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anno2/YearI/FirstSem/MCAD/altro/LittleBookOfSemaphores.pdf
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anno2/YearI/FirstSem/MCAD/altro/LittleBookOfSemaphores.pdf
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anno2/YearI/FirstSem/MCAD/altro/hw3sol..htm
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anno2/YearI/FirstSem/MCAD/altro/hw3sol..htm
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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
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<html>
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<head>
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<title>Homework 3 solutions</title>
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</head>
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<body>
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<h1>Homework 3 solutions</h1>
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<ol start=1 type=1>
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<li class=MsoNormal style='mso-list:l1 level1 lfo3;tab-stops:list .5in'><b>7.1.
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Busy Waiting<br>
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<br>
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</b><span style='font-weight:normal'><i>Busy waiting</i></span> is
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continuously executing (using the CPU) while waiting for something to
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happen. This is typically implemented by spinning; e.g. testing a variable
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in a tight loop until it changes. <br>
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<br>
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The alternative to busy waiting is <i>blocking</i><span style='font-style:
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normal'>, where the waiting process is suspended an other processes can
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execute while the process is waiting.<br style='mso-special-character:
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line-break'>
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<![if !supportLineBreakNewLine]><br style='mso-special-character:line-break'>
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<![endif]></span></li>
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<li class=MsoNormal style='mso-list:l1 level1 lfo3;tab-stops:list .5in'><b>7.2.
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Spinlocks<br>
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<br>
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</b><span style='font-weight:normal'>Spinlocks are not appropriate for
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uniprocessor systems because by definition only one process is executing
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at a time. As a result, no other process can ever change release the lock
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– the scheduler has to intervene and switch out the spinning process
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first. As a result, the spinning process might as well suspend itself
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instead of spinning. Or, locking should disable interrupts to prevent the scheduler
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from running, so that the process is guaranteed that it can execute the
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entire critical section.<br>
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<br>
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On a 2 CPU multiprocessor, for example, a process scheduled on processor 1
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can release the lock while a process on processor 2 is spinning on it. No
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interrupt is needed. Thus, spinlocks are not necessarily wasteful on a
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multiprocessor.</span></li>
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</ol>
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<p class=MsoNormal>Synchronization Problems</p>
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<p class=MsoNormal>Notes:</p>
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<p class=MsoNormal style='margin-left:.5in;text-indent:-.25in;mso-list:l2 level1 lfo7;
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tab-stops:list .5in'><![if !supportLists]><span style='font-family:"Times New Roman"'>-<span
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style='font:7.0pt "Times New Roman"'>
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</span></span><![endif]>Please use real binary semaphores, counting semaphores,
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or monitors. If you invent your own synchronization method, there is a 99% chance
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you will get it wrong. Plus it makes it very hard to grade - we look very
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closely and are more likely to find problems. If you must do this, provide an
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implementation of your construct, because otherwise we don't know if:</p>
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<p class=MsoNormal style='margin-left:1.0in;text-indent:-.25in;mso-list:l2 level2 lfo7;
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tab-stops:list 1.0in'><![if !supportLists]><span style='font-family:"Courier New"'>o<span
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style='font:7.0pt "Times New Roman"'>
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</span></span><![endif]>It is possible to implement</p>
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<p class=MsoNormal style='margin-left:1.0in;text-indent:-.25in;mso-list:l2 level2 lfo7;
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tab-stops:list 1.0in'><![if !supportLists]><span style='font-family:"Courier New"'>o<span
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style='font:7.0pt "Times New Roman"'>
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</span></span><![endif]>How it works</p>
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<p class=MsoNormal style='margin-left:.5in;text-indent:-.25in;mso-list:l2 level1 lfo7;
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tab-stops:list .5in'><![if !supportLists]><span style='font-family:"Times New Roman"'>-<span
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style='font:7.0pt "Times New Roman"'>
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</span></span><![endif]>If you use semaphores, you must specify initial values
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for your semaphores (mandatory).</p>
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<p class=MsoNormal style='margin-left:.5in;text-indent:-.25in;mso-list:l2 level1 lfo7;
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tab-stops:list .5in'><![if !supportLists]><span style='font-family:"Times New Roman"'>-<span
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style='font:7.0pt "Times New Roman"'>
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</span></span><![endif]>If you have a shared state, you should say what mutex
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protects the shared state (not mandatory)</p>
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<p class=MsoNormal style='margin-left:.5in;text-indent:-.25in;mso-list:l2 level1 lfo7;
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tab-stops:list .5in'><![if !supportLists]><span style='font-family:"Times New Roman"'>-<span
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style='font:7.0pt "Times New Roman"'>
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</span></span><![endif]>With semaphores, if you wake up on one semaphore and then
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call signal() to wake up the next waiter you might wake yourself up -
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semaphores have history, so your next wait might be the one that awakes,
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because the other waiters might have been context switchted out before they
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call wait().</p>
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<p class=MsoNormal style='margin-left:.5in;text-indent:-.25in;mso-list:l2 level1 lfo7;
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tab-stops:list .5in'><![if !supportLists]><span style='font-family:"Times New Roman"'>-<span
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style='font:7.0pt "Times New Roman"'>
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</span></span><![endif]>With semaphores, you shouldn't wait while holding a
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mutex, unless the routine signaling you does not need the mutex to wake you up.</p>
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<p class=MsoNormal style='margin-left:.5in;text-indent:-.25in;mso-list:l2 level1 lfo7;
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tab-stops:list .5in'><![if !supportLists]><span style='font-family:"Times New Roman"'>-<span
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style='font:7.0pt "Times New Roman"'>
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</span></span><![endif]>With monitors, you don't need to grab a mutex (or,
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shudder, a condition variable) to access shared state. The monitor, by
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definition, ensures mutual exclusion.</p>
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<p class=MsoNormal style='margin-left:.5in;text-indent:-.25in;mso-list:l2 level1 lfo7;
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tab-stops:list .5in'><![if !supportLists]><span style='font-family:"Times New Roman"'>-<span
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style='font:7.0pt "Times New Roman"'>
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</span></span><![endif]>With monitors, wait() doesn't take any parameters - it
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just waits until somebody then calls signal().</p>
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<p class=MsoNormal style='margin-left:.5in;text-indent:-.25in;mso-list:l2 level1 lfo7;
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tab-stops:list .5in'><![if !supportLists]><span style='font-family:"Times New Roman"'>-<span
|
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style='font:7.0pt "Times New Roman"'>
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</span></span><![endif]>When writing synchronization code, you should try to
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minimize the number of context switches and the number of times a process is
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awoken when it doesn't need to be. Ideally, in a while() loop, a process should
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only be woken once.<br style='mso-special-character:line-break'>
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<![if !supportLineBreakNewLine]><br style='mso-special-character:line-break'>
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<![endif]></p>
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<ol start=3 type=1>
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<li class=MsoNormal style='mso-list:l1 level1 lfo3;tab-stops:list .5in'><b>7.8.
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Sleeping-barber problem.</b><span style='font-weight:normal'> </span></li>
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</ol>
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<h2 style='margin-left:.5in;mso-list:l0 level1 lfo10'><![if !supportLists]><span
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style='font-size:12.0pt'><span style='font:7.0pt "Times New Roman"'>
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</span></span><![endif]><span style='font-size:12.0pt'>Barbershop Requirements:<o:p></o:p></span></h2>
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<h3 style='margin-left:.5in;mso-list:l0 level1 lfo11'><![if !supportLists]><span
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style='font-size:10.0pt'>-<span style='font:7.0pt "Times New Roman"'>
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</span></span><![endif]><span style='font-size:10.0pt'>Barber sleeps if no
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customers waiting<o:p></o:p></span></h3>
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<h3 style='margin-left:.5in;mso-list:l0 level1 lfo11'><![if !supportLists]><span
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style='font-size:10.0pt'>-<span style='font:7.0pt "Times New Roman"'>
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</span></span><![endif]><span style='font-size:10.0pt'>Customers leave if no
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chairs for waiting<o:p></o:p></span></h3>
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<h3 style='margin-left:.5in;mso-list:l0 level1 lfo11'><![if !supportLists]><span
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style='font-size:10.0pt'>-<span style='font:7.0pt "Times New Roman"'>
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</span></span><![endif]><span style='font-size:10.0pt'>Waiting customers can't
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leave until haircut done.<o:p></o:p></span></h3>
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<h2 style='margin-left:.5in;mso-list:l0 level1 lfo10'><![if !supportLists]><span
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style='font-size:12.0pt'><span style='font:7.0pt "Times New Roman"'>
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</span></span><![endif]><span style='font-size:12.0pt'>Solution: we use
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semaphores<o:p></o:p></span></h2>
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<h3 style='margin-left:.5in;mso-list:l0 level1 lfo11'><![if !supportLists]><span
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style='font-size:10.0pt'>-<span style='font:7.0pt "Times New Roman"'>
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</span></span><![endif]><span style='font-size:10.0pt'>Mutex = 1<o:p></o:p></span></h3>
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<h3 style='margin-left:.5in;mso-list:l0 level1 lfo11'><![if !supportLists]><span
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style='font-size:10.0pt'>-<span style='font:7.0pt "Times New Roman"'>
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</span></span><![endif]><span style='font-size:10.0pt'>counting:
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barber_sleeping, customer_queue, cut_done<o:p></o:p></span></h3>
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<b>Barber Code</b><br>
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<br>
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<pre>
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wait(mutex)
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if (customers_waiting == 0) {
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signal(mutex);
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wait(barber_sleeping);
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wait(mutex);
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}
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customers_waiting--;
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signal(mutex);
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signal(customer_queue);
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do_cut_hair();
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signal(cut_done);
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</pre>
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<b>Customer Code</b><br>
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<pre>
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wait(mutex);
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if (customers_waiting == n) {
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signal(mutex);
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return;
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}
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customers_waiting++;
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if (customers_waiting == 1) {
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signal(barber_sleeping);
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}
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signal(mutex);
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wait(customer_queue);
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get_hair_cut();
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wait(cut_done);
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</pre>
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<b>As a monitor</b>
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<pre>
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monitor barbershop {
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int num_waiting;
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condition get_cut;
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condition barber_asleep;
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condition in_chair;
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condition cut_done;
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Barber routine
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barber() {
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while (1);
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while (num_waiting == 0) {
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barber_asleep.wait();
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}
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customer_waiting.signal();
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in_chair.wait();
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give_hait_cut();
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cut_done.signal();
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}
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Customer routine
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customer () {
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if (num_waiting == n) {
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return;
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}
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if (num_waiting == 0) {
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barber_asleep.signal();
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}
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customer_waiting.wait();
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in_char.signal();
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get_hair_cut();
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cut_done.wait();
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}
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</pre>
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<p style='margin-left:.5in;tab-stops:45.8pt 91.6pt 137.4pt 183.2pt 229.0pt 274.8pt 320.6pt 366.4pt 412.2pt 458.0pt 503.8pt 549.6pt 595.4pt 641.2pt 687.0pt 732.8pt'><![if !supportEmptyParas]> <![endif]><o:p></o:p></p>
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|
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<p style='margin-left:.5in;text-indent:-.25in;mso-list:l1 level1 lfo3;
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tab-stops:list .5in'><![if !supportLists]>7.<span style='font:7.0pt "Times New Roman"'>
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</span><![endif]><b>7.9. The Cigarette-Smokers Problem.</b><span
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style='font-weight:normal'> </span></p>
|
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|
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<p style='margin-left:.5in;tab-stops:45.8pt 91.6pt 137.4pt 183.2pt 229.0pt 274.8pt 320.6pt 366.4pt 412.2pt 458.0pt 503.8pt 549.6pt 595.4pt 641.2pt 687.0pt 732.8pt'>Semaphores
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are a convenient mechanism for implementing this, because they remember what
|
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elements are on the table.</p>
|
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|
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<p style='margin-left:.5in;tab-stops:45.8pt 91.6pt 137.4pt 183.2pt 229.0pt 274.8pt 320.6pt 366.4pt 412.2pt 458.0pt 503.8pt 549.6pt 595.4pt 641.2pt 687.0pt 732.8pt'><span
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style='font-family:"Courier New"'>Sempahore TobaccoAndPaper = 0;<br>
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Sempahore PaperAndMatches = 0;<br>
|
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Semaphore MatchesAndTobacco = 0;<br>
|
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Sempaphore DoneSmoking = 1;<o:p></o:p></span></p>
|
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|
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<p class=MsoNormal style='margin-left:.5in;tab-stops:45.8pt 91.6pt 137.4pt 183.2pt 229.0pt 274.8pt 320.6pt 366.4pt 412.2pt 458.0pt 503.8pt 549.6pt 595.4pt 641.2pt 687.0pt 732.8pt'><span
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style='font-family:"Courier New"'>void agent() <br>
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{<br>
|
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wait(DoneSmoking);<br>
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int r = rand() % 3;<o:p></o:p></span></p>
|
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|
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<p class=MsoNormal style='margin-left:.5in;tab-stops:45.8pt 91.6pt 137.4pt 183.2pt 229.0pt 274.8pt 320.6pt 366.4pt 412.2pt 458.0pt 503.8pt 549.6pt 595.4pt 641.2pt 687.0pt 732.8pt'><span
|
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style='font-family:"Courier New"'><br>
|
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//<br>
|
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// Signal which ever combination was<br>
|
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// chosen.<br>
|
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//<br style='mso-special-character:line-break'>
|
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<![if !supportLineBreakNewLine]><br style='mso-special-character:line-break'>
|
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<![endif]><o:p></o:p></span></p>
|
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|
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<p class=MsoNormal style='margin-left:.5in;tab-stops:45.8pt 91.6pt 137.4pt 183.2pt 229.0pt 274.8pt 320.6pt 366.4pt 412.2pt 458.0pt 503.8pt 549.6pt 595.4pt 641.2pt 687.0pt 732.8pt'><span
|
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style='font-family:"Courier New"'><br>
|
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switch( r ) {<br>
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case 0:
|
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signal(TobaccoAndPaper);<br>
|
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break;<br>
|
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case 1:
|
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signal(PaperAndMatches);<br>
|
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break;<br>
|
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case 2:
|
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signal(MatchesAndTobacco);<br>
|
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break;<br>
|
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}<br>
|
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}<br>
|
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<br>
|
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void Smoker_A()<br>
|
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{<br>
|
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while(true) {<br>
|
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<br>
|
||||
//<br>
|
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// Wait for our
|
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two ingredients<br>
|
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//<br>
|
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<br>
|
||||
|
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wait(TobaccoAndPaper);<br>
|
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smoke();<br>
|
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<br>
|
||||
//<br>
|
||||
// Signal that
|
||||
we're done smoking<br>
|
||||
// so the next
|
||||
agent can put down<br>
|
||||
// ingredients.<br>
|
||||
//<br>
|
||||
<br>
|
||||
|
||||
signal(DoneSmoking);<br>
|
||||
}<br>
|
||||
}<br style='mso-special-character:line-break'>
|
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<![if !supportLineBreakNewLine]><br style='mso-special-character:line-break'>
|
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<![endif]></span></p>
|
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|
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<p class=MsoNormal style='margin-left:.5in;tab-stops:45.8pt 91.6pt 137.4pt 183.2pt 229.0pt 274.8pt 320.6pt 366.4pt 412.2pt 458.0pt 503.8pt 549.6pt 595.4pt 641.2pt 687.0pt 732.8pt'>Smoker
|
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b and c are similar.<span style='font-family:"Courier New"'><o:p></o:p></span></p>
|
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|
||||
<p style='margin-left:.5in;tab-stops:45.8pt 91.6pt 137.4pt 183.2pt 229.0pt 274.8pt 320.6pt 366.4pt 412.2pt 458.0pt 503.8pt 549.6pt 595.4pt 641.2pt 687.0pt 732.8pt'><span
|
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style="mso-spacerun: yes"> </span></p>
|
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|
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<pre style='margin-left:.5in'><span style="mso-spacerun: yes"> </span></pre>
|
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|
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<p style='margin-left:.5in;text-indent:-.25in;mso-list:l1 level1 lfo3;
|
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tab-stops:list .5in'><![if !supportLists]>8.<span style='font:7.0pt "Times New Roman"'>
|
||||
</span><![endif]><b>7.15. File-synchronization</b><span style='font-weight:
|
||||
normal'> </span></p>
|
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|
||||
<p style='margin-left:.5in;tab-stops:45.8pt 91.6pt 137.4pt 183.2pt 229.0pt 274.8pt 320.6pt 366.4pt 412.2pt 458.0pt 503.8pt 549.6pt 595.4pt 641.2pt 687.0pt 732.8pt'>
|
||||
From the book you should know how to use a <i>conditional-wait</i><span
|
||||
style='font-style:normal'> construct (priority-based signaling), so we'll use it here. We'll assume for simplicity that no
|
||||
process has an id greater than or equal to n (or we'd just print an error
|
||||
message). </span></p>
|
||||
|
||||
<pre style='margin-left:.5in'><![if !supportEmptyParas]> <![endif]><o:p></o:p></pre><pre
|
||||
style='margin-left:.5in'> type file = monitor</pre><pre style='margin-left:
|
||||
.5in'><span style="mso-spacerun: yes"> </span>var space_available: binary condition</pre><pre
|
||||
style='margin-left:.5in'><span style="mso-spacerun: yes"> </span>total: integer</pre><pre
|
||||
style='margin-left:.5in'> </pre><pre style='margin-left:.5in'><span style="mso-spacerun: yes"> </span>procedure entry file_open(id)</pre><pre
|
||||
style='margin-left:.5in'><span style="mso-spacerun: yes"> </span>begin</pre><pre
|
||||
style='margin-left:.5in'><span style="mso-spacerun: yes"> </span>while (total + id >= n)</pre><pre
|
||||
style='margin-left:.5in'><span style="mso-spacerun: yes"> </span>space_available.wait(id)</pre><pre
|
||||
style='margin-left:.5in'><span style="mso-spacerun: yes"> </span>total = total + id;</pre><pre
|
||||
style='margin-left:.5in'><span style="mso-spacerun: yes"> </span>if (total < n - 1)</pre><pre
|
||||
style='margin-left:.5in'><span style="mso-spacerun: yes"> </span>space_available.signal();</pre><pre
|
||||
style='margin-left:.5in'><span style="mso-spacerun: yes"> </span>end</pre><pre
|
||||
style='margin-left:.5in'><![if !supportEmptyParas]> <![endif]><o:p></o:p></pre><pre
|
||||
style='margin-left:.5in'><span style="mso-spacerun: yes"> </span>procedure entry file_close(id)</pre><pre
|
||||
style='margin-left:.5in'><span style="mso-spacerun: yes"> </span>begin</pre><pre
|
||||
style='margin-left:.5in'><span style="mso-spacerun: yes"> </span>total = total - id;</pre><pre
|
||||
style='margin-left:.5in'><span style="mso-spacerun: yes"> </span>space_available.signal();</pre><pre
|
||||
style='margin-left:.5in'><span style="mso-spacerun: yes"> </span>end</pre>
|
||||
|
||||
<p class=MsoNormal style='margin-left:.5in;tab-stops:45.8pt 91.6pt 137.4pt 183.2pt 229.0pt 274.8pt 320.6pt 366.4pt 412.2pt 458.0pt 503.8pt 549.6pt 595.4pt 641.2pt 687.0pt 732.8pt'>What
|
||||
happens here? As long as the incoming processes find adequate space available
|
||||
(i.e. sums less than n), they will claim that space and open the file. Note
|
||||
that since we're using a binary condition, we can signal() at will even if
|
||||
nothing is waiting. If a process finds inadequate space, it will block. When a
|
||||
process is done, it wakes up the process with the smallest id (the most likely
|
||||
to fit in the available space). This process, upon waking, then signals the
|
||||
next process if space is still available, but only after successfully claiming
|
||||
its own space. At some point (quite possibly with the first process), the space
|
||||
made available may not be enough for the waking process, and so a process will
|
||||
be woken up prematurely. This is what the loop is for; it will immediately
|
||||
block again. </p>
|
||||
|
||||
|
||||
|
||||
<ol start=6 type=1>
|
||||
<li class=MsoNormal style='mso-list:l1 level1 lfo3;tab-stops:list .5in'><b>8.4
|
||||
b<br>
|
||||
</b>install traffic lights :)
|
||||
<br>
|
||||
<li class=MsoNormal style='mso-list:l1 level1 lfo3;tab-stops:list .5in'><b>8.8
|
||||
<br>
|
||||
</b>The system can always make progress in all possible
|
||||
allocation scenarios (use pigeonhole principle to show this).
|
||||
</ol>
|
||||
<hr>
|
||||
</body>
|
||||
</html>
|
||||
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Reference in a new issue