本文主要是介绍《ACE程序员指南》中领导者与跟随者模式示例代码中的bug和解决方案,希望对大家解决编程问题提供一定的参考价值,需要的开发者们随着小编来一起学习吧!
《ACE程序员指南》中线程池一章中提到了两个模式:
半同步半异步模式和领导者和跟随者模式,对于后者
书中给出了一个示例程序,为了方便说明,我把示例代码贴在这里:
#include "ace/config-lite.h"
#if defined (ACE_HAS_THREADS)
#include "ace/OS_NS_string.h"
#include "ace/OS_NS_sys_time.h"
#include "ace/Task.h"
#include "ace/Containers.h"
#include "ace/Synch.h"
// Listing 4 code/ch16
class Follower
{
public:
Follower (ACE_Thread_Mutex &leader_lock)
: cond_(leader_lock)
{
owner_ = ACE_Thread::self ();
}
//FUZZ: disable check_for_lack_ACE_OS
int wait (void)
{
return this->cond_.wait ();
}
int signal (void)
{
return this->cond_.signal ();
}
//FUZZ: enable check_for_lack_ACE_OS
ACE_thread_t owner (void)
{
return this->owner_;
}
private:
ACE_Condition<ACE_Thread_Mutex> cond_;
ACE_thread_t owner_;
};
// Listing 4
// Listing 1 code/ch16
class LF_ThreadPool : public ACE_Task<ACE_MT_SYNCH>
{
public:
LF_ThreadPool () : shutdown_(0), current_leader_(0)
{
ACE_TRACE (ACE_TEXT ("LF_ThreadPool::TP"));
}
virtual int svc (void);
void shut_down (void)
{
shutdown_ = 1;
}
private:
int become_leader (void);
Follower *make_follower (void);
int elect_new_leader (void);
int leader_active (void)
{
ACE_TRACE (ACE_TEXT ("LF_ThreadPool::leader_active"));
return this->current_leader_ != 0;
}
void leader_active (ACE_thread_t leader)
{
ACE_TRACE (ACE_TEXT ("LF_ThreadPool::leader_active"));
this->current_leader_ = leader;
}
void process_message (ACE_Message_Block *mb);
int done (void)
{
return (shutdown_ == 1);
}
private:
int shutdown_;
ACE_thread_t current_leader_;
ACE_Thread_Mutex leader_lock_;
ACE_Unbounded_Queue<Follower*> followers_;
ACE_Thread_Mutex followers_lock_;
static long LONG_TIME;
};
// Listing 1
// Listing 2 code/ch16
int
LF_ThreadPool::svc (void)
{
ACE_TRACE (ACE_TEXT ("LF_ThreadPool::svc"));
while (!done ())
{
become_leader (); // Block until this thread is the leader.
ACE_Message_Block *mb = 0;
ACE_Time_Value tv (LONG_TIME);
tv += ACE_OS::gettimeofday ();
// Get a message, elect new leader, then process message.
if (this->getq (mb, &tv) < 0)
{
if (elect_new_leader () == 0)
break;
continue;
}
elect_new_leader ();
process_message (mb);
}
return 0;
}
// Listing 2
// Listing 3 code/ch16
int
LF_ThreadPool::become_leader (void)
{
ACE_TRACE (ACE_TEXT ("LF_ThreadPool::become_leader"));
ACE_GUARD_RETURN
(ACE_Thread_Mutex, leader_mon, this->leader_lock_, -1);
if (leader_active ())
{
Follower *fw = make_follower ();
{
// Wait until told to do so.
while (leader_active ())
fw->wait ();
}
delete fw;
}
ACE_DEBUG ((LM_DEBUG, ACE_TEXT ("(%t) Becoming the leader/n")));
// Mark yourself as the active leader.
leader_active (ACE_Thread::self ());
return 0;
}
Follower*
LF_ThreadPool::make_follower (void)
{
ACE_TRACE (ACE_TEXT ("LF_ThreadPool::make_follower"));
ACE_GUARD_RETURN
(ACE_Thread_Mutex, follower_mon, this->followers_lock_, 0);
Follower *fw;
ACE_NEW_RETURN (fw, Follower (this->leader_lock_), 0);
this->followers_.enqueue_tail (fw);
return fw;
}
// Listing 3
// Listing 5 code/ch16
int
LF_ThreadPool::elect_new_leader (void)
{
ACE_TRACE (ACE_TEXT ("LF_ThreadPool::elect_new_leader"));
ACE_GUARD_RETURN
(ACE_Thread_Mutex, leader_mon, this->leader_lock_, -1);
leader_active (0);
// Wake up a follower
if (!followers_.is_empty ())
{
ACE_GUARD_RETURN (ACE_Thread_Mutex,
follower_mon,
this->followers_lock_,
-1);
// Get the old follower.
Follower *fw;
if (this->followers_.dequeue_head (fw) != 0)
return -1;
ACE_DEBUG ((LM_DEBUG,
ACE_TEXT ("(%t) Resigning and Electing %d/n"),
fw->owner ()));
return (fw->signal () == 0) ? 0 : -1;
}
else
{
ACE_DEBUG
((LM_ERROR, ACE_TEXT ("(%t) Oops no followers left/n")));
return -1;
}
}
// Listing 5
void
LF_ThreadPool::process_message (ACE_Message_Block *mb)
{
ACE_TRACE (ACE_TEXT ("LF_ThreadPool::process_message"));
int msgId;
ACE_OS::memcpy (&msgId, mb->rd_ptr (), sizeof(int));
mb->release ();
ACE_DEBUG ((LM_DEBUG,
ACE_TEXT ("(%t) Started processing message:%d/n"),
msgId));
ACE_OS::sleep (1);
ACE_DEBUG ((LM_DEBUG,
ACE_TEXT ("(%t) Finished processing message:%d/n"),
msgId));
}
long LF_ThreadPool::LONG_TIME = 5L;
int ACE_TMAIN (int, ACE_TCHAR *[])
{
LF_ThreadPool tp;
tp.activate (THR_NEW_LWP| THR_JOINABLE, 5);
// Wait for a few seconds...
ACE_OS::sleep (2);
ACE_Time_Value tv (1L);
ACE_Message_Block *mb;
for (int i = 0; i < 30; i++)
{
ACE_NEW_RETURN (mb, ACE_Message_Block (sizeof(int)), -1);
ACE_OS::memcpy (mb->wr_ptr (), &i, sizeof(int));
ACE_OS::sleep (tv);
// Add a new work item.
tp.putq (mb);
}
ACE_Thread_Manager::instance ()->wait ();
ACE_OS::sleep (10);
return 0;
}
#else
#include "ace/OS_main.h"
#include "ace/OS_NS_stdio.h"
int ACE_TMAIN (int, ACE_TCHAR *[])
{
ACE_OS::puts (ACE_TEXT ("This example requires threads."));
return 0;
}
#endif /* ACE_HAS_THREADS */
我简单解释一下程序的思路:首先创建一个线程池,这个线程池中包括5个线程,每个线程都试图成为领导者,
但领导者只能有一个,因此一旦某个线程竞争成为领导者,其他线程只能把自己定位为跟随者,
并把与之相应的跟随者对象放在一个跟随者队列中,然后等待机会成为领导者,等待时跟随者线程处于挂起状态。
那个竞争中成为领导者的线程,试图从消息队列中抓起消息,如果消息队列中没有消息,该线程也会处于挂起状态。
一旦客户把一个消息放入队列(相当于添加了一个任务),领导者线程会被激活,并从消息队列中获取一条消息,
然后它做的第一件事情是找一个新领导者,寻找算法就是简单的从跟随者队列的头部取出跟随者,
指定它为新领导者,并激活该跟随者线程,当完成这些事情后,再去处理从队列中获取的消息,去执行相应的任务。
等它完成任务处理后,它再去努力成为领导者,如果发现目前已经有领导者,只好把自己定位为跟随者,并把与之相
对应的跟随者对象放入跟随者队列,等待机会成为领导者。
我在Windows平台上测试上述代码,确定是有问题的,最开始处理几条消息,领导者和跟随者的切换还是对的,
但到后来,有几个线程就永远也无法成为领导者了,最后这有两个线程在轮流坐庄,这个显然不是我们期望的结果。
我仔细的研究了一下线程运行的中间过程,找到了问题的原因。
elect_new_leader 函数在选择完跟随者线程作为新领导者后,并没有在函数中直接修改current_leader_变量为
新领导者的值,而是把该变量设置为0,修改current_leader_变量为新领导者的值事情延迟到新领导者线程被激活后
再去在该线程中执行,这样中间就有一个时间差,这就给了老领导者一个机会偷窃领导者的位置。
假设老领者已经完成了任务,这时新领导者的线程刚刚被操作系统激活,还没有来得及把current_leader_标记为
自己的Threadid,老领导者发现领导者的位置还没有人,直接把自己设置为领导者,而新领导者本来作为正统的被
选出来的领导者,在做设置领导者前,发现位置上已经有领导了,只好又继续等待,但因为elect_new_leader 函数
已经把它的跟随者对象从队列中移除,这样它永远没有机会成为领导者,而只能一直挂起等待了。
找到问题的原因后,修改bug并不复杂,只要在elect_new_leader 函数中把current_leader_改成新领导者的线程id
就不会出问题了,修改后的代码如下:
#include "ace/config-lite.h"
#if defined (ACE_HAS_THREADS)
#include "ace/OS_NS_string.h"
#include "ace/OS_NS_sys_time.h"
#include "ace/Task.h"
#include "ace/Containers.h"
#include "ace/Synch.h"
#include <sstream>
using namespace std;
// Listing 4 code/ch16
class Follower
{
public:
Follower (ACE_Thread_Mutex &leader_lock)
: cond_(leader_lock)
{
owner_ = ACE_Thread::self ();
}
//FUZZ: disable check_for_lack_ACE_OS
int wait (void)
{
return this->cond_.wait ();
}
int signal (void)
{
return this->cond_.signal ();
}
//FUZZ: enable check_for_lack_ACE_OS
ACE_thread_t owner (void)
{
return this->owner_;
}
private:
ACE_Condition<ACE_Thread_Mutex> cond_;
ACE_thread_t owner_;
};
// Listing 4
// Listing 1 code/ch16
class LF_ThreadPool : public ACE_Task<ACE_MT_SYNCH>
{
public:
LF_ThreadPool () : shutdown_(0), current_leader_(0)
{
ACE_TRACE (ACE_TEXT ("LF_ThreadPool::TP"));
}
virtual int svc (void);
void shut_down (void)
{
shutdown_ = 1;
}
private:
int become_leader (void);
Follower *make_follower (void);
int elect_new_leader (void);
int leader_active (void)
{
ACE_TRACE (ACE_TEXT ("LF_ThreadPool::leader_active"));
return this->current_leader_ != 0;
}
void leader_active (ACE_thread_t leader)
{
ACE_TRACE (ACE_TEXT ("LF_ThreadPool::leader_active"));
this->current_leader_ = leader;
}
void process_message (ACE_Message_Block *mb);
int done (void)
{
return (shutdown_ == 1);
}
private:
int shutdown_;
ACE_thread_t current_leader_;
ACE_Thread_Mutex leader_lock_;
ACE_Unbounded_Queue<Follower*> followers_;
ACE_Thread_Mutex followers_lock_;
static long LONG_TIME;
};
// Listing 1
// Listing 2 code/ch16
int
LF_ThreadPool::svc (void)
{
ACE_TRACE (ACE_TEXT ("LF_ThreadPool::svc"));
while (!done ())
{
become_leader (); // Block until this thread is the leader.
ACE_Message_Block *mb = 0;
ACE_Time_Value tv (LONG_TIME);
tv += ACE_OS::gettimeofday ();
// Get a message, elect new leader, then process message.
if (this->getq (mb, &tv) < 0)
{
elect_new_leader ();
break;
}
elect_new_leader ();
process_message (mb);
}
return 0;
}
// Listing 2
// Listing 3 code/ch16
int
LF_ThreadPool::become_leader (void)
{
ACE_TRACE (ACE_TEXT ("LF_ThreadPool::become_leader"));
ACE_GUARD_RETURN
(ACE_Thread_Mutex, leader_mon, this->leader_lock_, -1);
if (leader_active ())
{
Follower *fw = make_follower ();
{
// Wait until told to do so.
while (leader_active ())
{
fw->wait ();
ACE_DEBUG ((LM_DEBUG, ACE_TEXT ("(%t) thread activate!/n")));
break;
}
}
delete fw;
}
ACE_DEBUG ((LM_DEBUG, ACE_TEXT ("(%t) Becoming the leader/n")));
// Mark yourself as the active leader.
leader_active (ACE_Thread::self ());
return 0;
}
Follower*
LF_ThreadPool::make_follower (void)
{
ACE_TRACE (ACE_TEXT ("LF_ThreadPool::make_follower"));
ACE_GUARD_RETURN
(ACE_Thread_Mutex, follower_mon, this->followers_lock_, 0);
Follower *fw;
ACE_NEW_RETURN (fw, Follower (this->leader_lock_), 0);
this->followers_.enqueue_tail (fw);
ACE_DEBUG ((LM_DEBUG, ACE_TEXT ("(%t) thread follower enter queue/n")));
return fw;
}
// Listing 3
// Listing 5 code/ch16
int
LF_ThreadPool::elect_new_leader (void)
{
ACE_TRACE (ACE_TEXT ("LF_ThreadPool::elect_new_leader"));
ACE_GUARD_RETURN
(ACE_Thread_Mutex, leader_mon, this->leader_lock_, -1);
leader_active (0);
// Wake up a follower
if (!followers_.is_empty ())
{
ACE_GUARD_RETURN (ACE_Thread_Mutex,
follower_mon,
this->followers_lock_,
-1);
// Get the old follower.
Follower *fw;
if (this->followers_.dequeue_head (fw) != 0)
return -1;
leader_active (fw->owner());
ACE_DEBUG ((LM_DEBUG,
ACE_TEXT ("(%t) Resigning and Electing %d/n"),
fw->owner ()));
return (fw->signal () == 0) ? 0 : -1;
}
else
{
ACE_DEBUG
((LM_ERROR, ACE_TEXT ("(%t) Oops no followers left/n")));
return -1;
}
}
// Listing 5
void
LF_ThreadPool::process_message (ACE_Message_Block *mb)
{
ACE_TRACE (ACE_TEXT ("LF_ThreadPool::process_message"));
int msgId;
ACE_OS::memcpy (&msgId, mb->rd_ptr (), sizeof(int));
mb->release ();
ACE_DEBUG ((LM_DEBUG,
ACE_TEXT ("(%t) Started processing message:%d/n"),
msgId));
ACE_OS::sleep (1);
ACE_DEBUG ((LM_DEBUG,
ACE_TEXT ("(%t) Finished processing message:%d/n"),
msgId));
}
long LF_ThreadPool::LONG_TIME = 5L;
int ACE_TMAIN (int, ACE_TCHAR *[])
{
LF_ThreadPool tp;
tp.activate (THR_NEW_LWP| THR_JOINABLE, 5);
// Wait for a few seconds...
ACE_OS::sleep (2);
ACE_Time_Value tv (1L);
ACE_Message_Block *mb;
for (int i = 0; i < 30; i++)
{
ACE_NEW_RETURN (mb, ACE_Message_Block (sizeof(int)), -1);
ACE_OS::memcpy (mb->wr_ptr (), &i, sizeof(int));
ACE_OS::sleep (tv);
// Add a new work item.
tp.putq (mb);
}
ACE_Thread_Manager::instance ()->wait ();
ACE_OS::sleep (10);
return 0;
}
#else
#include "ace/OS_main.h"
#include "ace/OS_NS_stdio.h"
int ACE_TMAIN (int, ACE_TCHAR *[])
{
ACE_OS::puts (ACE_TEXT ("This example requires threads."));
return 0;
}
#endif /* ACE_HAS_THREADS */
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