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POWhy does locking slow down this sequential file parser?
 

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  1. POWhy does locking slow down this sequential file parser?
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    <p>I wrote a simple reader and parser for a graph file format. The problem is that it is incredibly slow. Here are the relevant methods:</p> <pre><code>Graph METISGraphReader::read(std::string path) { METISParser parser(path); std::pair&lt;int64_t, int64_t&gt; header = parser.getHeader(); int64_t n = header.first; int64_t m = header.second; Graph G(n); node u = 0; while (parser.hasNext()) { u += 1; std::vector&lt;node&gt; adjacencies = parser.getNext(); for (node v : adjacencies) { if (! G.hasEdge(u, v)) { G.insertEdge(u, v); } } } return G; } std::vector&lt;node&gt; METISParser::getNext() { std::string line; bool comment = false; do { comment = false; std::getline(this-&gt;graphFile, line); // check for comment line starting with '%' if (line[0] == '%') { comment = true; TRACE("comment line found"); } else { return parseLine(line); } } while (comment); } static std::vector&lt;node&gt; parseLine(std::string line) { std::stringstream stream(line); std::string token; char delim = ' '; std::vector&lt;node&gt; adjacencies; // split string and push adjacent nodes while (std::getline(stream, token, delim)) { node v = atoi(token.c_str()); adjacencies.push_back(v); } return adjacencies; } </code></pre> <p>To diagnose why it is so slow, I ran it in a profiler (Apple Instruments). The results were surprising: It's slow because of locking overhead. The program spends over 90% of its time in <code>pthread_mutex_lock</code> and <code>_pthread_cond_wait</code>.</p> <p><img src="https://i.stack.imgur.com/k4CFw.png" alt="Instruments"></p> <p>I have no idea where the locking overhead comes from, but I need to get rid of it. Can you suggest next steps?</p> <p>EDIT: See the call stack expanded for <code>_pthread_con_wait</code>. I cannot figure out the source of the locking overhead by looking at this:</p> <p><img src="https://i.stack.imgur.com/e0ek5.png" alt="enter image description here"></p>
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    1. CO@KonradRudolph I read from a file, a `std::ifstream`. Why do you think I read from stdin?
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    2. COBecause I’m a knob.
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    3. COHmm, (why) do you link your code against OpenMP? Does log4cxx bring this dependency?
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