Java线程源码解析之yield和sleep
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概述
由于Thread的yield和sleep有一定的相似性,因此放在一起进行分析。yield会释放CPU资源,让优先级更高(至少是相同)的线程获得执行机会;sleep当传入参数为0时,和yield相同;当传入参数大于0时,也是释放CPU资源,当可以让其它任何优先级的线程获得执行机会;
假设当前进程只有main线程,当调用yield之后,main线程会继续运行,因为没有比它优先级更高的线程;而调用sleep之后,mian线程会进入TIMED_WAITING状态,不会继续运行;
yield
Thread.sleep底层是通过JVM_Yield方法实现的(见jvm.cpp):
JVM_ENTRY(void, JVM_Yield(JNIEnv *env, jclass threadClass)) JVMWrapper("JVM_Yield"); //检查是否设置了DontYieldALot参数,默认为fasle //如果设置为true,直接返回 if (os::dont_yield()) return; //如果ConvertYieldToSleep=true(默认为false),调用os::sleep,否则调用os::yield if (ConvertYieldToSleep) { os::sleep(thread, MinSleepInterval, false);//sleep 1ms } else { os::yield(); }JVM_END
从上面知道,实际上调用的是os::yield:
//sched_yield是linux kernel提供的API,它会使调用线程放弃CPU使用权,加入到同等优先级队列的末尾;//如果调用线程是优先级最高的唯一线程,yield方法返回后,调用线程会继续运行;//因此可以知道,对于和调用线程相同或更高优先级的线程来说,yield方法会给予了它们一次运行的机会;void os::yield() { sched_yield();}
sleep
Thread.sleep最终调用JVM_Sleep方法:
JVM_ENTRY(void, JVM_Sleep(JNIEnv* env, jclass threadClass, jlong millis)) JVMWrapper("JVM_Sleep"); if (millis < 0) {//参数校验 THROW_MSG(vmSymbols::java_lang_IllegalArgumentException(), "timeout value is negative"); } //如果线程已经中断,抛出中断异常,关于中断的实现,在另一篇文章中会讲解 if (Thread::is_interrupted (THREAD, true) && !HAS_PENDING_EXCEPTION) { THROW_MSG(vmSymbols::java_lang_InterruptedException(), "sleep interrupted"); } //设置线程状态为SLEEPING JavaThreadSleepState jtss(thread); EventThreadSleep event; if (millis == 0) { //如果设置了ConvertSleepToYield(默认为true),和yield效果相同 if (ConvertSleepToYield) { os::yield(); } else {//否则调用os::sleep方法 ThreadState old_state = thread->osthread()->get_state(); thread->osthread()->set_state(SLEEPING); os::sleep(thread, MinSleepInterval, false);//sleep 1ms thread->osthread()->set_state(old_state); } } else {//参数大于0 //保存初始状态,返回时恢复原状态 ThreadState old_state = thread->osthread()->get_state(); //osthread->thread status mapping: // NEW->NEW //RUNNABLE->RUNNABLE //BLOCKED_ON_MONITOR_ENTER->BLOCKED //IN_OBJECT_WAIT,PARKED->WAITING //SLEEPING,IN_OBJECT_WAIT_TIMED,PARKED_TIMED->TIMED_WAITING //TERMINATED->TERMINATED thread->osthread()->set_state(SLEEPING); //调用os::sleep方法,如果发生中断,抛出异常 if (os::sleep(thread, millis, true) == OS_INTRPT) { if (!HAS_PENDING_EXCEPTION) { if (event.should_commit()) { event.set_time(millis); event.commit(); } THROW_MSG(vmSymbols::java_lang_InterruptedException(), "sleep interrupted"); } } thread->osthread()->set_state(old_state);//恢复osThread状态 } if (event.should_commit()) { event.set_time(millis); event.commit(); }JVM_END
os::sleep的源码如下:
int os::sleep(Thread* thread, jlong millis, bool interruptible) { assert(thread == Thread::current(), "thread consistency check"); //线程有如下几个成员变量: //ParkEvent * _ParkEvent ; // for synchronized() //ParkEvent * _SleepEvent ; // for Thread.sleep //ParkEvent * _MutexEvent ; // for native internal Mutex/Monitor //ParkEvent * _MuxEvent ; // for low-level muxAcquire-muxRelease ParkEvent * const slp = thread->_SleepEvent ; slp->reset() ; OrderAccess::fence() ;//如果millis>0,传入interruptible=true,否则为false if (interruptible) { jlong prevtime = javaTimeNanos(); for (;;) { if (os::is_interrupted(thread, true)) {//判断是否中断 return OS_INTRPT; } jlong newtime = javaTimeNanos();//获取当前时间 //如果linux不支持monotonic lock,有可能出现newtime<prevtime if (newtime - prevtime < 0) { assert(!Linux::supports_monotonic_clock(), "time moving backwards"); } else { millis -= (newtime - prevtime) / NANOSECS_PER_MILLISEC; } if(millis <= 0) { return OS_OK; } prevtime = newtime; { assert(thread->is_Java_thread(), "sanity check"); JavaThread *jt = (JavaThread *) thread; ThreadBlockInVM tbivm(jt); OSThreadWaitState osts(jt->osthread(), false ); jt->set_suspend_equivalent(); slp->park(millis); jt->check_and_wait_while_suspended(); } } } else {//如果interruptible=false //设置osthread的状态为CONDVAR_WAIT OSThreadWaitState osts(thread->osthread(), false ); jlong prevtime = javaTimeNanos(); for (;;) { jlong newtime = javaTimeNanos(); if (newtime - prevtime < 0) { assert(!Linux::supports_monotonic_clock(), "time moving backwards"); } else { millis -= (newtime - prevtime) / NANOSECS_PER_MILLISEC; } if(millis <= 0) break ; prevtime = newtime; slp->park(millis);//底层调用pthread_cond_timedwait实现 } return OS_OK ; }}
通过阅读源码知道,原来sleep是通过pthread_cond_timedwait实现的,那么为什么不通过linux的sleep实现呢?
- pthread_cond_timedwait既可以堵塞在某个条件变量上,也可以设置超时时间;
- sleep不能及时唤醒线程,最小精度为秒;
可以看出pthread_cond_timedwait使用灵活,而且时间精度更高;
# 例子
通过strace可以查看代码的系统调用情况,建立两个类,一个调用Thread.sleep(),一个调用Thread.yield(),查看其系统调用情况:
- Thread.sleep(0)
Thread.sleep(0);System.out.println("hello");
可以看到sched_yield的系统调用 - Thread.sleep(nonzero)
Thread.sleep(1000);System.out.println("hello");
在其中并没有看到pthread_cond_timedwait的调用,其实Java的线程有可两种实现方式:
- LinuxThreads
- NPTL(Native POSIX Thread Library)
可以通过如下命令查看到底是使用哪种线程实现:// NPTL or LinuxThreads?static bool is_LinuxThreads() { return !_is_NPTL; }static bool is_NPTL() { return _is_NPTL; }
getconf GNU_LIBPTHREAD_VERSION
关于两者之间的区别,请查看wiki。由于我的机器上采用的是2,因此无法看到ppthread_cond_timedwait的调用;
ppthread_cond_timedwait采用futex(Fast Userspace muTEXes)实现,因而可以看到对futex的调用;
关于JVM是如何决定采用哪种实现方式,可以查看如下方法(os_linux.cpp):
// detecting pthread libraryvoid os::Linux::libpthread_init() { // Save glibc and pthread version strings. Note that _CS_GNU_LIBC_VERSION // and _CS_GNU_LIBPTHREAD_VERSION are supported in glibc >= 2.3.2. Use a // generic name for earlier versions. // Define macros here so we can build HotSpot on old systems.# ifndef _CS_GNU_LIBC_VERSION# define _CS_GNU_LIBC_VERSION 2# endif# ifndef _CS_GNU_LIBPTHREAD_VERSION# define _CS_GNU_LIBPTHREAD_VERSION 3# endif size_t n = confstr(_CS_GNU_LIBC_VERSION, NULL, 0); if (n > 0) { char *str = (char *)malloc(n, mtInternal); confstr(_CS_GNU_LIBC_VERSION, str, n); os::Linux::set_glibc_version(str); } else { // _CS_GNU_LIBC_VERSION is not supported, try gnu_get_libc_version() static char _gnu_libc_version[32]; jio_snprintf(_gnu_libc_version, sizeof(_gnu_libc_version), "glibc %s %s", gnu_get_libc_version(), gnu_get_libc_release()); os::Linux::set_glibc_version(_gnu_libc_version); } //系统函数confstr获取C库信息 n = confstr(_CS_GNU_LIBPTHREAD_VERSION, NULL, 0); if (n > 0) { char *str = (char *)malloc(n, mtInternal); confstr(_CS_GNU_LIBPTHREAD_VERSION, str, n); // Vanilla RH-9 (glibc 2.3.2) has a bug that confstr() always tells // us "NPTL-0.29" even we are running with LinuxThreads. Check if this // is the case. LinuxThreads has a hard limit on max number of threads. // So sysconf(_SC_THREAD_THREADS_MAX) will return a positive value. // On the other hand, NPTL does not have such a limit, sysconf() // will return -1 and errno is not changed. Check if it is really NPTL. if (strcmp(os::Linux::glibc_version(), "glibc 2.3.2") == 0 && strstr(str, "NPTL") && sysconf(_SC_THREAD_THREADS_MAX) > 0) { free(str); os::Linux::set_libpthread_version("linuxthreads"); } else { os::Linux::set_libpthread_version(str); } } else { // glibc before 2.3.2 only has LinuxThreads. os::Linux::set_libpthread_version("linuxthreads"); } if (strstr(libpthread_version(), "NPTL")) { os::Linux::set_is_NPTL(); } else { os::Linux::set_is_LinuxThreads(); } // LinuxThreads have two flavors: floating-stack mode, which allows variable // stack size; and fixed-stack mode. NPTL is always floating-stack. if (os::Linux::is_NPTL() || os::Linux::supports_variable_stack_size()) { os::Linux::set_is_floating_stack(); }}
- Thread.yield
Thread.yield();System.out.println("hello");
和Thread.sleep(0)相同;
参考资料
- Linux 线程模型的比较:LinuxThreads 和 NPTL
作者:allanYan
链接:http://www.jianshu.com/p/0964124ae822
來源:简书
著作权归作者所有。商业转载请联系作者获得授权,非商业转载请注明出处。
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