We are testing what happens, if the user dynamically changes core count.

[root@mwm-c12 cpu_offline]# cat a.java
import java.util.concurrent.atomic.AtomicInteger;
import java.util.concurrent.CyclicBarrier;

public class a implements Runnable {
  private static final AtomicInteger ai = new AtomicInteger(0);
  private static int i = 0;
  private static final CyclicBarrier cb = new CyclicBarrier(2, new Runnable(){
       public void run() {
         System.out.println( "CPUs now: " + Runtime.getRuntime().availableProcessors() + " i: " + a.i + " ai: " + ai.get() );
       }
     });
  public static void main( String [] args ) throws Exception {
    a a = new a();

    new Thread(a).start();
    a.run();
  }

  public void run() {
    for(int j = 0; j < 100; j++) {
      for( int k = 0; k < 10000000; k++ ) {
        synchronized(this) {
          i++;
        }
      }

      for( int k = 0; k < 10000000; k++ ) {
        ai.getAndIncrement();
      }

      try{
        cb.await();
      }catch(Exception e){
      }
    }
  }
}


[root@mwm-c12 cpu_offline]# for ((i=1; i<24; i++)); do echo /sys/devices/system/cpu/cpu${i}/online; echo 0 > /sys/devices/system/cpu/cpu${i}/online; done
[root@mwm-c12 cpu_offline]# /home/mwm_jms/JMSB/bin/jvm/jdk-mwm1/bin/java a | tee a.out & sleep 3; for ((i=1; i<24; i++)); do echo /sys/devices/system/cpu/cpu${i}/online; echo 1 > /sys/devices/system/cpu/cpu${i}/online; done


Which basically disables 23 out of 24 cores. The program then starts on just one CPU, and 3 seconds later we turn the 23 offline CPUs back online. At this moment the JVM crashes with a core dump.

CPUs now: 11 i: 420000000 ai: 420000000
CPUs now: 13 i: 440000000 ai: 440000000
#
# A fatal error has been detected by the Java Runtime Environment:
#
# SIGSEGV (0xb) at pc=0x00007f172a1fbe60, pid=30015, tid=1102403904
#
# JRE version: 7.0-b141
# Java VM: Java HotSpot(TM) 64-Bit Server VM (21.0-b11 mixed mode linux-amd64 compressed oops)
# Problematic frame:
# V [libjvm.so+0x6d5e60] ObjectMonitor::UnlinkAfterAcquire(Thread*, ObjectWaiter*)+0x50
#
# Core dump written. Default location: /home/mwm_jms/alex/cpu_offline/core or core.30015
#
# An error report file with more information is saved as:
# /home/mwm_jms/alex/cpu_offline/hs_err_pid30015.log
#
# If you would like to submit a bug report, please visit:
# http://bugreport.sun.com/bugreport/crash.jsp


Other crash points are possible:

#
# A fatal error has been detected by the Java Runtime Environment:
#
# Internal Error (synchronizer.cpp:1286), pid=4893, tid=1086261568
# guarantee(object->mark() == markOopDesc::INFLATING()) failed: invariant
#
# JRE version: 7.0-b141
# Java VM: Java HotSpot(TM) 64-Bit Server VM (21.0-b11 mixed mode linux-amd64 compressed oops)
# Core dump written. Default location: /home/mwm_jms/alex/cpu_offline/core or core.4893
#
# If you would like to submit a bug report, please visit:
# http://bugreport.sun.com/bugreport/crash.jsp
#

Core dumps are available on request. (Or tell me where to upload them - 3.9M zipped)