[Boost-bugs] [Boost C++ Libraries] #12120: Performance improvement in thread/barrier.hpp

Subject: [Boost-bugs] [Boost C++ Libraries] #12120: Performance improvement in thread/barrier.hpp
From: Boost C++ Libraries (noreply_at_[hidden])
Date: 2016-04-11 19:50:03

#12120: Performance improvement in thread/barrier.hpp
------------------------------------------+--------------------------
 Reporter:  Ronald Holthuizen <ronald@…>  |      Owner:  anthonyw
     Type:  Bugs                          |     Status:  new
Milestone:  To Be Determined              |  Component:  thread
  Version:  Boost 1.61.0                  |   Severity:  Optimization
 Keywords:  barrier wait performance      |
------------------------------------------+--------------------------
 The current Boost code in thread/barrier.hpp has this code:

 ---------------------
     bool wait()
     {
       boost::unique_lock < boost::mutex > lock(m_mutex);
       unsigned int gen = m_generation;

       if (--m_count == 0)
       {
         m_generation++;
         m_count = static_cast<unsigned int>(fct_());
         BOOST_ASSERT(m_count != 0);
         m_cond.notify_all();
         return true;
       }

       while (gen == m_generation)
         m_cond.wait(lock);
       return false;
     }
 ------------------
 In the line m_cond.notify_all(); m_mutex is locked while the other threads
 are notified. This is a performance anti-pattern: the notified threads
 will first have to wait for the mutex to be released.

 By adding the line:
 lock.unlock();
 before the m_cond.notify_all(); the performance of the barrier class is
 improved.

 More info why this is better, see sample in:
 http://en.cppreference.com/w/cpp/thread/condition_variable

-- 
Ticket URL: <https://svn.boost.org/trac/boost/ticket/12120>
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