97 lines
2.2 KiB
HTML
97 lines
2.2 KiB
HTML
|
<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>
|
||
|
|
|
||
|
|
|
||
|
|
|
||
|
|
|
||
|
|
|
||
|
|
|