Performance improvement in thread/barrier.hpp

[Ronald Holthuizen <ronald@…>]

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;

}

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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