VMThread - VM Operations#
VM Operations#
VMThread 线程作为协调者(coordinator) ，循环监听 safepoint request 队列中的 VM_Operation 请求，并执行队列中的操作。
以下是部分 VM_Operation 的类型 ：
src/hotspot/share/runtime/vmOperation.hpp
// VM_Operation 的类型
// The following classes are used for operations
// initiated by a Java thread but that must
// take place in the VMThread.
#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(Halt) \
template(SafepointALot) \
template(Cleanup) \
template(ThreadDump) \
template(PrintThreads) \
template(FindDeadlocks) \
template(ClearICs) \
template(ForceSafepoint) \
template(DeoptimizeFrame) \
template(DeoptimizeAll) \
template(ZombieAll) \
template(Verify) \
template(HeapDumper) \
template(CollectForMetadataAllocation) \
template(CollectForCodeCacheAllocation) \
template(GC_HeapInspection) \
template(GenCollectFull) \
template(GenCollectForAllocation) \
template(ParallelGCFailedAllocation) \
template(ParallelGCSystemGC) \
template(G1CollectForAllocation) \
template(G1CollectFull) \
template(G1PauseRemark) \
template(G1PauseCleanup) \
template(G1TryInitiateConcMark) \
template(ZMarkEndOld) \
template(ZMarkEndYoung) \
template(ZMarkFlushOperation) \
template(ZMarkStartYoung) \
template(ZMarkStartYoungAndOld) \
template(ZRelocateStartOld) \
template(ZRelocateStartYoung) \
template(ZRendezvousGCThreads) \
template(ZVerifyOld) \
template(XMarkStart) \
template(XMarkEnd) \
template(XRelocateStart) \
template(XVerify) \
template(HandshakeAllThreads) \
template(PopulateDumpSharedSpace) \
template(JNIFunctionTableCopier) \
template(RedefineClasses) \
template(GetObjectMonitorUsage) \
template(GetAllStackTraces) \
template(GetThreadListStackTraces) \
template(VirtualThreadGetStackTrace) \
template(VirtualThreadGetFrameCount) \
template(ChangeBreakpoints) \
template(GetOrSetLocal) \
template(VirtualThreadGetOrSetLocal) \
template(VirtualThreadGetCurrentLocation) \
template(ChangeSingleStep) \
template(SetNotifyJvmtiEventsMode) \
template(HeapWalkOperation) \
template(HeapIterateOperation) \
template(ReportJavaOutOfMemory) \
template(JFRCheckpoint) \
template(ShenandoahFullGC) \
template(ShenandoahInitMark) \
template(ShenandoahFinalMarkStartEvac) \
template(ShenandoahInitUpdateRefs) \
template(ShenandoahFinalUpdateRefs) \
template(ShenandoahFinalRoots) \
template(ShenandoahDegeneratedGC) \
template(Exit) \
template(LinuxDllLoad) \
template(WhiteBoxOperation) \
template(JVMCIResizeCounters) \
template(ClassLoaderStatsOperation) \
template(ClassLoaderHierarchyOperation) \
template(DumpHashtable) \
template(CleanClassLoaderDataMetaspaces) \
template(PrintCompileQueue) \
template(PrintClassHierarchy) \
template(PrintClasses) \
template(ICBufferFull) \
template(PrintMetadata) \
template(GTestExecuteAtSafepoint) \
template(GTestStopSafepoint) \
template(JFROldObject) \
template(JvmtiPostObjectFree) \
template(RendezvousGCThreads)
class VM_Operation : public StackObj {
public:
enum VMOp_Type {
VM_OPS_DO(VM_OP_ENUM) // 使用了上面的 #define VM_OPS_DO(template)
VMOp_Terminating
};
private:
Thread* _calling_thread;
// The VM operation name array
static const char* _names[];
public:
VM_Operation() : _calling_thread(nullptr) {}
// Called by VM thread - does in turn invoke doit(). Do not override this
void evaluate();
// evaluate() is called by the VMThread and in turn calls doit().
// If the thread invoking VMThread::execute((VM_Operation*) is a JavaThread,
// doit_prologue() is called in that thread before transferring control to
// the VMThread.
// If doit_prologue() returns true the VM operation will proceed, and
// doit_epilogue() will be called by the JavaThread once the VM operation
// completes. If doit_prologue() returns false the VM operation is cancelled.
virtual void doit() = 0;
virtual bool doit_prologue() { return true; };
virtual void doit_epilogue() {};
// An operation can either be done inside a safepoint
// or concurrently with Java threads running.
virtual bool evaluate_at_safepoint() const { return true; }
图:VM Operations#
VM Thread#
VM Thread 全局变量#
src/hotspot/share/runtime/vmThread.cpp
VMThread* VMThread::_vm_thread = nullptr;
VM_Operation* VMThread::_cur_vm_operation = nullptr;
VM_Operation* VMThread::_next_vm_operation = &cleanup_op; // Prevent any thread from setting an operation until VM thread is ready.
全局锁：
src/hotspot/share/runtime/mutexLocker.hpp
extern Monitor* VMOperation_lock; // a lock on queue of vm_operations waiting to execute
VM Thread 监听 Safepoint Request#
src/hotspot/share/runtime/vmThread.cpp
void VMThread::loop() {
assert(_cur_vm_operation == nullptr, "no current one should be executing");
SafepointSynchronize::init(_vm_thread);
// Need to set a calling thread for ops not passed
// via the normal way.
cleanup_op.set_calling_thread(_vm_thread);
safepointALot_op.set_calling_thread(_vm_thread);
while (true) {
if (should_terminate()) break;
wait_for_operation();
if (should_terminate()) break;
assert(_next_vm_operation != nullptr, "Must have one");
inner_execute(_next_vm_operation);
}
}
可能是分配内存失败触发 GC，也可能是其它原因，Java 线程向 VM Thread 提出了进入 safepoint 的请求(VM_Operation)，请求中带上 safepoint operation 参数，参数其实是 STOP THE WORLD(STW) 后要执行的 Callback 操作 。
void VMThread::inner_execute(VM_Operation* op) {
...
if (_cur_vm_operation->evaluate_at_safepoint() &&
!SafepointSynchronize::is_at_safepoint()) {
SafepointSynchronize::begin(); // <<<----
if (has_timeout_task) {
_timeout_task->arm(_cur_vm_operation->name());
}
end_safepoint = true;
}
evaluate_operation(_cur_vm_operation); // <<<----
if (end_safepoint) {
if (has_timeout_task) {
_timeout_task->disarm();
}
SafepointSynchronize::end(); // <<<----
}
...
}
更详细可见本书的 Safepoint 一节。
VM Operations 与 Safepoints 的关系#
VMThread 会一直等待 VM Operation 出现在 VMOperationQueue 中，然后执行这些VM Operation。通常，这些操作需要虚拟机到达 Safepoint 后才能执行，因此会转给 VMThread。简单地说，当虚拟机处于 Safepoint 时，虚拟机内的所有线程都会被阻塞(blocked)，并且在 Safepoint 期间，在执行 native code 的任何线程都无法返回到虚拟机。这意味着在执行 VM operation 时，不会有线程修改 Java 堆，而且所有线程都处于这样一种状态：它们的 Java stack 是不变的，可以被 GC 线程等检查。
大家最熟悉的 VM operation 是 GC，或者更具体地说是许多 GC 算法中常见的 “Stop The World ”阶段的 GC。但除了 GC 以外，还有许多其他基于 Safepoint 的 VM operation ，例如：有偏见的锁定撤销(biased locking revocation)、thread stack dumps、thread suspension 或 thread stopping（即 java.lang.Thread.stop() 方法）以及通过 JVMTI 请求的许多观察/修改操作。
许多 VM operation 是同步的，即请求者会阻塞直到操作完成，但也有一些是异步或并发的，即请求者可以与 VMThread 并行（当然，前提是没有启动 Safepoint ）。
Safepoint 是通过一种基于轮询的合作机制发起的。简单来说，每隔一段时间就会有一个线程查询 “我是否应该在 Safepoint 阻塞 ？高效地完成这个查询并不简单。在线程状态转换过程中，就是经常查询的地方。一旦发起 Safepoint 请求，VMThread 必须等待所有线程都处于 Safepoint 安全状态后，才能继续执行 VM operation 。在 Safepoint 期间，Threads_lock 用于阻塞任何正在运行的线程，VMThread 最终会在 VM Operation 执行完毕后释放 Threads_lock。
关于 VM Operations 与 Safepoints 的关系，更详细可见本书的 Safepoint 一节。
一个应用线程 Request VM Operation#
应用线程如需要执行 VM Operation 时，就会向 VM Thread 提交 VM Operation 请求。如内存分配失败，并触发 GC 时。
src/hotspot/share/runtime/vmThread.cpp
void VMThread::execute(VM_Operation* op) {
Thread* t = Thread::current();
if (t->is_VM_thread()) {
op->set_calling_thread(t);
((VMThread*)t)->inner_execute(op);
return;
}
// Avoid re-entrant attempts to gc-a-lot
SkipGCALot sgcalot(t);
// JavaThread or WatcherThread
if (t->is_Java_thread()) {
JavaThread::cast(t)->check_for_valid_safepoint_state();
}
// New request from Java thread, evaluate prologue
if (!op->doit_prologue()) { // <<<< (prologue:序幕),是 virtual function，具体实现由 VM Operation 子类实现
return; // op was cancelled
}
op->set_calling_thread(t);
wait_until_executed(op); // <<<< 提交 VM Operation Request，并等待 VM Operation Request 执行完成
op->doit_epilogue(); // <<<< epilogue:结语，相对于上面的 prologue:序幕, 是 virtual function，具体实现由 VM Operation 子类实现
}
一个具体实验示例见本书的 GC 主要流程 - 内存分配失败触发 GC 。
void VM_Operation::set_calling_thread(Thread* thread) {
_calling_thread = thread;
}
src/hotspot/share/runtime/vmThread.cpp
void VMThread::wait_until_executed(VM_Operation* op) {
MonitorLocker ml(VMOperation_lock,
Thread::current()->is_Java_thread() ?
Mutex::_safepoint_check_flag :
Mutex::_no_safepoint_check_flag);
{
TraceTime timer("Installing VM operation", TRACETIME_LOG(Trace, vmthread));
while (true) {
if (VMThread::vm_thread()->set_next_operation(op)) { // <<<< 提交 VM Operation Request
ml.notify_all();
break;
}
// Wait to install this operation as the next operation in the VM Thread
log_trace(vmthread)("A VM operation already set, waiting");
ml.wait();
}
}
{
// Wait until the operation has been processed
TraceTime timer("Waiting for VM operation to be completed", TRACETIME_LOG(Trace, vmthread));
// _next_vm_operation is cleared holding VMOperation_lock after it has been
// executed. We wait until _next_vm_operation is not our op.
while (_next_vm_operation == op) {
// VM Thread can process it once we unlock the mutex on wait.
ml.wait();// 并等待 VM Operation Request 执行完成
}
}
}
bool VMThread::set_next_operation(VM_Operation *op) {
if (_next_vm_operation != nullptr) {
return false;
}
log_debug(vmthread)("Adding VM operation: %s", op->name());
_next_vm_operation = op;
HOTSPOT_VMOPS_REQUEST(
(char *) op->name(), strlen(op->name()),
op->evaluate_at_safepoint() ? 0 : 1); // <<<< 提交 VM Operation Request
return true;
}
本以为 vm operations 应该是以队列保存，但实现代码可以看出，是一个变量： _next_vm_operation 。即不是以队列支持缓存 VM Operations，而只支持单个，这样的设计当然是有考虑的。
void VMThread::wait_until_executed(VM_Operation* op) {
MonitorLocker ml(VMOperation_lock,
Thread::current()->is_Java_thread() ?
Mutex::_safepoint_check_flag :
Mutex::_no_safepoint_check_flag);
{
TraceTime timer("Installing VM operation", TRACETIME_LOG(Trace, vmthread));
while (true) {
if (VMThread::vm_thread()->set_next_operation(op)) {
ml.notify_all();
break;
}
// Wait to install this operation as the next operation in the VM Thread
log_trace(vmthread)("A VM operation already set, waiting");
ml.wait();
}
}
{
// Wait until the operation has been processed
TraceTime timer("Waiting for VM operation to be completed", TRACETIME_LOG(Trace, vmthread));
// _next_vm_operation is cleared holding VMOperation_lock after it has been
// executed. We wait until _next_vm_operation is not our op.
while (_next_vm_operation == op) {
// VM Thread can process it once we unlock the mutex on wait.
ml.wait();
}
}
}
本书的实验 GC 主要流程 - 内存分配失败触发 GC 中，用 gdb 输出 VMThread 和 VM Operation Request Thread(发起VM Operation 请求的应用线程) 的 call stack 可以直观说明线程的交互：
VM Operation Request Thread(发起VM Operation 请求的应用线程) 的 call stack:
libc.so.6!__futex_abstimed_wait_common64(int private, _Bool cancel, const struct timespec * abstime, int op, unsigned int expected, unsigned int * futex_word) (futex-internal.c:57)
libc.so.6!__futex_abstimed_wait_common(_Bool cancel, int private, const struct timespec * abstime, clockid_t clockid, unsigned int expected, unsigned int * futex_word) (futex-internal.c:87)
libc.so.6!__GI___futex_abstimed_wait_cancelable64(unsigned int * futex_word, unsigned int expected, clockid_t clockid, const struct timespec * abstime, int private) (futex-internal.c:139)
libc.so.6!__pthread_cond_wait_common(pthread_mutex_t * mutex, pthread_cond_t * cond) (pthread_cond_wait.c:503)
libc.so.6!___pthread_cond_wait(pthread_cond_t * cond, pthread_mutex_t * mutex) (pthread_cond_wait.c:627)
libjvm.so!PlatformMonitor::wait(PlatformMonitor * const this, uint64_t millis) (/jdk/src/hotspot/os/posix/os_posix.cpp:1900)
libjvm.so!Monitor::wait(Monitor * const this, uint64_t timeout) (/jdk/src/hotspot/share/runtime/mutex.cpp:254)
libjvm.so!MonitorLocker::wait(MonitorLocker * const this, int64_t timeout) (/jdk/src/hotspot/share/runtime/mutexLocker.hpp:255)
libjvm.so!VMThread::wait_until_executed(VM_Operation * op) (/jdk/src/hotspot/share/runtime/vmThread.cpp:377)
libjvm.so!VMThread::execute(VM_Operation * op) (/jdk/src/hotspot/share/runtime/vmThread.cpp:555)
libjvm.so!GenCollectedHeap::mem_allocate_work(GenCollectedHeap * const this, size_t size, bool is_tlab) (/jdk/src/hotspot/share/gc/shared/genCollectedHeap.cpp:355)
libjvm.so!GenCollectedHeap::mem_allocate(GenCollectedHeap * const this, size_t size, bool * gc_overhead_limit_was_exceeded) (/jdk/src/hotspot/share/gc/shared/genCollectedHeap.cpp:398)
libjvm.so!MemAllocator::mem_allocate_outside_tlab(const MemAllocator * const this, MemAllocator::Allocation & allocation) (/jdk/src/hotspot/share/gc/shared/memAllocator.cpp:240)
libjvm.so!MemAllocator::mem_allocate_slow(const MemAllocator * const this, MemAllocator::Allocation & allocation) (/jdk/src/hotspot/share/gc/shared/memAllocator.cpp:348)
libjvm.so!MemAllocator::mem_allocate(const MemAllocator * const this, MemAllocator::Allocation & allocation) (/jdk/src/hotspot/share/gc/shared/memAllocator.cpp:360)
libjvm.so!MemAllocator::allocate(const MemAllocator * const this) (/jdk/src/hotspot/share/gc/shared/memAllocator.cpp:367)
libjvm.so!CollectedHeap::array_allocate(CollectedHeap * const this, Klass * klass, size_t size, int length, bool do_zero, JavaThread * __the_thread__) (/jdk/src/hotspot/share/gc/shared/collectedHeap.inline.hpp:41)
libjvm.so!TypeArrayKlass::allocate_common(TypeArrayKlass * const this, int length, bool do_zero, JavaThread * __the_thread__) (/jdk/src/hotspot/share/oops/typeArrayKlass.cpp:93)
libjvm.so!TypeArrayKlass::allocate(TypeArrayKlass * const this, int length, JavaThread * __the_thread__) (/jdk/src/hotspot/share/oops/typeArrayKlass.hpp:68)
libjvm.so!oopFactory::new_typeArray(BasicType type, int length, JavaThread * __the_thread__) (/jdk/src/hotspot/share/memory/oopFactory.cpp:93)
VMThread 接收与处理 VM Operation Request 的 call stack:
libjvm.so!VM_GenCollectForAllocation::doit(VM_GenCollectForAllocation * const this) (/jdk/src/hotspot/share/gc/shared/gcVMOperations.cpp:199)
libjvm.so!VM_Operation::evaluate(VM_Operation * const this) (/jdk/src/hotspot/share/runtime/vmOperations.cpp:71)
libjvm.so!VMThread::evaluate_operation(VMThread * const this, VM_Operation * op) (/jdk/src/hotspot/share/runtime/vmThread.cpp:281)
libjvm.so!VMThread::inner_execute(VMThread * const this, VM_Operation * op) (/jdk/src/hotspot/share/runtime/vmThread.cpp:435)
libjvm.so!VMThread::loop(VMThread * const this) (/jdk/src/hotspot/share/runtime/vmThread.cpp:502)
libjvm.so!VMThread::run(VMThread * const this) (/jdk/src/hotspot/share/runtime/vmThread.cpp:175)
libjvm.so!Thread::call_run(Thread * const this) (/jdk/src/hotspot/share/runtime/thread.cpp:217)
libjvm.so!thread_native_entry(Thread * thread) (/jdk/src/hotspot/os/linux/os_linux.cpp:778)
libc.so.6!start_thread(void * arg) (pthread_create.c:442)
libc.so.6!clone3() (clone3.S:81)