prior question), which involves lots of reading and experimentation.

Sun Java5+ JVMs attempt to automatically select the optimal GC strategy and parameters based on their environment, which is great, but I can't figure out how to query the running JVM to find out what those parameters are.

Ideally, I'd like to see what values of the various GC-related -XX options are being used, as selected automatically by the VM. If I had that, I could have a baseline to begin tweaking.

Anyone know to recover these values from a running VM?

Answer 1

-XX:+PrintCommandLineFlags Prints flags passed on command line or configured by the ergonomics (auto-sizing) features.

-XX:+PrintFlagsInitial Dumps ALL the default flags and values.

-XX:+PrintFlagsFinal Dumps ALL the flags after processing command line and ergonomics.

So I think the latter will do for you, just add it to your command line script.

Answer 2

Check out the HotSpotDiagnosticMBean

The following example will print out the value of an option as well as whether the value is DEFAULT or VM_CREATION:

import java.lang.management.ManagementFactory;

import javax.management.ObjectName;
import javax.management.openmbean.CompositeData;
import javax.management.openmbean.CompositeDataSupport;

public class HotSpotTest {

    public static void main(String [] args) throws Exception {
        printHotSpotOption("MaxHeapFreeRatio");
        printHotSpotOption("SurvivorRatio");
        printHotSpotOptions();
    }

    private static void printHotSpotOption(String option) throws Exception {
        ObjectName name = new ObjectName("com.sun.management:type=HotSpotDiagnostic");
        String operationName = "getVMOption";
        Object [] params = new Object [] {option};
        String [] signature = new String[] {String.class.getName()};
        Object result = ManagementFactory.getPlatformMBeanServer().invoke(name, operationName, params, signature);
        CompositeDataSupport data = (CompositeDataSupport) result;

        System.out.println(option);
        System.out.println("- Value: "+data.get("value"));
        System.out.println("- Origin: "+data.get("origin"));
    }

    private static void printHotSpotOptions() throws Exception {
        ObjectName name = new ObjectName("com.sun.management:type=HotSpotDiagnostic");
        String attributeName = "DiagnosticOptions";
        Object result = ManagementFactory.getPlatformMBeanServer().getAttribute(name, attributeName);
        CompositeData [] array = (CompositeData[]) result;
        for (CompositeData d : array) {
            System.out.println(d.get("name"));
            System.out.println("- Value: "+d.get("value"));
            System.out.println("- Origin: "+d.get("origin"));
        }
    }
}