JVM记录一次偏向锁撤销导致系统停顿的问题

1.现象

 某次上线公司内部系统时候，发现系统反应很慢，停顿时间过长。

2.问题排查

遇到这种情况，推测是频繁GC导致，首先是查看日志（配置参数-XX:+PrintGCApplicationStoppedTime），结果如下：

Total time for which application threads were stopped: 0.0050077 seconds, Stopping threads took: 0.0009924Total time for which application threads were stopped: 0. 0048003 seconds, Stopping threads took: 0.000722Total time for which application threads were stopped: 0. 0042130 seconds, Stopping threads took: 0.0006124Total time for which application threads were stopped: 0.0167637 seconds, Stopping threads took: 0. 0002226Total time for which appLication threads were stopped: 0.0041316 seconds, stopping threads took: 0.0003492Total time for which application threads were stopped: 0.0116966 seconds, Stopping threads took: 0.0004518Total time for which application threads were stopped: 0.0031024 seconds, Stopping threads took: 0.0002000Total time for which application threads were stopped: 0.0031730 seconds, Stopping threads took: 0. 0001507Total time for which application threads were stopped: 0.0643528 seconds, Stopping threads took: 0.0005159Total time for which application threads were stopped: 0.0051813 seconds, Stopping threads took: 0.001691Total time for which application threads were stopped: 0.0041419 seconds, Stopping threads took: 0. 0005032Total time for which application threads were stopped: 0. 0036222 seconds, Stopping threads took: 0. 0005483

可以看出这是stop the world过于频繁导致，在测试环境复现此情况，配置下面JVM参数打印出stop the world的原因。

-XX:+PrintSafepointStatistics-XX:+PrintSafepointStatisticsCount=1-XX:+SafepointTimeout-XX:SafepointTimeoutDelay=200-XX:+UnlockDiagnosticVMOptions-XX:-DisplayVMOutput-XX:+LogVMOutput-XX:LogFile=/XXXX.log

最终在日志中发现是频繁的RevokeBias（偏向锁撤销）导致的STW时间过长，从而导致系统停顿。

3.问题解决

1）为什么频繁的偏向锁撤销会导致STW时间增加呢？阅读偏向锁源码可以知道：偏向锁的撤销需要等待全局安全点（safe point）,暂停持有偏向锁的线程，检查持有偏向锁的线程状态。首先遍历当前JVM的所有线程，如果能找到偏向线程，则说明偏向的线程还存活，此时检查线程是否在执行同步代码块中的代码，如果是，则升级为轻量级锁，进行CAS竞争锁。可以看出撤销偏向锁的时候会导致stop the word。

static BiasedLocking::Condition revoke_bias(oop obj, bool allow_rebias, bool is_bulk, JavaThread* requesting_thread) {markOop mark = obj->mark();// 如果对象不是偏向锁，直接返回 NOT_BIASEDif (!mark->has_bias_pattern()) {...return BiasedLocking::NOT_BIASED;}uint age = mark->age();markOop biased_prototype = markOopDesc::biased_locking_prototype()->set_age(age);//匿名偏向模式（101）markOop unbiased_prototype = markOopDesc::prototype()->set_age(age);//无锁模式（001）...JavaThread* biased_thread = mark->biased_locker();if (biased_thread == NULL) {// 匿名偏向。if (!allow_rebias) {obj->set_mark(unbiased_prototype);// 如果不允许重偏向，则将对象的 mark word 设置为无锁模式}...return BiasedLocking::BIAS_REVOKED;}/*判断偏向线程是否还存活*/bool thread_is_alive = false;if (requesting_thread == biased_thread) {// 如果当前线程就是偏向线程thread_is_alive = true;} else {// 当前线程不是偏向线程，遍历当前 jvm 的所有线程，如果能找到，则说明偏向的线程还存活for (JavaThread* cur_thread = Threads::first(); cur_thread != NULL; cur_thread = cur_thread->next()) {if (cur_thread == biased_thread) {thread_is_alive = true;break;}}}// 如果偏向的线程已经不存活了if (!thread_is_alive) {// 如果允许重偏向，则将对象 mark word 设置为匿名偏向状态，否则设置为无锁状态if (allow_rebias) {obj->set_mark(biased_prototype);} else {obj->set_mark(unbiased_prototype);}...return BiasedLocking::BIAS_REVOKED;}/*线程还存活则遍历线程栈中所有的 lock record*/GrowableArray* cached_monitor_info = get_or_compute_monitor_info(biased_thread);BasicLock* highest_lock = NULL;for (int i = 0; i < cached_monitor_info->length(); i++) {MonitorInfo* mon_info = cached_monitor_info->at(i);// 如果能找到对应的 lock record，说明偏向所有者正在持有锁if (mon_info->owner() == obj) {.../*升级为轻量级锁，修改栈中所有关联该锁的 lock record，先处理所有锁重入的情况，轻量级锁的 displaced mark word 为 NULL，表示锁重入*/markOop mark = markOopDesc::encode((BasicLock*) NULL);highest_lock = mon_info->lock();highest_lock->set_displaced_header(mark);} else {...}}/* highest_lock 如果非空，则它是最早关联该锁的 lock record，这个 lock record 是线程彻底退出该锁的最后一个 lock record，所以要设置 lock record 的 displaced mark word 为无锁状态的 mark word，并让锁对象的 mark word 指向当前 lock record*/if (highest_lock != NULL) {highest_lock->set_displaced_header(unbiased_prototype);obj->release_set_mark(markOopDesc::encode(highest_lock));...} else {// 偏向所有者没有在持有锁...if (allow_rebias) {obj->set_mark(biased_prototype);// 设置为匿名偏向状态} else {obj->set_mark(unbiased_prototype);// 将 mark word 设置为无锁状态}}return BiasedLocking::BIAS_REVOKED;}

2）定位到问题了，此时可以有两种思路：第一种，减少竞争锁的线程数量，第二种，关闭偏向锁，JVM参数-XX:-UseBiasedLocking 。经过修改代码，系统停顿时间显著减少。

小结一下：我们需要结合自身应用并发情况，来评估偏向锁带来的收益。偏向锁主要影响重启后短时间内的负载尖刺，平滑流量场景影响不大。

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