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POMultithreading: classical Producer Consumer algorithm
 

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    <p>Something I don't get about the classical algorithm for the Producer-Consumer problem (from Wikipedia:)</p> <pre>semaphore mutex = 1 semaphore fillCount = 0 semaphore emptyCount = BUFFER_SIZE procedure producer() { while (true) { item = produceItem() down(emptyCount) down(mutex) putItemIntoBuffer(item) up(mutex) up(fillCount) } up(fillCount) //the consumer may not finish before the producer. } procedure consumer() { while (true) { down(fillCount) down(mutex) item = removeItemFromBuffer() up(mutex) up(emptyCount) consumeItem(item) } } </pre> <p>I note that both producers and consumers lock 'mutex' prior to messing with the buffer, and unlock it thereafter. If that is the case, i.e. only a single thread is accessing the buffer at any given moment, I don't really see how the above algo differs from a simple solution that entails only putting a guarding mutex over the buffer:</p> <pre> semaphore mutex = 1 procedure producer() { while (true) { item = produceItem() flag = true while (flag) { down(mutex) if (bufferNotFull()) { putItemIntoBuffer(item) flag = false } up(mutex) } } } procedure consumer() { while (true) { flag = true while (flag) { down(mutex) if (bufferNotEmpty()) { item = removeItemFromBuffer() flag = false } up(mutex) } consumeItem(item) } } </pre> <p>Only thing I can think of that necessitates using the 'fillCount' and 'emptyCount' semaphores is <em>scheduling</em>.</p> <p>Maybe the first algo is for making sure that in a state where 5 consumers are waiting on an empty buffer (zero 'fillCount'), it is assured that when a new producer comes along, it will go past its "down(emptyCount)" statement quickly and get the 'mutex' quickly.</p> <p>(whereas in the other solution the consumers will needlessly get the 'mutex' only to relinquish it until the new producer gets it and inserts an item).</p> <p>Am I right? Am I missing something?</p>
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