static volatile long gc_memoryUsedAfterGC;
static volatile long gc_lastStart;
static volatile long gc_duration;
static Lock gc_lock = lock();

static void gc() {
  callOpt(javax(), 'cleanWeakMaps);
  {
    lock gc_lock;
    gc_lastStart = sysNow();
    System.gc();
    gc_duration = sysNow()-gc_lastStart;
    gc_memoryUsedAfterGC = usedMemory();
  }
  vmBus_send('gcDone);
}