JVM学习 : VM_Thread

代码图片来自：https://blog.csdn.net/qq_31865983/article/details/103788358

VM_Thread 就是大家平时说的 JVM线程，只有一个实例，也就是虚拟机创建过程中只会被创建一次（C++层面），并且在虚拟机销毁的时候被销毁

具体的作用是 开启一个无限循环（while (true)）, 然后不断地从一个 VM_Operation 队列中取出 VM_Operation 并且执行，如果没有 VM_Operation 就等待一会

VM_Operation 是通过其他线程 放入到队列中的，所以类似 生产者-消费者 模式。VM_Operation 有许多种，大概有四类

从注释上看，应该都是一些堆内存的分配操作？

细分的话，具体操作如下：

#define VM_OP_ENUM(type)   VMOp_##type,
// Note: When new VM_XXX comes up, add 'XXX' to the template table.
#define VM_OPS_DO(template)                       
  template(Dummy)                                 
  template(ThreadStop)                            
  template(ThreadDump)                            
  template(PrintThreads)                          
  template(FindDeadlocks)                         
  template(ForceSafepoint)                        
  template(ForceAsyncSafepoint)                   
  template(Deoptimize)                            
  template(DeoptimizeFrame)                       
  template(DeoptimizeAll)                         
  template(ZombieAll)                             
  template(UnlinkSymbols)                         
  template(Verify)                                
  template(PrintJNI)                              
  template(HeapDumper)                            
  template(DeoptimizeTheWorld)                    
  template(CollectForMetadataAllocation)          
  template(GC_HeapInspection)                     
  template(GenCollectFull)                        
  template(GenCollectFullConcurrent)              
  template(GenCollectForAllocation)               
  template(ParallelGCFailedAllocation)            
  template(ParallelGCSystemGC)                    
  template(CGC_Operation)                         
  template(CMS_Initial_Mark)                      
  template(CMS_Final_Remark)                      
  template(G1CollectFull)                         
  template(G1CollectForAllocation)                
  template(G1IncCollectionPause)                  
  template(DestroyAllocationContext)              
  template(EnableBiasedLocking)                   
  template(RevokeBias)                            
  template(BulkRevokeBias)                        
  template(PopulateDumpSharedSpace)               
  template(JNIFunctionTableCopier)                
  template(RedefineClasses)                       
  template(GetOwnedMonitorInfo)                   
  template(GetObjectMonitorUsage)                 
  template(GetCurrentContendedMonitor)            
  template(GetStackTrace)                         
  template(GetMultipleStackTraces)                
  template(GetAllStackTraces)                     
  template(GetThreadListStackTraces)              
  template(GetFrameCount)                         
  template(GetFrameLocation)                      
  template(ChangeBreakpoints)                     
  template(GetOrSetLocal)                         
  template(GetCurrentLocation)                    
  template(EnterInterpOnlyMode)                   
  template(ChangeSingleStep)                      
  template(HeapWalkOperation)                     
  template(HeapIterateOperation)                  
  template(ReportJavaOutOfMemory)                 
  template(JFRCheckpoint)                         
  template(Exit)                                  
  template(LinuxDllLoad)                          
  template(RotateGCLog)                           
  template(WhiteBoxOperation)                     
  template(ClassLoaderStatsOperation)             

比较值得注意的是 加载Linux 的 动态链接库：

比较重要的就是GC操作:

每个线程通过 VMThread::execute 把 VM_Operation 放入队列的之前会通过doit_prologue方法做检查，检查是否能把这个 VM_Operation 放入队列。

因为VM_Operation有可能会被多个Java线程入队，但是实际上只用一个入队就够了。

有一种比较典型的情况就是GC操作可能会被多个Java线程入队，但实际只用GC一次就够了。

实际执行 VM_Operation 是在 VM_Operation::evaluate 中

具体过程伪代码：

　　Java方法投递该VM_operation

　　VMThread::execute (VM_Operation op)

　　不是 VM_Thread来执行投递方法：

　　　　if (op::doit_prologue()) {

　　　　     enqueue(op);

　　　　　 op::doit_epilogue();

　　　　}

　　如果是VM_Thread的话，可以直接执行evaluate方法

void VMThread::execute(VM_Operation* op) {
  Thread* t = Thread::current();

  if (!t->is_VM_thread()) {
    SkipGCALot sgcalot(t);    // avoid re-entrant attempts to gc-a-lot
    // JavaThread or WatcherThread
    bool concurrent = op->evaluate_concurrently();
    // only blocking VM operations need to verify the caller's safepoint state:
    if (!concurrent) {
      t->check_for_valid_safepoint_state(true);
    }

    // New request from Java thread, evaluate prologue
    if (!op->doit_prologue()) {
      return;   // op was cancelled
    }

    // Setup VM_operations for execution
    op->set_calling_thread(t, Thread::get_priority(t));

    // It does not make sense to execute the epilogue, if the VM operation object is getting
    // deallocated by the VM thread.
    bool execute_epilog = !op->is_cheap_allocated();
    assert(!concurrent || op->is_cheap_allocated(), "concurrent => cheap_allocated");

    // Get ticket number for non-concurrent VM operations
    int ticket = 0;
    if (!concurrent) {
      ticket = t->vm_operation_ticket();
    }

    // Add VM operation to list of waiting threads. We are guaranteed not to block while holding the
    // VMOperationQueue_lock, so we can block without a safepoint check. This allows vm operation requests
    // to be queued up during a safepoint synchronization.
    {
      VMOperationQueue_lock->lock_without_safepoint_check();
      bool ok = _vm_queue->add(op);
    op->set_timestamp(os::javaTimeMillis());
      VMOperationQueue_lock->notify();
      VMOperationQueue_lock->unlock();
      // VM_Operation got skipped
      if (!ok) {
        assert(concurrent, "can only skip concurrent tasks");
        if (op->is_cheap_allocated()) delete op;
        return;
      }
    }

    if (!concurrent) {
      // Wait for completion of request (non-concurrent)
      // Note: only a JavaThread triggers the safepoint check when locking
      MutexLocker mu(VMOperationRequest_lock);
      while(t->vm_operation_completed_count() < ticket) {
        VMOperationRequest_lock->wait(!t->is_Java_thread());
      }
    }

    if (execute_epilog) {
      op->doit_epilogue();
    }
  } else {
    // invoked by VM thread; usually nested VM operation
    assert(t->is_VM_thread(), "must be a VM thread");
    VM_Operation* prev_vm_operation = vm_operation();
    if (prev_vm_operation != NULL) {
      // Check the VM operation allows nested VM operation. This normally not the case, e.g., the compiler
      // does not allow nested scavenges or compiles.
      if (!prev_vm_operation->allow_nested_vm_operations()) {
        fatal(err_msg("Nested VM operation %s requested by operation %s",
                      op->name(), vm_operation()->name()));
      }
      op->set_calling_thread(prev_vm_operation->calling_thread(), prev_vm_operation->priority());
    }

    EventMark em("Executing %s VM operation: %s", prev_vm_operation ? "nested" : "", op->name());

    // Release all internal handles after operation is evaluated
    HandleMark hm(t);
    _cur_vm_operation = op;

    if (op->evaluate_at_safepoint() && !SafepointSynchronize::is_at_safepoint()) {
      SafepointSynchronize::begin();
      op->evaluate();
      SafepointSynchronize::end();
    } else {
      op->evaluate();
    }

    // Free memory if needed
    if (op->is_cheap_allocated()) delete op;

    _cur_vm_operation = prev_vm_operation;
  }
}

进入队列之后的 op 们，会在 VM_Thread run方法 的 loop 循环中被取出执行，当然，这个队列被取出时要加锁 MutexLockerEx（具体通过pthread_cond_wait等pthread库函数实现）

run方法是在新的操作系统线程中执行的。

具体在Threads::create_vm中体现：

　　　　　　

具体os是怎么开始run方法的？

　　起始是有一个 叫 java_start 的方法，调用了 run 方法

　　然后 os_start 调用 pthread_start 创建一个操作系统层面的线程去执行 java_start