-@node Project 1--Threads, Project 2--User Programs, Pintos Tour, Top
+@node Project 1--Threads
@chapter Project 1: Threads
In this assignment, we give you a minimally functional thread system.
Your job is to extend the functionality of this system to gain a
-better understanding of synchronization problems. Additionally, you
-will use at least part of this increased functionality in future
-assignments.
+better understanding of synchronization problems.
-You will be working in primarily in the @file{threads} directory for
+You will be working primarily in the @file{threads} directory for
this assignment, with some work in the @file{devices} directory on the
side. Compilation should be done in the @file{threads} directory.
-Before you read the description of this project, you should read all
-of the following sections: @ref{Introduction}, @ref{Coding Standards},
-@ref{Project Documentation}, @ref{Debugging Tools}, and
-@ref{Development Tools}. You should at least skim the material in
-@ref{Threads Tour}. To complete this project you will also need to
-read @ref{Multilevel Feedback Scheduling}.
+Before you read the description of this project, you should read all of
+the following sections: @ref{Introduction}, @ref{Coding Standards},
+@ref{Debugging Tools}, and @ref{Development Tools}. You should at least
+skim the material from @ref{Pintos Loading} through @ref{Memory
+Allocation}, especially @ref{Synchronization}. To complete this project
+you will also need to read @ref{4.4BSD Scheduler}.
+
+@menu
+* Project 1 Background::
+* Project 1 Requirements::
+* Project 1 FAQ::
+@end menu
+
+@node Project 1 Background
+@section Background
+
@menu
* Understanding Threads::
-* Project 1 Code::
-* Debugging versus Testing::
-* Tips::
-* Problem 1-1 Alarm Clock::
-* Problem 1-2 Join::
-* Problem 1-3 Priority Scheduling::
-* Problem 1-4 Advanced Scheduler::
-* Threads FAQ::
+* Project 1 Source Files::
+* Project 1 Synchronization::
+* Development Suggestions::
@end menu
@node Understanding Threads
-@section Understanding Threads
+@subsection Understanding Threads
-The first step is to read and understand the initial thread system.
-Pintos, by default, implements thread creation and thread completion,
+The first step is to read and understand the code for the initial thread
+system.
+Pintos already implements thread creation and thread completion,
a simple scheduler to switch between threads, and synchronization
-primitives (semaphores, locks, and condition variables).
+primitives (semaphores, locks, condition variables, and optimization
+barriers).
-However, there's a lot of magic going on in some of this code, so if
+Some of this code might seem slightly mysterious. If
you haven't already compiled and run the base system, as described in
the introduction (@pxref{Introduction}), you should do so now. You
-can read through parts of the source code by hand to see what's going
+can read through parts of the source code to see what's going
on. If you like, you can add calls to @func{printf} almost
anywhere, then recompile and run to see what happens and in what
order. You can also run the kernel in a debugger and set breakpoints
at interesting spots, single-step through code and examine data, and
-so on. @xref{i386-elf-gdb}, for more information.
+so on.
When a thread is created, you are creating a new context to be
-scheduled. You provide a function to be run in this context as an
-argument to @func{thread_create}. The first time the thread is
-scheduled and runs, it will start from the beginning of that function
-and execute it in the context. When that function returns, that thread
-completes. Each thread, therefore, acts like a mini-program running
+scheduled. You provide a function to be run in this context as an
+argument to @func{thread_create}. The first time the thread is
+scheduled and runs, it starts from the beginning of that function
+and executes in that context. When the function returns, the thread
+terminates. Each thread, therefore, acts like a mini-program running
inside Pintos, with the function passed to @func{thread_create}
acting like @func{main}.
-At any given time, Pintos is running exactly one thread, with the
-others switched out. The scheduler decides which thread to run next
-when it needs to switch between them. (If no thread is ready to run
-at any given time, then the special ``idle'' thread runs.) The
-synchronization primitives are used to force context switches when one
+At any given time, exactly one thread runs and the rest, if any,
+become inactive. The scheduler decides which thread to
+run next. (If no thread is ready to run
+at any given time, then the special ``idle'' thread, implemented in
+@func{idle}, runs.)
+Synchronization primitives can force context switches when one
thread needs to wait for another thread to do something.
-The exact mechanics of a context switch are pretty gruesome and have
-been provided for you in @file{threads/switch.S} (this is 80@var{x}86
-assembly; don't worry about understanding it). It involves saving the
-state of the currently running thread and restoring the state of the
+The mechanics of a context switch are
+in @file{threads/switch.S}, which is 80@var{x}86
+assembly code. (You don't have to understand it.) It saves the
+state of the currently running thread and restores the state of the
thread we're switching to.
-Using the @command{gdb} debugger, slowly trace through a context
-switch to see what happens (@pxref{i386-elf-gdb}). You can set a
-breakpoint on the @func{schedule} function to start out, and then
-single-step from there.@footnote{@command{gdb} might tell you that
-@func{schedule} doesn't exist, which is arguably a @command{gdb} bug.
+Using the GDB debugger, slowly trace through a context
+switch to see what happens (@pxref{GDB}). You can set a
+breakpoint on @func{schedule} to start out, and then
+single-step from there.@footnote{GDB might tell you that
+@func{schedule} doesn't exist, which is arguably a GDB bug.
You can work around this by setting the breakpoint by filename and
line number, e.g.@: @code{break thread.c:@var{ln}} where @var{ln} is
-the line number of the first declaration in @func{schedule}.
-Alternatively you can recompile with optimization turned off, by
-removing @samp{-O3} from the @code{CFLAGS} line in
-@file{Make.config}.} Be sure to keep track of each thread's address
+the line number of the first declaration in @func{schedule}.} Be sure
+to keep track of each thread's address
and state, and what procedures are on the call stack for each thread.
You will notice that when one thread calls @func{switch_threads},
another thread starts running, and the first thing the new thread does
-is to return from @func{switch_threads}. We realize this comment will
-seem cryptic to you at this point, but you will understand threads
-once you understand why the @func{switch_threads} that gets called is
-different from the @func{switch_threads} that returns.
+is to return from @func{switch_threads}. You will understand the thread
+system once you understand why and how the @func{switch_threads} that
+gets called is different from the @func{switch_threads} that returns.
+@xref{Thread Switching}, for more information.
@strong{Warning}: In Pintos, each thread is assigned a small,
fixed-size execution stack just under @w{4 kB} in size. The kernel
-does try to detect stack overflow, but it cannot always succeed. You
+tries to detect stack overflow, but it cannot do so perfectly. You
may cause bizarre problems, such as mysterious kernel panics, if you
declare large data structures as non-static local variables,
e.g. @samp{int buf[1000];}. Alternatives to stack allocation include
-the page allocator in @file{threads/palloc.c} and the block allocator
-in @file{threads/malloc.c}. Note that the page allocator doles out
-@w{4 kB} chunks and that @func{malloc} has a @w{2 kB} block size
-limit. If you need larger chunks, consider using a linked structure
-instead.
+the page allocator and the block allocator (@pxref{Memory Allocation}).
-@node Project 1 Code
-@section Code
+@node Project 1 Source Files
+@subsection Source Files
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
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.
+beginning of the kernel. @xref{Pintos Loader}, for details. 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
+of the kernel image. @xref{Pintos Loader}, for details. 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
@itemx init.h
Kernel initialization, including @func{main}, the kernel's ``main
program.'' You should look over @func{main} at least to see what
-gets initialized.
+gets initialized. You might want to add your own initialization code
+here. @xref{Kernel Initialization}, for details.
@item thread.c
@itemx thread.h
-Basic thread support. Much of your work will take place in these
-files. @file{thread.h} defines @struct{thread}, which you will
-modify in the first three projects.
+Basic thread support. Much of your work will take place in these files.
+@file{thread.h} defines @struct{thread}, which you are likely to modify
+in all four projects. See @ref{struct thread} and @ref{Threads} for
+more information.
@item switch.S
@itemx switch.h
Assembly language routine for switching threads. Already discussed
-above.
+above. @xref{Thread Functions}, for more information.
@item palloc.c
@itemx palloc.h
Page allocator, which hands out system memory in multiples of 4 kB
-pages.
+pages. @xref{Page Allocator}, for more information.
@item malloc.c
@itemx malloc.h
-A very simple implementation of @func{malloc} and @func{free} for
-the kernel.
+A simple implementation of @func{malloc} and @func{free} for
+the kernel. @xref{Block Allocator}, for more information.
@item interrupt.c
@itemx interrupt.h
Basic interrupt handling and functions for turning interrupts on and
-off.
+off. @xref{Interrupt Handling}, for more information.
-@item intr-stubs.pl
+@item intr-stubs.S
@itemx intr-stubs.h
-A Perl program that outputs assembly for low-level interrupt handling.
+Assembly code for low-level interrupt handling. @xref{Interrupt
+Infrastructure}, for more information.
@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.
+Basic synchronization primitives: semaphores, locks, condition
+variables, and optimization barriers. You will need to use these for
+synchronization in all
+four projects. @xref{Synchronization}, for more information.
@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.
+@item vaddr.h
+@itemx pte.h
+Functions and macros for working with virtual addresses and page table
+entries. These will be more important to you in project 3. For now,
+you can ignore them.
+
+@item flags.h
+Macros that define a few bits in the 80@var{x}86 ``flags'' register.
+Probably of no interest. See @bibref{IA32-v1}, section 3.4.3, ``EFLAGS
+Register,'' for more information.
@end table
@menu
@end menu
@node devices code
-@subsection @file{devices} code
+@subsubsection @file{devices} code
The basic threaded kernel also includes these files in the
@file{devices} directory:
@item timer.c
@itemx timer.h
System timer that ticks, by default, 100 times per second. You will
-modify this code in Problem 1-1.
+modify this code in this project.
@item vga.c
@itemx vga.h
VGA display driver. Responsible for writing text to the screen.
-You should have no need to look at this code. @func{printf} will
-call into the VGA display driver for you, so there's little reason to
+You should have no need to look at this code. @func{printf}
+calls into the VGA display driver for you, so there's little reason to
call this code yourself.
@item serial.c
@itemx serial.h
Serial port driver. Again, @func{printf} calls this code for you,
-so you don't need to do so yourself. Feel free to look through it if
-you're curious.
+so you don't need to do so yourself.
+It handles serial input by passing it to the input layer (see below).
@item disk.c
@itemx disk.h
Supports reading and writing sectors on up to 4 IDE disks. This won't
actually be used until project 2.
+@item kbd.c
+@itemx kbd.h
+Keyboard driver. Handles keystrokes passing them to the input layer
+(see below).
+
+@item input.c
+@itemx input.h
+Input layer. Queues input characters passed along by the keyboard or
+serial drivers.
+
@item intq.c
@itemx intq.h
Interrupt queue, for managing a circular queue that both kernel
threads and interrupt handlers want to access. Used by the keyboard
and serial drivers.
+
+@item rtc.c
+@itemx rtc.h
+Real-time clock driver, to enable the kernel to determine the current
+date and time. By default, this is only used by @file{thread/init.c}
+to choose an initial seed for the random number generator.
+
+@item speaker.c
+@itemx speaker.h
+Driver that can produce tones on the PC speaker.
+
+@item pit.c
+@itemx pit.h
+Code to configure the 8254 Programmable Interrupt Timer. This code is
+used by both @file{devices/timer.c} and @file{devices/speaker.c}
+because each device uses one of the PIT's output channel.
@end table
@node lib files
-@subsection @file{lib} files
+@subsubsection @file{lib} files
Finally, @file{lib} and @file{lib/kernel} contain useful library
routines. (@file{lib/user} will be used by user programs, starting in
@itemx stdlib.h
@itemx string.c
@itemx string.h
-Implementation of the standard C library. @xref{C99}, for information
+A subset of the standard C library. @xref{C99}, for
+information
on a few recently introduced pieces of the C library that you might
not have encountered before. @xref{Unsafe String Functions}, for
information on what's been intentionally left out for safety.
@item random.c
@itemx random.h
-Pseudo-random number generator.
+Pseudo-random number generator. The actual sequence of random values
+will not vary from one Pintos run to another, unless you do one of
+three things: specify a new random seed value on the @option{-rs}
+kernel command-line option on each run, or use a simulator other than
+Bochs, or specify the @option{-r} option to @command{pintos}.
@item round.h
Macros for rounding.
@item kernel/bitmap.c
@itemx kernel/bitmap.h
Bitmap implementation. You can use this in your code if you like, but
-you probably won't have any need for project 1.
+you probably won't have any need for it in project 1.
@item kernel/hash.c
@itemx kernel/hash.h
@item kernel/console.c
@itemx kernel/console.h
+@item kernel/stdio.h
Implements @func{printf} and a few other functions.
@end table
-@node Debugging versus Testing
-@section Debugging versus Testing
-
-When you're debugging code, it's useful to be able to be able to run a
-program twice and have it do exactly the same thing. On second and
-later runs, you can make new observations without having to discard or
-verify your old observations. This property is called
-``reproducibility.'' The simulator we use, Bochs, can be set up for
-reproducibility, and that's the way that @command{pintos} invokes it.
-
-Of course, a simulation can only be reproducible from one run to the
-next if its input is the same each time. For simulating an entire
-computer, as we do, this means that every part of the computer must be
-the same. For example, you must use the same disks, the same version
-of Bochs, and you must not hit any keys on the keyboard (because you
-could not be sure to hit them at exactly the same point each time)
-during the runs.
-
-While reproducibility is useful for debugging, it is a problem for
-testing thread synchronization, an important part of this project. In
-particular, when Bochs is set up for reproducibility, timer interrupts
-will come at perfectly reproducible points, and therefore so will
-thread switches. That means that running the same test several times
-doesn't give you any greater confidence in your code's correctness
-than does running it only once.
-
-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 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
-
-There should be no busy-waiting in any of your solutions to this
-assignment. Furthermore, resist the temptation to directly disable
-interrupts in your solution by calling @func{intr_disable} or
-@func{intr_set_level}, although you may find doing so to be useful
-while debugging. Instead, use semaphores, locks and condition
-variables to solve synchronization problems. Hint: read the comments
-in @file{threads/synch.h} if you're unsure what synchronization
-primitives may be used in what situations.
-
-Given some designs of some problems, there may be one or two instances
-in which it is appropriate to directly change the interrupt levels
-instead of relying on the given synchroniztion primitives. This must
-be justified in your @file{DESIGNDOC} file. If you're not sure you're
-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 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 1-1 (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 @func{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 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 1-3 fully working before you begin to tackle
-Problem 1-4.
-
-@node Problem 1-1 Alarm Clock
-@section Problem 1-1: Alarm Clock
-
-Improve the implementation of the timer device defined in
-@file{devices/timer.c} by reimplementing @func{timer_sleep}.
-Threads call @code{timer_sleep(@var{x})} to suspend execution until
-time has advanced by at least @w{@var{x} timer ticks}. This is
-useful for threads that operate in real-time, for example, for
-blinking the cursor once per second. There is no requirement that
-threads start running immediately after waking up; just put them on
-the ready queue after they have waited for approximately the right
-amount of time.
-
-A working implementation of this function is provided. However, the
-version provided is poor, because it ``busy waits,'' that is, it spins
-in a tight loop checking the current time until the current time has
-advanced far enough. This is undesirable because it wastes time that
-could potentially be used more profitably by another thread. Your
-solution should not busy wait.
-
-The argument to @func{timer_sleep} is expressed in timer ticks, not
-in milliseconds or another unit. There are @code{TIMER_FREQ} timer
+@node Project 1 Synchronization
+@subsection Synchronization
+
+Proper synchronization is an important part of the solutions to these
+problems. Any synchronization problem can be easily solved by turning
+interrupts off: while interrupts are off, there is no concurrency, so
+there's no possibility for race conditions. Therefore, it's tempting to
+solve all synchronization problems this way, but @strong{don't}.
+Instead, use semaphores, locks, and condition variables to solve the
+bulk of your synchronization problems. Read the tour section on
+synchronization (@pxref{Synchronization}) or the comments in
+@file{threads/synch.c} if you're unsure what synchronization primitives
+may be used in what situations.
+
+In the Pintos projects, the only class of problem best solved by
+disabling interrupts is coordinating data shared between a kernel thread
+and an interrupt handler. Because interrupt handlers can't sleep, they
+can't acquire locks. This means that data shared between kernel threads
+and an interrupt handler must be protected within a kernel thread by
+turning off interrupts.
+
+This project only requires accessing a little bit of thread state from
+interrupt handlers. For the alarm clock, the timer interrupt needs to
+wake up sleeping threads. In the advanced scheduler, the timer
+interrupt needs to access a few global and per-thread variables. When
+you access these variables from kernel threads, you will need to disable
+interrupts to prevent the timer interrupt from interfering.
+
+When you do turn off interrupts, take care to do so for the least amount
+of code possible, or you can end up losing important things such as
+timer ticks or input events. Turning off interrupts also increases the
+interrupt handling latency, which can make a machine feel sluggish if
+taken too far.
+
+The synchronization primitives themselves in @file{synch.c} are
+implemented by disabling interrupts. You may need to increase the
+amount of code that runs with interrupts disabled here, but you should
+still try to keep it to a minimum.
+
+Disabling interrupts can be useful for debugging, if you want to make
+sure that a section of code is not interrupted. You should remove
+debugging code before turning in your project. (Don't just comment it
+out, because that can make the code difficult to read.)
+
+There should be no busy waiting in your submission. A tight loop that
+calls @func{thread_yield} is one form of busy waiting.
+
+@node Development Suggestions
+@subsection Development Suggestions
+
+In the past, many groups divided the assignment into pieces, then each
+group member worked on his or her piece until just before the
+deadline, at which time the group reconvened to combine their code and
+submit. @strong{This is a bad idea. We do not recommend this
+approach.} Groups that do this often find that two changes conflict
+with each other, requiring lots of last-minute debugging. Some groups
+who have done this have turned in code that did not even compile or
+boot, much less pass any tests.
+
+@localcvspolicy{}
+
+You should expect to run into bugs that you simply don't understand
+while working on this and subsequent projects. When you do,
+reread the appendix on debugging tools, which is filled with
+useful debugging tips that should help you to get back up to speed
+(@pxref{Debugging Tools}). Be sure to read the section on backtraces
+(@pxref{Backtraces}), which will help you to get the most out of every
+kernel panic or assertion failure.
+
+@node Project 1 Requirements
+@section Requirements
+
+@menu
+* Project 1 Design Document::
+* Alarm Clock::
+* Priority Scheduling::
+* Advanced Scheduler::
+@end menu
+
+@node Project 1 Design Document
+@subsection Design Document
+
+Before you turn in your project, you must copy @uref{threads.tmpl, , the
+project 1 design document template} into your source tree under the name
+@file{pintos/src/threads/DESIGNDOC} and fill it in. We recommend that
+you read the design document template before you start working on the
+project. @xref{Project Documentation}, for a sample design document
+that goes along with a fictitious project.
+
+@node Alarm Clock
+@subsection Alarm Clock
+
+Reimplement @func{timer_sleep}, defined in @file{devices/timer.c}.
+Although a working implementation is provided, it ``busy waits,'' that
+is, it spins in a loop checking the current time and calling
+@func{thread_yield} until enough time has gone by. Reimplement it to
+avoid busy waiting.
+
+@deftypefun void timer_sleep (int64_t @var{ticks})
+Suspends execution of the calling thread until time has advanced by at
+least @w{@var{x} timer ticks}. Unless the system is otherwise idle, the
+thread need not wake up after exactly @var{x} ticks. Just put it on
+the ready queue after they have waited for the right amount of time.
+
+@func{timer_sleep} is useful for threads that operate in real-time,
+e.g.@: for blinking the cursor once per second.
+
+The argument to @func{timer_sleep} is expressed in timer ticks, not in
+milliseconds or any another unit. There are @code{TIMER_FREQ} timer
ticks per second, where @code{TIMER_FREQ} is a macro defined in
-@code{devices/timer.h}.
-
-@node Problem 1-2 Join
-@section Problem 1-2: Join
-
-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 @var{A} is the running thread and @var{B} is the argument,
-then we say that ``@var{A} joins @var{B}.''
-
-Incidentally, we don't use @code{struct thread *} as
-@func{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(@var{B})} and
-@code{thread_join(@var{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 @func{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 @func{thread_join} to have the same restriction.
-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 @struct{thread},
-whether it is ever joined or not, and regardless of whether the child
-exits before or after its parent. That is, a thread should be freed
-exactly once in all cases.
-
-Joining a given thread is idempotent. That is, joining a thread T
-multiple times is equivalent to joining it once, because T has already
-exited at the time of the later joins. Thus, joins on T after the
-first should return immediately.
-
-Calling @func{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 (@var{A} joins
-@var{B}, then @var{B} joins @var{C}), multiple joins (@var{A} joins
-@var{B}, then @var{A} joins @var{C}), and so on. Does your join work
-if @func{thread_join} is called on a thread that has not yet been
-scheduled for the first time? You should handle all of these cases.
-Write test code that demonstrates the cases your join works for.
-Don't overdo the output volume, please!
-
-Be careful to program this function correctly. You will need its
-functionality for project 2.
-
-Once you've implemented @func{thread_join}, define
-@code{THREAD_JOIN_IMPLEMENTED} in @file{constants.h}.
-@xref{Conditional Compilation}, for more information.
-
-@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
-@func{thread_set_priority} to set the priority of the running thread
-and @func{thread_get_priority} to get the running thread's priority.
-(This API only allows a thread to examine and modify 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).
+@code{devices/timer.h}. The default value is 100. We don't recommend
+changing this value, because any change is likely to cause many of
+the tests to fail.
+@end deftypefun
+
+Separate functions @func{timer_msleep}, @func{timer_usleep}, and
+@func{timer_nsleep} do exist for sleeping a specific number of
+milliseconds, microseconds, or nanoseconds, respectively, but these will
+call @func{timer_sleep} automatically when necessary. You do not need
+to modify them.
+
+If your delays seem too short or too long, reread the explanation of the
+@option{-r} option to @command{pintos} (@pxref{Debugging versus
+Testing}).
+
+The alarm clock implementation is not needed for later projects,
+although it could be useful for project 4.
+
+@node Priority Scheduling
+@subsection Priority Scheduling
+
+Implement priority scheduling in Pintos.
+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
+threads are waiting for a lock, semaphore, or condition variable, the
+highest priority waiting thread should be awakened first. A thread
+may raise or lower its own priority at any time, but lowering its
+priority such that it no longer has the highest priority must cause it
+to immediately yield the CPU.
+
+Thread priorities range from @code{PRI_MIN} (0) to @code{PRI_MAX} (63).
+Lower numbers correspond to lower priorities, so that priority 0
+is the lowest priority and priority 63 is the highest.
The initial thread priority is passed as an argument to
@func{thread_create}. If there's no reason to choose another
-priority, use @code{PRI_DEFAULT} (29). The @code{PRI_} macros are
+priority, use @code{PRI_DEFAULT} (31). 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
-threads are waiting for a lock, semaphore or condition variable, the
-highest priority waiting thread should be woken up first. A thread
-may set its priority at any time.
-
-One issue with priority scheduling is ``priority inversion'': if a
-high priority thread needs to wait for a low priority thread (for
-instance, for a lock held by a low priority thread, or in
-@func{thread_join} for a thread to complete), and a middle priority
-thread is on the ready list, then the high priority thread will never
-get the CPU because the low priority thread will not get any CPU time.
-A partial fix for this problem is to have the waiting thread
-``donate'' its priority to the low priority thread while it is holding
-the lock, then recall the donation once it has acquired the lock.
-Implement this fix.
-
-You will need to account for all different orders that priority
-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 @var{H}, @var{M}, and @var{L}, respectively, if @var{H} is
-waiting on a lock that @var{M} holds and @var{M} is waiting on a lock
-that @var{L} holds, then both @var{M} and @var{L} should be boosted to
-@var{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
-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
-
-Implement Solaris's multilevel feedback queue scheduler (MLFQS) to
-reduce the average response time for running jobs on your system.
-@xref{Multilevel Feedback Scheduling}, for a detailed description of
-the MLFQS requirements.
+One issue with priority scheduling is ``priority inversion''. Consider
+high, medium, and low priority threads @var{H}, @var{M}, and @var{L},
+respectively. If @var{H} needs to wait for @var{L} (for instance, for a
+lock held by @var{L}), and @var{M} is on the ready list, then @var{H}
+will never get the CPU because the low priority thread will not get any
+CPU time. A partial fix for this problem is for @var{H} to ``donate''
+its priority to @var{L} while @var{L} is holding the lock, then recall
+the donation once @var{L} releases (and thus @var{H} acquires) the lock.
+
+Implement priority donation. You will need to account for all different
+situations in which priority donation is required. Be sure to handle
+multiple donations, in which multiple priorities are donated to a single
+thread. You must also handle nested donation: if @var{H} is waiting on
+a lock that @var{M} holds and @var{M} is waiting on a lock that @var{L}
+holds, then both @var{M} and @var{L} should be boosted to @var{H}'s
+priority. If necessary, you may impose a reasonable limit on depth of
+nested priority donation, such as 8 levels.
+
+You must implement priority donation for locks. You need not
+implement priority donation for the other Pintos synchronization
+constructs. You do need to implement priority scheduling in all
+cases.
-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
-provided test).
+Finally, implement the following functions that allow a thread to
+examine and modify its own priority. Skeletons for these functions are
+provided in @file{threads/thread.c}.
-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).
+@deftypefun void thread_set_priority (int @var{new_priority})
+Sets the current thread's priority to @var{new_priority}. If the
+current thread no longer has the highest priority, yields.
+@end deftypefun
-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.
+@deftypefun int thread_get_priority (void)
+Returns the current thread's priority. In the presence of priority
+donation, returns the higher (donated) priority.
+@end deftypefun
-@node Threads FAQ
-@section FAQ
+You need not provide any interface to allow a thread to directly modify
+other threads' priorities.
-@enumerate 1
-@item
-@b{I am adding a new @file{.h} or @file{.c} file. How do I fix the
-@file{Makefile}s?}@anchor{Adding c or h Files}
+The priority scheduler is not used in any later project.
-To add a @file{.c} file, edit the top-level @file{Makefile.build}.
-You'll want to add your file to variable @samp{@var{dir}_SRC}, where
-@var{dir} is the directory where you added the file. For this
-project, that means you should add it to @code{threads_SRC}, or
-possibly @code{devices_SRC} if you put in the @file{devices}
-directory. Then run @code{make}. If your new file doesn't get
-compiled, run @code{make clean} and then try again.
+@node Advanced Scheduler
+@subsection Advanced Scheduler
-When you modify the top-level @file{Makefile.build}, the modified
-version should be automatically copied to
-@file{threads/build/Makefile} when you re-run make. The opposite is
-not true, so any changes will be lost the next time you run @code{make
-clean} from the @file{threads} directory. Therefore, you should
-prefer to edit @file{Makefile.build} (unless your changes are meant to
-be truly temporary).
+Implement a multilevel feedback queue scheduler similar to the
+4.4@acronym{BSD} scheduler to
+reduce the average response time for running jobs on your system.
+@xref{4.4BSD Scheduler}, for detailed requirements.
+
+Like the priority scheduler, the advanced scheduler chooses the thread
+to run based on priorities. However, the advanced scheduler does not do
+priority donation. Thus, we recommend that you have the priority
+scheduler working, except possibly for priority donation, before you
+start work on the advanced scheduler.
+
+You must write your code to allow us to choose a scheduling algorithm
+policy at Pintos startup time. By default, the priority scheduler
+must be active, but we must be able to choose the 4.4@acronym{BSD}
+scheduler
+with the @option{-mlfqs} kernel option. Passing this
+option sets @code{thread_mlfqs}, declared in @file{threads/thread.h}, to
+true when the options are parsed by @func{parse_options}, which happens
+early in @func{main}.
+
+When the 4.4@acronym{BSD} scheduler is enabled, threads no longer
+directly control their own priorities. The @var{priority} argument to
+@func{thread_create} should be ignored, as well as any calls to
+@func{thread_set_priority}, and @func{thread_get_priority} should return
+the thread's current priority as set by the scheduler.
+
+The advanced scheduler is not used in any later project.
+
+@node Project 1 FAQ
+@section FAQ
-There is no need to edit the @file{Makefile}s to add a @file{.h} file.
+@table @b
+@item How much code will I need to write?
-@item
-@b{How do I write my test cases?}
+Here's a summary of our reference solution, produced by the
+@command{diffstat} program. The final row gives total lines inserted
+and deleted; a changed line counts as both an insertion and a deletion.
-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.
+The reference solution represents just one possible solution. Many
+other solutions are also possible and many of those differ greatly from
+the reference solution. Some excellent solutions may not modify all the
+files modified by the reference solution, and some may modify files not
+modified by the reference solution.
-@item
-@b{Why can't I disable interrupts?}
-
-Turning off interrupts should only be done for short amounts of time,
-or else you end up losing important things such as disk or input
-events. Turning off interrupts also increases the interrupt handling
-latency, which can make a machine feel sluggish if taken too far.
-Therefore, in general, setting the interrupt level should be used
-sparingly. Also, any synchronization problem can be easily solved by
-turning interrupts off, since while interrupts are off, there is no
-concurrency, so there's no possibility for race condition.
-
-To make sure you understand concurrency well, we are discouraging you
-from taking this shortcut at all in your solution. If you are unable
-to solve a particular synchronization problem with semaphores, locks,
-or conditions, or think that they are inadequate for a particular
-reason, you may turn to disabling interrupts. If you want to do this,
-we require in your design document a complete justification and
-scenario (i.e.@: exact sequence of events) to show why interrupt
-manipulation is the best solution. If you are unsure, the TAs can
-help you determine if you are using interrupts too haphazardly. We
-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.
+@verbatim
+ devices/timer.c | 42 +++++-
+ threads/fixed-point.h | 120 ++++++++++++++++++
+ threads/synch.c | 88 ++++++++++++-
+ threads/thread.c | 196 ++++++++++++++++++++++++++----
+ threads/thread.h | 23 +++
+ 5 files changed, 440 insertions(+), 29 deletions(-)
+@end verbatim
-@item
-@b{Where might interrupt-level manipulation be appropriate?}
+@file{fixed-point.h} is a new file added by the reference solution.
-You might find it necessary in some solutions to the Alarm problem.
+@item How do I update the @file{Makefile}s when I add a new source file?
-You might want it at one small point for the priority scheduling
-problem. Note that it is not required to use interrupts for these
-problems. There are other, equally correct solutions that do not
-require interrupt manipulation. However, if you do manipulate
-interrupts and @strong{correctly and fully document it} in your design
-document, we will allow limited use of interrupt disabling.
+@anchor{Adding Source Files}
+To add a @file{.c} file, edit the top-level @file{Makefile.build}.
+Add the new file to variable @samp{@var{dir}_SRC}, where
+@var{dir} is the directory where you added the file. For this
+project, that means you should add it to @code{threads_SRC} or
+@code{devices_SRC}. Then run @code{make}. If your new file
+doesn't get
+compiled, run @code{make clean} and then try again.
-@item
-@b{What does ``warning: no previous prototype for `@var{function}''
-mean?}
+When you modify the top-level @file{Makefile.build} and re-run
+@command{make}, the modified
+version should be automatically copied to
+@file{threads/build/Makefile}. The converse is
+not true, so any changes will be lost the next time you run @code{make
+clean} from the @file{threads} directory. Unless your changes are
+truly temporary, you should prefer to edit @file{Makefile.build}.
+
+A new @file{.h} file does not require editing the @file{Makefile}s.
+
+@item What does @code{warning: no previous prototype for `@var{func}'} mean?
It means that you defined a non-@code{static} function without
preceding it by a prototype. Because non-@code{static} functions are
the problem, add a prototype in a header file that you include, or, if
the function isn't actually used by other @file{.c} files, make it
@code{static}.
-@end enumerate
-@menu
-* Problem 1-1 Alarm Clock FAQ::
-* Problem 1-2 Join FAQ::
-* Problem 1-3 Priority Scheduling FAQ::
-* Problem 1-4 Advanced Scheduler FAQ::
-@end menu
+@item What is the interval between timer interrupts?
+
+Timer interrupts occur @code{TIMER_FREQ} times per second. You can
+adjust this value by editing @file{devices/timer.h}. The default is
+100 Hz.
-@node Problem 1-1 Alarm Clock FAQ
-@subsection Problem 1-1: Alarm Clock FAQ
+We don't recommend changing this value, because any changes are likely
+to cause many of the tests to fail.
-@enumerate 1
-@item
-@b{Why can't I use most synchronization primitives in an interrupt
-handler?}
+@item How long is a time slice?
-As you've discovered, you cannot sleep in an external interrupt
-handler. Since many lock, semaphore, and condition variable functions
-attempt to sleep, you won't be able to call those in
-@func{timer_interrupt}. You may still use those that never sleep.
+There are @code{TIME_SLICE} ticks per time slice. This macro is
+declared in @file{threads/thread.c}. The default is 4 ticks.
-Having said that, you need to make sure that global data does not get
-updated by multiple threads simultaneously executing
-@func{timer_sleep}. Here are some pieces of information to think
-about:
+We don't recommend changing this value, because any changes are likely
+to cause many of the tests to fail.
-@enumerate a
-@item
-Interrupts are turned off while @func{timer_interrupt} runs. This
-means that @func{timer_interrupt} will not be interrupted by a
-thread running in @func{timer_sleep}.
+@item How do I run the tests?
-@item
-A thread in @func{timer_sleep}, however, can be interrupted by a
-call to @func{timer_interrupt}, except when that thread has turned
-off interrupts.
+@xref{Testing}.
-@item
-Examples of synchronization mechanisms have been presented in lecture.
-Going over these examples should help you understand when each type is
-useful or needed.
-@end enumerate
+@item Why do I get a test failure in @func{pass}?
-@item
-@b{What about timer overflow due to the fact that times are defined as
-integers? Do I need to check for that?}
+@anchor{The pass function fails}
+You are probably looking at a backtrace that looks something like this:
-Don't worry about the possibility of timer values overflowing. Timer
-values are expressed as signed 63-bit numbers, which at 100 ticks per
-second should be good for almost 2,924,712,087 years.
+@example
+0xc0108810: debug_panic (lib/kernel/debug.c:32)
+0xc010a99f: pass (tests/threads/tests.c:93)
+0xc010bdd3: test_mlfqs_load_1 (...threads/mlfqs-load-1.c:33)
+0xc010a8cf: run_test (tests/threads/tests.c:51)
+0xc0100452: run_task (threads/init.c:283)
+0xc0100536: run_actions (threads/init.c:333)
+0xc01000bb: main (threads/init.c:137)
+@end example
-@item
-@b{The test program mostly works but reports a few out-of-order
-wake ups. I think it's a problem in the test program. What gives?}
+This is just confusing output from the @command{backtrace} program. It
+does not actually mean that @func{pass} called @func{debug_panic}. In
+fact, @func{fail} called @func{debug_panic} (via the @func{PANIC}
+macro). GCC knows that @func{debug_panic} does not return, because it
+is declared @code{NO_RETURN} (@pxref{Function and Parameter
+Attributes}), so it doesn't include any code in @func{fail} to take
+control when @func{debug_panic} returns. This means that the return
+address on the stack looks like it is at the beginning of the function
+that happens to follow @func{fail} in memory, which in this case happens
+to be @func{pass}.
+
+@xref{Backtraces}, for more information.
-This test is inherently full of race conditions. On a real system it
-wouldn't work perfectly all the time either. However, you can help it
-work more reliably:
+@item How do interrupts get re-enabled in the new thread following @func{schedule}?
+
+Every path into @func{schedule} disables interrupts. They eventually
+get re-enabled by the next thread to be scheduled. Consider the
+possibilities: the new thread is running in @func{switch_thread} (but
+see below), which is called by @func{schedule}, which is called by one
+of a few possible functions:
@itemize @bullet
@item
-Make time slices longer by increasing @code{TIME_SLICE} in
-@file{timer.c} to a large value, such as 100.
+@func{thread_exit}, but we'll never switch back into such a thread, so
+it's uninteresting.
@item
-Make the timer tick more slowly by decreasing @code{TIMER_FREQ} in
-@file{timer.h} to its minimum value of 19.
+@func{thread_yield}, which immediately restores the interrupt level upon
+return from @func{schedule}.
@item
-Increase the serial output speed to the maximum of 115,200 bps by
-modifying the call to @func{set_serial} in @func{serial_init_poll} in
-@file{devices/serial.c}.
-@end itemize
+@func{thread_block}, which is called from multiple places:
-The former two changes are only desirable for testing problem 1-1. You
-should revert them before working on other parts of the project or turn
-in the project. The latter is harmless, so you can retain it or revert
-it at your option.
-@end enumerate
+@itemize @minus
+@item
+@func{sema_down}, which restores the interrupt level before returning.
-@node Problem 1-2 Join FAQ
-@subsection Problem 1-2: Join FAQ
+@item
+@func{idle}, which enables interrupts with an explicit assembly STI
+instruction.
-@enumerate 1
@item
-@b{Am I correct to assume that once a thread is deleted, it is no
-longer accessible by the parent (i.e.@: the parent can't call
-@code{thread_join(child)})?}
+@func{wait} in @file{devices/intq.c}, whose callers are responsible for
+re-enabling interrupts.
+@end itemize
+@end itemize
+
+There is a special case when a newly created thread runs for the first
+time. Such a thread calls @func{intr_enable} as the first action in
+@func{kernel_thread}, which is at the bottom of the call stack for every
+kernel thread but the first.
+@end table
-A parent joining a child that has completed should be handled
-gracefully and should act as a no-op.
-@end enumerate
+@menu
+* Alarm Clock FAQ::
+* Priority Scheduling FAQ::
+* Advanced Scheduler FAQ::
+@end menu
-@node Problem 1-3 Priority Scheduling FAQ
-@subsection Problem 1-3: Priority Scheduling FAQ
+@node Alarm Clock FAQ
+@subsection Alarm Clock FAQ
-@enumerate 1
-@item
-@b{Doesn't the priority scheduling lead to starvation? Or do I have to
-implement some sort of aging?}
+@table @b
+@item Do I need to account for timer values overflowing?
-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
-technique. Problem 4 will introduce a mechanism for dynamically
+Don't worry about the possibility of timer values overflowing. Timer
+values are expressed as signed 64-bit numbers, which at 100 ticks per
+second should be good for almost 2,924,712,087 years. By then, we
+expect Pintos to have been phased out of the @value{coursenumber} curriculum.
+@end table
+
+@node Priority Scheduling FAQ
+@subsection Priority Scheduling FAQ
+
+@table @b
+@item Doesn't priority scheduling lead to starvation?
+
+Yes, strict priority scheduling can lead to starvation
+because a thread will not run if any higher-priority thread is runnable.
+The advanced scheduler introduces a mechanism for dynamically
changing thread priorities.
-This sort of scheduling is valuable in real-time systems because it
+Strict priority scheduling is valuable in real-time systems because it
offers the programmer more control over which jobs get processing
time. High priorities are generally reserved for time-critical
tasks. It's not ``fair,'' but it addresses other concerns not
applicable to a general-purpose operating system.
-@item
-@b{After a lock has been released, does the program need to switch to
-the highest priority thread that needs the lock (assuming that its
-priority is higher than that of the current thread)?}
+@item What thread should run after a lock has been released?
When a lock is released, the highest priority thread waiting for that
-lock should be unblocked and put on the ready to run list. The
+lock should be unblocked and put on the list of ready threads. The
scheduler should then run the highest priority thread on the ready
list.
-@item
-@b{If a thread calls @func{thread_yield} and then it turns out that
-it has higher priority than any other threads, does the high-priority
-thread continue running?}
+@item If the highest-priority thread yields, does it continue running?
Yes. If there is a single highest-priority thread, it continues
running until it blocks or finishes, even if it calls
@func{thread_yield}.
+If multiple threads have the same highest priority,
+@func{thread_yield} should switch among them in ``round robin'' order.
-@item
-@b{If the highest priority thread is added to the ready to run list it
-should start execution immediately. Is it immediate enough if I
-wait until next timer interrupt occurs?}
+@item What happens to the priority of a donating thread?
-The highest priority thread should run as soon as it is runnable,
-preempting whatever thread is currently running.
+Priority donation only changes the priority of the donee
+thread. The donor thread's priority is unchanged.
+Priority donation is not additive: if thread @var{A} (with priority 5) donates
+to thread @var{B} (with priority 3), then @var{B}'s new priority is 5, not 8.
-@item
-@b{What happens to the priority of the donating thread? Do the priorities
-get swapped?}
+@item Can a thread's priority change while it is on the ready queue?
-No. Priority donation only changes the priority of the low-priority
-thread. The donating thread's priority stays unchanged. Also note
-that priorities aren't additive: if thread A (with priority 5) donates
-to thread B (with priority 3), then B's new priority is 5, not 8.
+Yes. Consider a ready, low-priority thread @var{L} that holds a lock.
+High-priority thread @var{H} attempts to acquire the lock and blocks,
+thereby donating its priority to ready thread @var{L}.
-@item
-@b{Can a thread's priority be changed while it is sitting on the ready
-queue?}
+@item Can a thread's priority change while it is blocked?
-Yes. Consider this case: low-priority thread L currently has a lock
-that high-priority thread H wants. H donates its priority to L (the
-lock holder). L finishes with the lock, and then loses the CPU and is
-moved to the ready queue. Now L's old priority is restored while it
-is in the ready queue.
+Yes. While a thread that has acquired lock @var{L} is blocked for any
+reason, its priority can increase by priority donation if a
+higher-priority thread attempts to acquire @var{L}. This case is
+checked by the @code{priority-donate-sema} test.
-@item
-@b{Can a thread's priority change while it is sitting on the queue of a
-semaphore?}
+@item Can a thread added to the ready list preempt the processor?
-Yes. Same scenario as above except L gets blocked waiting on a new
-lock when H restores its priority.
+Yes. If a thread added to the ready list has higher priority than the
+running thread, the correct behavior is to immediately yield the
+processor. It is not acceptable to wait for the next timer interrupt.
+The highest priority thread should run as soon as it is runnable,
+preempting whatever thread is currently running.
-@item
-@b{Why is pubtest3's FIFO test skipping some threads! I know my scheduler
-is round-robin'ing them like it's supposed to! Our output is like this:}
+@item How does @func{thread_set_priority} affect a thread receiving donations?
-@example
-Thread 0 goes.
-Thread 2 goes.
-Thread 3 goes.
-Thread 4 goes.
-Thread 0 goes.
-Thread 1 goes.
-Thread 2 goes.
-Thread 3 goes.
-Thread 4 goes.
-@end example
+It sets the thread's base priority. The thread's effective priority
+becomes the higher of the newly set priority or the highest donated
+priority. When the donations are released, the thread's priority
+becomes the one set through the function call. This behavior is checked
+by the @code{priority-donate-lower} test.
-@noindent @b{which repeats 5 times and then}
+@item Doubled test names in output make them fail.
+
+Suppose you are seeing output in which some test names are doubled,
+like this:
@example
-Thread 1 goes.
-Thread 1 goes.
-Thread 1 goes.
-Thread 1 goes.
-Thread 1 goes.
+(alarm-priority) begin
+(alarm-priority) (alarm-priority) Thread priority 30 woke up.
+Thread priority 29 woke up.
+(alarm-priority) Thread priority 28 woke up.
@end example
-This happens because context switches are being invoked by the test
-when it explicitly calls @func{thread_yield}. However, the time
-slice timer is still alive and so, every tick (by default), thread 1
-gets switched out (caused by @func{timer_interrupt} calling
-@func{intr_yield_on_return}) before it gets a chance to run its
-mainline. It is by coincidence that Thread 1 is the one that gets
-skipped in our example. If we use a different jitter value, the same
-behavior is seen where a thread gets started and switched out
-completely.
-
-Solution: Increase the value of @code{TIME_SLICE} in
-@file{devices/timer.c} to a very high value, such as 10000, to see
-that the threads will round-robin if they aren't interrupted.
-
-@item
-@b{What happens when a thread is added to the ready list which has
-higher priority than the currently running thread?}
-
-The correct behavior is to immediately yield the processor. Your
-solution must act this way.
-
-@item
-@b{What should @func{thread_get_priority} return in a thread while
-its priority has been increased by a donation?}
+What is happening is that output from two threads is being
+interleaved. That is, one thread is printing @code{"(alarm-priority)
+Thread priority 29 woke up.\n"} and another thread is printing
+@code{"(alarm-priority) Thread priority 30 woke up.\n"}, but the first
+thread is being preempted by the second in the middle of its output.
+
+This problem indicates a bug in your priority scheduler. After all, a
+thread with priority 29 should not be able to run while a thread with
+priority 30 has work to do.
+
+Normally, the implementation of the @code{printf()} function in the
+Pintos kernel attempts to prevent such interleaved output by acquiring
+a console lock during the duration of the @code{printf} call and
+releasing it afterwards. However, the output of the test name,
+e.g., @code{(alarm-priority)}, and the message following it is output
+using two calls to @code{printf}, resulting in the console lock being
+acquired and released twice.
+@end table
-The higher (donated) priority.
-@end enumerate
+@node Advanced Scheduler FAQ
+@subsection Advanced Scheduler FAQ
-@node Problem 1-4 Advanced Scheduler FAQ
-@subsection Problem 1-4: Advanced Scheduler FAQ
+@table @b
+@item How does priority donation interact with the advanced scheduler?
-@enumerate 1
-@item
-@b{What is the interval between timer interrupts?}
+It doesn't have to. We won't test priority donation and the advanced
+scheduler at the same time.
-Timer interrupts occur @code{TIMER_FREQ} times per second. You can
-adjust this value by editing @file{devices/timer.h}. The default is
-100 Hz.
+@item Can I use one queue instead of 64 queues?
-You can also adjust the number of timer ticks per time slice by
-modifying @code{TIME_SLICE} in @file{devices/timer.c}.
+Yes. In general, your implementation may differ from the description,
+as long as its behavior is the same.
-@item
-@b{Do I have to modify the dispatch table?}
+@item Some scheduler tests fail and I don't understand why. Help!
-No, although you are allowed to. It is possible to complete
-this problem (i.e.@: demonstrate response time improvement)
-without doing so.
+If your implementation mysteriously fails some of the advanced
+scheduler tests, try the following:
+@itemize
@item
-@b{When the scheduler changes the priority of a thread, how does this
-affect priority donation?}
-
-Short (official) answer: Don't worry about it. Your priority donation
-code may assume static priority assignment.
-
-Longer (unofficial) opinion: If you wish to take this into account,
-however, your design may end up being ``cleaner.'' You have
-considerable freedom in what actually takes place. I believe what
-makes the most sense is for scheduler changes to affect the
-``original'' (non-donated) priority. This change may actually be
-masked by the donated priority. Priority changes should only
-propagate with donations, not ``backwards'' from donees to donors.
+Read the source files for the tests that you're failing, to make sure
+that you understand what's going on. Each one has a comment at the
+top that explains its purpose and expected results.
@item
-@b{What is meant by ``static priority''?}
-
-Once thread A has donated its priority to thread B, if thread A's
-priority changes (due to the scheduler) while the donation still
-exists, you do not have to change thread B's donated priority.
-However, you are free to do so.
+Double-check your fixed-point arithmetic routines and your use of them
+in the scheduler routines.
@item
-@b{Do I have to make my dispatch table user-configurable?}
-
-No. Hard-coding the dispatch table values is fine.
-@end enumerate
+Consider how much work your implementation does in the timer
+interrupt. If the timer interrupt handler takes too long, then it
+will take away most of a timer tick from the thread that the timer
+interrupt preempted. When it returns control to that thread, it
+therefore won't get to do much work before the next timer interrupt
+arrives. That thread will therefore get blamed for a lot more CPU
+time than it actually got a chance to use. This raises the
+interrupted thread's recent CPU count, thereby lowering its priority.
+It can cause scheduling decisions to change. It also raises the load
+average.
+@end itemize
+@end table
projects / pintos-anon / blobdiff