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) \View Code
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;
}
}
View Code
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