Homework: Threads and Context Switching

Read: swtch.S and proc.c (focus on the code that switches between processes, specifically scheduler and sched). Also process creation: sys_fork() and copyproc().

Hand-In Procedure

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.


In this homework you will investigate how the kernel switches between two processes.


Suppose a process that is running in the kernel calls sched(), which ends up jumping into scheduler().

Turn in: Where is the stack that sched() executes on?

Turn in: Where is the stack that scheduler() executes on?

Turn in: When sched() calls swtch(), does that call to swtch() ever return? If so, when?

Now think back to lecture 2 and the invariants that gcc expects any function, including swtch, to maintain. Compare these invariants with what swtch actually implements, and the state that our kernel maintains in a struct context.

Turn in: Could swtch do less work and still be correct? Could we reduce the size of a struct context? Provide concrete examples if yes, or argue for why not.

Surround the call to swtch() in scheduler() with calls to cons_putc() like this:

      swtch(&c->context, &p->context);

Similarly, surround the call to swtch() in sched() with calls to cons_putc() like this:

  swtch(&cp->context, &cpus[cpu()].context);

Rebuild your kernel and boot it on bochs. With a few exceptions you should see a regular four-character pattern repeated over and over.

Turn in: What is the four-character pattern?

Turn in: The very first characters are ac. Why does this happen?

This completes the homework.