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POpthread scheduling problems
 

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  1. POpthread scheduling problems
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    <p>I have two threads in a producer-consumer pattern. Code works, but then the consumer thread will get starved, and then the producer thread will get starved.</p> <p>When working, program outputs:</p> <pre><code>Send Data...semValue = 1 Recv Data...semValue = 0 Send Data...semValue = 1 Recv Data...semValue = 0 Send Data...semValue = 1 Recv Data...semValue = 0 </code></pre> <p>Then something changes and threads get starved, program outputs:</p> <pre><code>Send Data...semValue = 1 Send Data...semValue = 2 Send Data...semValue = 3 ... Send Data...semValue = 256 Send Data...semValue = 257 Send Data...semValue = 258 Recv Data...semValue = 257 Recv Data...semValue = 256 Recv Data...semValue = 255 ... Recv Data...semValue = 0 Send Data...semValue = 1 Recv Data...semValue = 0 Send Data...semValue = 1 Recv Data...semValue = 0 </code></pre> <p>I know threads are scheduled by the OS, and can run at different rates and in random order. My question: When I do a YieldThread(calls <a href="http://www.kernel.org/doc/man-pages/online/pages/man3/pthread_yield.3.html" rel="nofollow noreferrer">pthread_yield</a>), shouldn’t the Talker give Listener a chance to run? Why am I getting this bizarre scheduling?</p> <p>Snippet of Code below. Thread class and Semaphore class are abstractions classes. I went ahead as stripped out the queue for data passing between the threads so I could eliminate that variable.</p> <pre><code>const int LOOP_FOREVER = 1; class Listener : public Thread { public: Listener(Semaphore* dataReadySemaphorePtr) : Thread("Listener"), dataReadySemaphorePtr(dataReadySemaphorePtr) { //Intentionally left blank. } private: void ThreadTask(void) { while(LOOP_FOREVER) { this-&gt;dataReadySemaphorePtr-&gt;Wait(); printf("Recv Data..."); YieldThread(); } } Semaphore* dataReadySemaphorePtr; }; class Talker : public Thread { public: Talker(Semaphore* dataReadySemaphorePtr) : Thread("Talker"), dataReadySemaphorePtr(dataReadySemaphorePtr) { //Intentionally left blank } private: void ThreadTask(void) { while(LOOP_FOREVER) { printf("Send Data..."); this-&gt;dataReadySemaphorePtr-&gt;Post(); YieldThread(); } } Semaphore* dataReadySemaphorePtr; }; int main() { Semaphore dataReadySemaphore(0); Listener listener(&amp;dataReadySemaphore); Talker talker(&amp;dataReadySemaphore); listener.StartThread(); talker.StartThread(); while (LOOP_FOREVER); //Wait here so threads can run } </code></pre>
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    1. USDennis Miller
    1. USDennis Miller
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    1. CONote that using printf or cout affects your multithreaded program: all standard IO operations use locks to prevent corruption of the buffers. At least they do on Windows and Linux. If they did not, you could not use printf from multiple threads at all.
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      1. USZan Lynx
    2. COYep, know about this. The printf's just make the problem appear sooner. I compile out all printf stdout in the production code, and when I do, it runs a lot longer before I start getting these scheduling problems.
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      1. USDennis Miller
 

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