96 lines
2.2 KiB
HTML
96 lines
2.2 KiB
HTML
<title>Homework: Threads and Context Switching</title>
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<html>
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<head>
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</head>
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<body>
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<h1>Homework: Threads and Context Switching</h1>
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<p>
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<b>Read</b>: swtch.S and proc.c (focus on the code that switches
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between processes, specifically <code>scheduler</code> and <code>sched</code>).
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<p>
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<b>Hand-In Procedure</b>
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<p>
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You are to turn in this homework during lecture. Please
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write up your answers to the exercises below and hand them in to a
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6.828 staff member at the beginning of lecture.
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<p>
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<b>Introduction</b>
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<p>
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In this homework you will investigate how the kernel switches between
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two processes.
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<p>
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<b>Assignment</b>:
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<p>
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Suppose a process that is running in the kernel
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calls <code>sched()</code>, which ends up jumping
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into <code>scheduler()</code>.
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<p>
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<b>Turn in</b>:
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Where is the stack that <code>sched()</code> executes on?
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<p>
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<b>Turn in</b>:
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Where is the stack that <code>scheduler()</code> executes on?
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<p>
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<b>Turn in:</b>
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When <code>sched()</code> calls <code>swtch()</code>,
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does that call to <code>swtch()</code> ever return? If so, when?
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<p>
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<b>Turn in:</b>
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Why does <code>swtch()</code> copy %eip from the stack into the
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context structure, only to copy it from the context
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structure to the same place on the stack
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when the process is re-activated?
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What would go wrong if <code>swtch()</code> just left the
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%eip on the stack and didn't store it in the context structure?
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<p>
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Surround the call to <code>swtch()</code> in <code>schedule()</code> with calls
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to <code>cons_putc()</code> like this:
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<pre>
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cons_putc('a');
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swtch(&cpus[cpu()].context, &p->context);
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cons_putc('b');
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</pre>
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<p>
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Similarly,
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surround the call to <code>swtch()</code> in <code>sched()</code> with calls
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to <code>cons_putc()</code> like this:
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<pre>
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cons_putc('c');
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swtch(&cp->context, &cpus[cpu()].context);
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cons_putc('d');
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</pre>
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<p>
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Rebuild your kernel and boot it on bochs.
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With a few exceptions
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you should see a regular four-character pattern repeated over and over.
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<p>
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<b>Turn in</b>: What is the four-character pattern?
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<p>
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<b>Turn in</b>: The very first characters are <code>ac</code>. Why does
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this happen?
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<p>
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<b>Turn in</b>: Near the start of the last line you should see
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<code>bc</code>. How could this happen?
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<p>
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<b>This completes the homework.</b>
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</body>
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