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| author | rsc <rsc> | 2008-09-03 04:50:04 +0000 |
|---|---|---|
| committer | rsc <rsc> | 2008-09-03 04:50:04 +0000 |
| commit | f53494c28e362fb7752bbc83417b9ba47cff0bf5 (patch) | |
| tree | 7a7474710c9553b0188796ba24ae3af992320153 /web/xv6-sched.html | |
| parent | ee3f75f229742a376bedafe34d0ba04995a942be (diff) | |
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diff --git a/web/xv6-sched.html b/web/xv6-sched.html new file mode 100644 index 0000000..f8b8b31 --- /dev/null +++ b/web/xv6-sched.html @@ -0,0 +1,96 @@ +<title>Homework: Threads and Context Switching</title> +<html> +<head> +</head> +<body> + +<h1>Homework: Threads and Context Switching</h1> + +<p> +<b>Read</b>: swtch.S and proc.c (focus on the code that switches +between processes, specifically <code>scheduler</code> and <code>sched</code>). + +<p> +<b>Hand-In Procedure</b> +<p> +You are to turn in this homework during lecture. Please +write up your answers to the exercises below and hand them in to a +6.828 staff member at the beginning of lecture. +<p> +<b>Introduction</b> + +<p> +In this homework you will investigate how the kernel switches between +two processes. + +<p> +<b>Assignment</b>: +<p> + +Suppose a process that is running in the kernel +calls <code>sched()</code>, which ends up jumping +into <code>scheduler()</code>. + +<p> +<b>Turn in</b>: +Where is the stack that <code>sched()</code> executes on? + +<p> +<b>Turn in</b>: +Where is the stack that <code>scheduler()</code> executes on? + +<p> +<b>Turn in:</b> +When <code>sched()</code> calls <code>swtch()</code>, +does that call to <code>swtch()</code> ever return? If so, when? + +<p> +<b>Turn in:</b> +Why does <code>swtch()</code> copy %eip from the stack into the +context structure, only to copy it from the context +structure to the same place on the stack +when the process is re-activated? +What would go wrong if <code>swtch()</code> just left the +%eip on the stack and didn't store it in the context structure? + +<p> +Surround the call to <code>swtch()</code> in <code>schedule()</code> with calls +to <code>cons_putc()</code> like this: +<pre> + cons_putc('a'); + swtch(&cpus[cpu()].context, &p->context); + cons_putc('b'); +</pre> +<p> +Similarly, +surround the call to <code>swtch()</code> in <code>sched()</code> with calls +to <code>cons_putc()</code> like this: + +<pre> + cons_putc('c'); + swtch(&cp->context, &cpus[cpu()].context); + cons_putc('d'); +</pre> +<p> +Rebuild your kernel and boot it on bochs. +With a few exceptions +you should see a regular four-character pattern repeated over and over. +<p> +<b>Turn in</b>: What is the four-character pattern? +<p> +<b>Turn in</b>: The very first characters are <code>ac</code>. Why does +this happen? +<p> +<b>Turn in</b>: Near the start of the last line you should see +<code>bc</code>. How could this happen? + +<p> +<b>This completes the homework.</b> + +</body> + + + + + + |