@menu
* Understanding Threads::
+* Project 1 Code::
* Debugging versus Testing::
* Tips::
* Problem 1-1 Alarm Clock::
limit. If you need larger chunks, consider using a linked structure
instead.
+@node Project 1 Code
+@section Code
+
+Here is a brief overview of the files in the @file{threads}
+directory. You will not need to modify most of this code, but the
+hope is that presenting this overview will give you a start on what
+code to look at.
+
+@table @file
+@item loader.S
+@itemx loader.h
+The kernel loader. Assembles to 512 bytes of code and data that the
+PC BIOS loads into memory and which in turn loads the kernel into
+memory, does basic processor initialization, and jumps to the
+beginning of the kernel. You should not need to look at this code or
+modify it.
+
+@item kernel.lds.S
+The linker script used to link the kernel. Sets the load address of
+the kernel and arranges for @file{start.S} to be at the very beginning
+of the kernel image. Again, you should not need to look at this code
+or modify it, but it's here in case you're curious.
+
+@item start.S
+Jumps to @code{main()}.
+
+@item init.c
+@itemx init.h
+Kernel initialization, including @code{main()}, the kernel's ``main
+program.'' You should look over @code{main()} at least to see what
+gets initialized.
+
+@item thread.c
+@itemx thread.h
+Basic thread support. Much of your work will take place in these
+files. @file{thread.h} defines @code{struct thread}, which you will
+modify in the first three projects.
+
+@item switch.S
+@itemx switch.h
+Assembly language routine for switching threads. Already discussed
+above.
+
+@item palloc.c
+@itemx palloc.h
+Page allocator, which hands out system memory one 4 kB page at a time.
+
+@item malloc.c
+@itemx malloc.h
+A very simple implementation of @code{malloc()} and @code{free()} for
+the kernel. The largest block that can be allocated is 2 kB.
+
+@item interrupt.c
+@itemx interrupt.h
+Basic interrupt handling and functions for turning interrupts on and
+off.
+
+@item intr-stubs.pl
+@itemx intr-stubs.h
+A Perl program that outputs assembly for low-level interrupt handling.
+
+@item synch.c
+@itemx synch.h
+Basic synchronization primitives: semaphores, locks, and condition
+variables. You will need to use these for synchronization through all
+four projects.
+
+@item test.c
+@itemx test.h
+Test code. For project 1, you will replace this file with your test
+cases.
+
+@item io.h
+Functions for I/O port access. This is mostly used by source code in
+the @file{devices} directory that you won't have to touch.
+
+@item mmu.h
+Functions and macros related to memory management, including page
+directories and page tables. This will be more important to you in
+project 3. For now, you can ignore it.
+@end table
+
+FIXME devices and lib directories?
+
@node Debugging versus Testing
@section Debugging versus Testing
So, to make your code easier to test, we've added a feature to Bochs
that makes timer interrupts come at random intervals, but in a
-perfectly predictable way. In particular, if you put a line
-@samp{ips-jitter: @var{seed}}, where @var{seed} is an integer, into
-your Bochs configuration file, then timer interrupts will come at
-irregularly spaced intervals. Within a single @var{seed} value,
-execution will still be reproducible, but timer behavior will change
-as @var{seed} is varied. Thus, for the highest degree of confidence
-you should test your code with many seed values.
+perfectly predictable way. In particular, if you invoke
+@command{pintos} with the option @option{-j @var{seed}}, timer
+interrupts will come at irregularly spaced intervals. Within a single
+@var{seed} value, execution will still be reproducible, but timer
+behavior will change as @var{seed} is varied. Thus, for the highest
+degree of confidence you should test your code with many seed values.
@node Tips
@section Tips
justified, ask!
While all parts of this assignment are required if you intend to earn
-full credit on this project, keep in mind that Problem 2 (Join) will
+full credit on this project, keep in mind that Problem 1-2 (Join) will
be needed for future assignments, so you'll want to get this one
right. We don't give out solutions, so you're stuck with your Join
-code for the whole quarter. Problem 1 (Alarm Clock) could be very
+code for the whole quarter. Problem 1-1 (Alarm Clock) could be very
handy, but not strictly required in the future. The upshot of all
this is that you should focus heavily on making sure that your
-implementation of Join works correctly, since if it's broken, you will
-need to fix it for future assignments. The other parts can be turned
-off in the future if you find you can't make them work quite right.
+implementation of @code{thread_join()} works correctly, since if it's
+broken, you will need to fix it for future assignments. The other
+parts can be turned off in the future if you find you can't make them
+work quite right.
-Also keep in mind that Problem 4 (the MLFQS) builds on the features you
-implement in Problem 3, so to avoid unnecessary code duplication, it
+Also keep in mind that Problem 1-4 (the MLFQS) builds on the features you
+implement in Problem 1-3, so to avoid unnecessary code duplication, it
would be a good idea to divide up the work among your team members
-such that you have Problem 3 fully working before you begin to tackle
-Problem 4.
+such that you have Problem 1-3 fully working before you begin to tackle
+Problem 1-4.
@node Problem 1-1 Alarm Clock
-@section Problem 1-2: Alarm Clock
+@section Problem 1-1: Alarm Clock
Improve the implementation of the timer device defined in
@file{devices/timer.c} by reimplementing @code{timer_sleep()}.
@node Problem 1-2 Join
@section Problem 1-2: Join
-Implement @code{thread_join(struct thread *)} in
-@file{threads/thread.c}. There is already a prototype for it in
-@file{threads/thread.h}, which you should not change. This function
-causes the currently running thread to block until thread passed as an
-argument exits. If A is the running thread and B is the argument,
-then we say that ``A joins B'' in this case.
+Implement @code{thread_join(tid_t)} in @file{threads/thread.c}. There
+is already a prototype for it in @file{threads/thread.h}, which you
+should not change. This function causes the currently running thread
+to block until the thread whose thread id is passed as an argument
+exits. If A is the running thread and B is the argument, then we say
+that ``A joins B'' in this case.
+
+Incidentally, we don't use @code{struct thread *} as
+@file{thread_join()}'s parameter type because a thread pointer is not
+unique over time. That is, when a thread dies, its memory may be,
+whether immediately or much later, reused for another thread. If
+thread A over time had two children B and C that were stored at the
+same address, then @code{thread_join(@r{B})} and
+@code{thread_join(@r{C})} would be ambiguous. Introducing a thread id
+or @dfn{tid}, represented by type @code{tid_t}, that is intentionally
+unique over time solves the problem. The provided code uses an
+@code{int} for @code{tid_t}, but you may decide you prefer to use some
+other type.
The model for @code{thread_join()} is the @command{wait} system call
in Unix-like systems. (Try reading the manpages.) That system call
can only be used by a parent process to wait for a child's death. You
should implement @code{thread_join()} to have the same restriction.
-That is, a thread may only join on its immediate children.
+That is, a thread may only join its immediate children.
A thread need not ever be joined. Your solution should properly free
all of a thread's resources, including its @code{struct thread},
exited at the time of the later joins. Thus, joins on T after the
first should return immediately.
-The behavior of calling @code{thread_join()} on an thread that is not
-the caller's child is undefined. You need not handle this case
-gracefully.
+Calling @code{thread_join()} on an thread that is not the caller's
+child should cause the caller to return immediately.
Consider all the ways a join can occur: nested joins (A joins B when B
is joined on C), multiple joins (A joins B, then A joins C), and so
Be careful to program this function correctly. You will need its
functionality for project 2.
-@node Problem 1-3 Priority Scheduling
-@section Problem 1-3 Priority Scheduling
+Once you've implemented @code{thread_join()}, define
+@code{THREAD_JOIN_IMPLEMENTED} in @file{constants.h}.
+@xref{Conditional Compilation}, for more information.
-Implement priority scheduling in Pintos. Priority
-scheduling is a key building block for real-time systems. Implement functions
-@code{thread_set_priority()} to set the priority of a thread and
-@code{thread_get_priority()} to get the priority of a thread. There
-are already prototypes for these functions in @file{threads/thread.h},
+@node Problem 1-3 Priority Scheduling
+@section Problem 1-3: Priority Scheduling
+
+Implement priority scheduling in Pintos. Priority scheduling is a key
+building block for real-time systems. Implement functions
+@code{thread_set_priority()} to set the priority of the running thread
+and @code{thread_get_priority()} to get the running thread's priority.
+(A thread can examine and modify only its own priority.) There are
+already prototypes for these functions in @file{threads/thread.h},
which you should not change.
+Thread priority ranges from @code{PRI_MIN} (0) to @code{PRI_MAX} (59).
+The initial thread priority is passed as an argument to
+@code{thread_create()}. If there's no reason to choose another
+priority, use @code{PRI_DEFAULT} (29). The @code{PRI_} macros are
+defined in @file{threads/thread.h}, and you should not change their
+values.
+
When a thread is added to the ready list that has a higher priority
than the currently running thread, the current thread should
immediately yield the processor to the new thread. Similarly, when
donation and inversion can occur. Be sure to handle multiple
donations, in which multiple priorities are donated to a thread. You
must also handle nested donation: given high, medium, and low priority
-threads H, M, and L, respectively, and supposing H is waiting on a
-lock that M holds and M is waiting on a lock that L holds, both M and
-L should be boosted to H's priority.
+threads H, M, and L, respectively, if H is waiting on a lock that M
+holds and M is waiting on a lock that L holds, then both M and L
+should be boosted to H's priority.
You only need to implement priority donation when a thread is waiting
-for a lock held by a lower-priority thread. You do not need to
+for a lock held by a lower-priority thread. You do not need to
implement this fix for semaphores, condition variables or joins.
However, you do need to implement priority scheduling in all cases.
@node Problem 1-4 Advanced Scheduler
-@section Problem 1-4 Advanced Scheduler
+@section Problem 1-4: Advanced Scheduler
Implement Solaris's multilevel feedback queue scheduler (MLFQS) to
reduce the average response time for running jobs on your system.
Demonstrate that your scheduling algorithm reduces response time
relative to the original Pintos scheduling algorithm (round robin) for
-at least one workload of your own design (i.e. in addition to the
+at least one workload of your own design (i.e.@: in addition to the
provided test).
You may assume a static priority for this problem. It is not necessary
to ``re-donate'' a thread's priority if it changes (although you are
free to do so).
+You must write your code so that we can turn the MLFQS on and off at
+compile time. By default, it must be off, but we must be able to turn
+it on by inserting the line @code{#define MLFQS 1} in
+@file{constants.h}. @xref{Conditional Compilation}, for details.
+
@node Threads FAQ
@section FAQ
There is no need to edit the @file{Makefile}s to add a @file{.h} file.
+@item
+@b{How do I write my test cases?}
+
+Test cases should be replacements for the existing @file{test.c}
+file. Put them in a @file{threads/testcases} directory.
+@xref{TESTCASE}, for more information.
+
@item
@b{If a thread finishes, should its children be terminated immediately,
or should they finish normally?}
want to emphasize that there are only limited cases where this is
appropriate.
+You might find @file{devices/intq.h} and its users to be an
+inspiration or source of rationale.
+
@item
-@b{Where might interrupt-level manipuation be appropriate?}
+@b{Where might interrupt-level manipulation be appropriate?}
You might find it necessary in some solutions to the Alarm problem.
@b{Doesn't the priority scheduling lead to starvation? Or do I have to
implement some sort of aging?}
-
It is true that strict priority scheduling can lead to starvation
because thread may not run if a higher-priority thread is runnable.
In this problem, don't worry about starvation or any sort of aging
solution must act this way.
@item
-@b{What range of priorities should be supported and what should the
-default priority of a thread be?}
+@b{What should @code{thread_get_priority()} return in a thread while
+its priority has been increased by a donation?}
-Your implementation should support priorities from 0 through 59 and
-the default priority of a thread should be 29.
+The higher (donated) priority.
@end enumerate
@item Advanced Scheduler FAQs
adjust this value by editing @file{devices/timer.h}. The default is
100 Hz.
+You can also adjust the number of timer ticks per time slice by
+modifying @code{TIME_SLICE} in @file{devices/timer.c}.
+
@item
@b{Do I have to modify the dispatch table?}
projects / pintos-anon / blobdiff