Update docs.

[pintos-anon] / doc / threads.texi

diff --git a/doc/threads.texi b/doc/threads.texi
index 7a048e7e8af35a8332e9d2bc4cd8d7b554b3cd5a..fbd8ac33e5c751add91b795952ffeb308fc46a79 100644 (file)
--- a/doc/threads.texi
+++ b/doc/threads.texi
@@ -13,6 +13,7 @@ side.  Compilation should be done in the @file{threads} directory.
 
 @menu
 * Understanding Threads::       
 
 @menu
 * Understanding Threads::       

+* Project 1 Code::              

 * Debugging versus Testing::    
 * Tips::                        
 * Problem 1-1 Alarm Clock::     
 * Debugging versus Testing::    
 * Tips::                        
 * Problem 1-1 Alarm Clock::     


@@ -20,7 +21,6 @@ side.  Compilation should be done in the @file{threads} directory.
 * Problem 1-3 Priority Scheduling::  
 * Problem 1-4 Advanced Scheduler::  
 * Threads FAQ::                 
 * Problem 1-3 Priority Scheduling::  
 * Problem 1-4 Advanced Scheduler::  
 * Threads FAQ::                 

-* Multilevel Feedback Scheduling::

 @end menu
 
 @node Understanding Threads
 @end menu
 
 @node Understanding Threads


@@ -30,13 +30,16 @@ The first step is to read and understand the initial thread system.
 Pintos, by default, implements thread creation and thread completion,
 a simple scheduler to switch between threads, and synchronization
 primitives (semaphores, locks, and condition variables). 
 Pintos, by default, implements thread creation and thread completion,
 a simple scheduler to switch between threads, and synchronization
 primitives (semaphores, locks, and condition variables). 

-@c FIXME: base system doesn't do anything. Debugger sucks.
-However, there's a lot of magic going on in some of this code, so you
-should compile and run the base system to see a simple test of the
-code.  You should read through the source code by hand to see what's
-going on.  If you like, you can add calls to @code{printf()} almost
+
+However, there's a lot of magic going on in some of this code, so 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
+on.  If you like, you can add calls to @code{printf()} almost

 anywhere, then recompile and run to see what happens and in what
 anywhere, then recompile and run to see what happens and in what

-order.
+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.

 
 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
 
 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


@@ -59,16 +62,19 @@ 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
 thread we're switching to.
 assembly; don't worry about understanding it).  It involves saving the
 state of the currently running thread and restoring the state of the
 thread we're switching to.

-@c FIXME 
-Slowly trace through a context switch to see what happens.  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 @code{switch_threads()}, another thread starts running,
-and the first thing the new thread does is to return from
-@code{switch_threads()}. We realize this comment will seem cryptic 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 @code{schedule()} function to start out, and then
+single-step from there.  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
+@code{switch_threads()}, another thread starts running, and the first
+thing the new thread does is to return from
+@code{switch_threads()}.  We realize this comment will seem cryptic to

 you at this point, but you will understand threads once you understand
 why the @code{switch_threads()} that gets called is different from the
 you at this point, but you will understand threads once you understand
 why the @code{switch_threads()} that gets called is different from the

-@code{switch_threads()} that returns.
+@code{switch_threads()} that returns.  @c FIXME

 
 @strong{Warning}: In Pintos, each thread is assigned a small,
 fixed-size execution stack just under @w{4 kB} in size.  The kernel
 
 @strong{Warning}: In Pintos, each thread is assigned a small,
 fixed-size execution stack just under @w{4 kB} in size.  The kernel


@@ -82,6 +88,94 @@ in @file{threads/malloc.c}.  Note that the page allocator doles out
 limit.  If you need larger chunks, consider using a linked structure
 instead.
 
 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 paging.c
+@itemx paging.h
+Initializes the kernel page table.  FIXME
+
+@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
 
 @node Debugging versus Testing
 @section Debugging versus Testing
 


@@ -111,6 +205,7 @@ 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.
 
 doesn't give you any greater confidence in your code's correctness
 than does running it only once.
 

+FIXME

 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
 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


@@ -140,24 +235,25 @@ 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
 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
 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
 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
 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
 
 @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()}.
 
 Improve the implementation of the timer device defined in
 @file{devices/timer.c} by reimplementing @code{timer_sleep()}.


@@ -182,18 +278,30 @@ in milliseconds or some other unit.
 @node Problem 1-2 Join
 @section Problem 1-2: Join
 
 @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.
 
 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},
 
 A thread need not ever be joined.  Your solution should properly free
 all of a thread's resources, including its @code{struct thread},


@@ -206,9 +314,8 @@ 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.
 
 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
 
 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


@@ -221,15 +328,23 @@ Be careful to program this function correctly.  You will need its
 functionality for project 2.
 
 @node Problem 1-3 Priority Scheduling
 functionality for project 2.
 
 @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 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},
+@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.
 
 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
 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


@@ -253,17 +368,17 @@ 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
 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
 
 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
 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.
 
 Implement Solaris's multilevel feedback queue scheduler (MLFQS) to
 reduce the average response time for running jobs on your system.


@@ -272,14 +387,19 @@ the MLFQS requirements.
 
 Demonstrate that your scheduling algorithm reduces response time
 relative to the original Pintos scheduling algorithm (round robin) for
 
 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).
 
 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).
 

-@node Threads FAQ, Multilevel Feedback Scheduling, Problem 1-4 Advanced Scheduler, Project 1--Threads
+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
 
 @enumerate 1
 @section FAQ
 
 @enumerate 1


@@ -308,6 +428,13 @@ be truly temporary).
 
 There is no need to edit the @file{Makefile}s to add a @file{.h} file.
 
 
 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?}
 @item
 @b{If a thread finishes, should its children be terminated immediately,
 or should they finish normally?}


@@ -340,8 +467,11 @@ help you determine if you are using interrupts too haphazardly.  We
 want to emphasize that there are only limited cases where this is
 appropriate.
 
 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
 @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.
 
 
 You might find it necessary in some solutions to the Alarm problem.
 


@@ -415,7 +545,6 @@ gracefully and should act as a no-op.
 @b{Doesn't the priority scheduling lead to starvation? Or do I have to
 implement some sort of aging?}
 
 @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
 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


@@ -529,11 +658,10 @@ The correct behavior is to immediately yield the processor.  Your
 solution must act this way.
 
 @item
 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
 @end enumerate
 
 @item Advanced Scheduler FAQs


@@ -546,6 +674,9 @@ 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.
 
 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?}
 
 @item
 @b{Do I have to modify the dispatch table?}
 


@@ -582,5 +713,3 @@ However, you are free to do so.
 No.  Hard-coding the dispatch table values is fine.
 @end enumerate
 @end enumerate
 No.  Hard-coding the dispatch table values is fine.
 @end enumerate
 @end enumerate

-
-@include mlfqs.texi