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POperformance of boost::threads notify_one vs notify_all
 

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  1. POperformance of boost::threads notify_one vs notify_all
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    <p>I have an implementation of Semaphore to manage a shared resource using boost::threads. My implementation of the Semaphore is as shown below.</p> <pre><code>void releaseResource() { boost::unique_lock&lt;boost::mutex&gt; lock(mutex_); boost::thread::id curr_thread = boost::this_thread::get_id(); // scan through to identify the current thread for (unsigned int i=0; i&lt;org_count;++i) { if (res_data[i].thread_id == curr_thread) { res_data[i].thread_id = boost::thread::id(); res_data[i].available = true; break; } } ++count_; condition_.notify_one(); } unsigned int acquireResource() { boost::unique_lock&lt;boost::mutex&gt; lock(mutex_); while(count_ == 0) { // put thread to sleep until resource becomes available condition_.wait(lock); } --count_; // Scan through the resource list and hand a index for the resource unsigned int res_ctr; for (unsigned int i=0; i&lt;org_count;++i) { if (res_data[i].available) { res_data[i].thread_id = boost::this_thread::get_id(); res_data[i].available = false; res_ctr = i; break; } } return res_ctr; } </code></pre> <p>My question is regarding the performance degradation that I notice when there are more threads than the number of resources available. If I use <code>notify_all</code> to wake threads up instead of <code>notify_one</code> as shown in the <code>releaseResource()</code> code, I see that the performance improves. Has anyone else experienced something similar?</p> <p>I am using <strong>Windows 7</strong> and <strong>boost 1.52</strong>.</p>
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    1. COIt seems that [boost 1.52.0 implementation](http://www.boost.org/doc/libs/1_52_0/boost/thread/win32/condition_variable.hpp) has slightly complex task in `notify_one` than `notify_all`, for `std::vector<> generations` member, `vector::erase + remove_if` in `notify_one` vs. only `vector::clear` in `notify_all`.
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    2. COThank you for the remark. Can you tell me if I can use `notify_all` instead of `notify_one` for the code specified above?
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    3. COI just wanted to make a note for other users that the issue mentioned here got fixed in boost 1.53.
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