1 @node Project 1--Threads, Project 2--User Programs, Pintos Tour, Top
2 @chapter Project 1: Threads
4 In this assignment, we give you a minimally functional thread system.
5 Your job is to extend the functionality of this system to gain a
6 better understanding of synchronization problems. Additionally, you
7 will use at least part of this increased functionality in future
10 You will be working in primarily in the @file{threads} directory for
11 this assignment, with some work in the @file{devices} directory on the
12 side. Compilation should be done in the @file{threads} directory.
14 Before you read the description of this project, you should read all
15 of the following sections: @ref{Introduction}, @ref{Coding Standards},
16 @ref{Project Documentation}, @ref{Debugging Tools}, and
17 @ref{Development Tools}. You should at least skim the material in
18 @ref{Threads Tour}. To complete this project you will also need to
19 read @ref{Multilevel Feedback Scheduling}.
22 * Understanding Threads::
24 * Debugging versus Testing::
26 * Problem 1-1 Alarm Clock::
28 * Problem 1-3 Priority Scheduling::
29 * Problem 1-4 Advanced Scheduler::
33 @node Understanding Threads
34 @section Understanding Threads
36 The first step is to read and understand the initial thread system.
37 Pintos, by default, implements thread creation and thread completion,
38 a simple scheduler to switch between threads, and synchronization
39 primitives (semaphores, locks, and condition variables).
41 However, there's a lot of magic going on in some of this code, so if
42 you haven't already compiled and run the base system, as described in
43 the introduction (@pxref{Introduction}), you should do so now. You
44 can read through parts of the source code by hand to see what's going
45 on. If you like, you can add calls to @func{printf} almost
46 anywhere, then recompile and run to see what happens and in what
47 order. You can also run the kernel in a debugger and set breakpoints
48 at interesting spots, single-step through code and examine data, and
49 so on. @xref{i386-elf-gdb}, for more information.
51 When a thread is created, you are creating a new context to be
52 scheduled. You provide a function to be run in this context as an
53 argument to @func{thread_create}. The first time the thread is
54 scheduled and runs, it will start from the beginning of that function
55 and execute it in the context. When that function returns, that thread
56 completes. Each thread, therefore, acts like a mini-program running
57 inside Pintos, with the function passed to @func{thread_create}
58 acting like @func{main}.
60 At any given time, Pintos is running exactly one thread, with the
61 others switched out. The scheduler decides which thread to run next
62 when it needs to switch between them. (If no thread is ready to run
63 at any given time, then the special ``idle'' thread runs.) The
64 synchronization primitives are used to force context switches when one
65 thread needs to wait for another thread to do something.
67 The exact mechanics of a context switch are pretty gruesome and have
68 been provided for you in @file{threads/switch.S} (this is 80@var{x}86
69 assembly; don't worry about understanding it). It involves saving the
70 state of the currently running thread and restoring the state of the
71 thread we're switching to.
73 Using the @command{gdb} debugger, slowly trace through a context
74 switch to see what happens (@pxref{i386-elf-gdb}). You can set a
75 breakpoint on the @func{schedule} function to start out, and then
76 single-step from there.@footnote{@command{gdb} might tell you that
77 @func{schedule} doesn't exist, which is arguably a @command{gdb} bug.
78 You can work around this by setting the breakpoint by filename and
79 line number, e.g.@: @code{break thread.c:@var{ln}} where @var{ln} is
80 the line number of the first declaration in @func{schedule}.} Be sure
81 to keep track of each thread's address
82 and state, and what procedures are on the call stack for each thread.
83 You will notice that when one thread calls @func{switch_threads},
84 another thread starts running, and the first thing the new thread does
85 is to return from @func{switch_threads}. We realize this comment will
86 seem cryptic to you at this point, but you will understand threads
87 once you understand why the @func{switch_threads} that gets called is
88 different from the @func{switch_threads} that returns.
90 @strong{Warning}: In Pintos, each thread is assigned a small,
91 fixed-size execution stack just under @w{4 kB} in size. The kernel
92 does try to detect stack overflow, but it cannot always succeed. You
93 may cause bizarre problems, such as mysterious kernel panics, if you
94 declare large data structures as non-static local variables,
95 e.g. @samp{int buf[1000];}. Alternatives to stack allocation include
96 the page allocator in @file{threads/palloc.c} and the block allocator
97 in @file{threads/malloc.c}. Note that the page allocator doles out
98 @w{4 kB} chunks and that @func{malloc} has a @w{2 kB} block size
99 limit. If you need larger chunks, consider using a linked structure
105 Here is a brief overview of the files in the @file{threads}
106 directory. You will not need to modify most of this code, but the
107 hope is that presenting this overview will give you a start on what
113 The kernel loader. Assembles to 512 bytes of code and data that the
114 PC BIOS loads into memory and which in turn loads the kernel into
115 memory, does basic processor initialization, and jumps to the
116 beginning of the kernel. You should not need to look at this code or
120 The linker script used to link the kernel. Sets the load address of
121 the kernel and arranges for @file{start.S} to be at the very beginning
122 of the kernel image. Again, you should not need to look at this code
123 or modify it, but it's here in case you're curious.
126 Jumps to @func{main}.
130 Kernel initialization, including @func{main}, the kernel's ``main
131 program.'' You should look over @func{main} at least to see what
136 Basic thread support. Much of your work will take place in these
137 files. @file{thread.h} defines @struct{thread}, which you will
138 modify in the first three projects.
142 Assembly language routine for switching threads. Already discussed
147 Page allocator, which hands out system memory in multiples of 4 kB
152 A very simple implementation of @func{malloc} and @func{free} for
157 Basic interrupt handling and functions for turning interrupts on and
162 A Perl program that outputs assembly for low-level interrupt handling.
166 Basic synchronization primitives: semaphores, locks, and condition
167 variables. You will need to use these for synchronization through all
172 Test code. For project 1, you will replace this file with your test
176 Functions for I/O port access. This is mostly used by source code in
177 the @file{devices} directory that you won't have to touch.
180 Functions and macros related to memory management, including page
181 directories and page tables. This will be more important to you in
182 project 3. For now, you can ignore it.
191 @subsection @file{devices} code
193 The basic threaded kernel also includes these files in the
194 @file{devices} directory:
199 System timer that ticks, by default, 100 times per second. You will
200 modify this code in Problem 1-1.
204 VGA display driver. Responsible for writing text to the screen.
205 You should have no need to look at this code. @func{printf} will
206 call into the VGA display driver for you, so there's little reason to
207 call this code yourself.
211 Serial port driver. Again, @func{printf} calls this code for you,
212 so you don't need to do so yourself. Feel free to look through it if
217 Supports reading and writing sectors on up to 4 IDE disks. This won't
218 actually be used until project 2.
222 Interrupt queue, for managing a circular queue that both kernel
223 threads and interrupt handlers want to access. Used by the keyboard
228 @subsection @file{lib} files
230 Finally, @file{lib} and @file{lib/kernel} contain useful library
231 routines. (@file{lib/user} will be used by user programs, starting in
232 project 2, but it is not part of the kernel.) Here's a few more
249 Implementation of the standard C library. @xref{C99}, for information
250 on a few recently introduced pieces of the C library that you might
251 not have encountered before. @xref{Unsafe String Functions}, for
252 information on what's been intentionally left out for safety.
256 Functions and macros to aid debugging. @xref{Debugging Tools}, for
261 Pseudo-random number generator.
267 System call numbers. Not used until project 2.
271 Doubly linked list implementation. Used all over the Pintos code, and
272 you'll probably want to use it a few places yourself in project 1.
274 @item kernel/bitmap.c
275 @itemx kernel/bitmap.h
276 Bitmap implementation. You can use this in your code if you like, but
277 you probably won't have any need for project 1.
281 Hash table implementation. Likely to come in handy for project 3.
283 @item kernel/console.c
284 @itemx kernel/console.h
285 Implements @func{printf} and a few other functions.
288 @node Debugging versus Testing
289 @section Debugging versus Testing
291 When you're debugging code, it's useful to be able to be able to run a
292 program twice and have it do exactly the same thing. On second and
293 later runs, you can make new observations without having to discard or
294 verify your old observations. This property is called
295 ``reproducibility.'' The simulator we use, Bochs, can be set up for
296 reproducibility, and that's the way that @command{pintos} invokes it
299 Of course, a simulation can only be reproducible from one run to the
300 next if its input is the same each time. For simulating an entire
301 computer, as we do, this means that every part of the computer must be
302 the same. For example, you must use the same disks, the same version
303 of Bochs, and you must not hit any keys on the keyboard (because you
304 could not be sure to hit them at exactly the same point each time)
307 While reproducibility is useful for debugging, it is a problem for
308 testing thread synchronization, an important part of this project. In
309 particular, when Bochs is set up for reproducibility, timer interrupts
310 will come at perfectly reproducible points, and therefore so will
311 thread switches. That means that running the same test several times
312 doesn't give you any greater confidence in your code's correctness
313 than does running it only once.
315 So, to make your code easier to test, we've added a feature, called
316 ``jitter,'' to Bochs, that makes timer interrupts come at random
317 intervals, but in a perfectly predictable way. In particular, if you
318 invoke @command{pintos} with the option @option{-j @var{seed}}, timer
319 interrupts will come at irregularly spaced intervals. Within a single
320 @var{seed} value, execution will still be reproducible, but timer
321 behavior will change as @var{seed} is varied. Thus, for the highest
322 degree of confidence you should test your code with many seed values.
324 On the other hand, when Bochs runs in reproducible mode, timings are not
325 realistic, meaning that a ``one-second'' delay may be much shorter or
326 even much longer than one second. You can invoke @command{pintos} with
327 a different option, @option{-r}, to make it set up Bochs for realistic
328 timings, in which a one-second delay should take approximately one
329 second of real time. Simulation in real-time mode is not reproducible,
330 and options @option{-j} and @option{-r} are mutually exclusive.
337 There should be no busy waiting in any of your solutions to this
338 assignment. We consider a tight loop that calls @func{thread_yield}
339 to be one form of busy waiting.
342 Proper synchronization is an important part of the solutions to these
343 problems. It is tempting to synchronize all your code by turning off
344 interrupts with @func{intr_disable} or @func{intr_set_level}, because
345 this eliminates concurrency and thus the possibility for race
346 conditions, but @strong{don't}. Instead, use semaphores, locks, and
347 condition variables to solve the bulk of your synchronization
348 problems. Read the tour section on synchronization
349 (@pxref{Synchronization}) or the comments in @file{threads/synch.c} if
350 you're unsure what synchronization primitives may be used in what
353 You might run into a few situations where interrupt disabling is the
354 best way to handle synchronization. If so, you need to explain your
355 rationale in your design documents. If you're unsure whether a given
356 situation justifies disabling interrupts, talk to the TAs, who can
357 help you decide on the right thing to do.
359 Disabling interrupts can be useful for debugging, if you want to make
360 sure that a section of code is not interrupted. You should remove
361 debugging code before turning in your project.
364 All parts of this assignment are required if you intend to earn full
365 credit on this project. However, some will be more important in
370 Problem 1-1 (Alarm Clock) could be handy for later projects, but it is
371 not strictly required.
374 Problem 1-2 (Join) will be needed for future projects. We don't give
375 out solutions, so to avoid extra work later you should make sure that
376 your implementation of @func{thread_join} works correctly.
379 Problems 1-3 and 1-4 won't be needed for later projects.
383 Problem 1-4 (MLFQS) builds on the features you implement in Problem
384 1-3. You should have Problem 1-3 fully working before you begin to
388 In the past, many groups divided the assignment into pieces, then each
389 group member worked on his or her piece until just before the
390 deadline, at which time the group reconvened to combine their code and
391 submit. @strong{This is a bad idea. We do not recommend this
392 approach.} Groups that do this often find that two changes conflict
393 with each other, requiring lots of last-minute debugging. Some groups
394 who have done this have turned in code that did not even successfully
397 Instead, we recommend integrating your team's changes early and often,
398 using a source code control system such as CVS (@pxref{CVS}) or a
399 group collaboration site such as SourceForge (@pxref{SourceForge}).
400 This is less likely to produce surprises, because everyone can see
401 everyone else's code as it is written, instead of just when it is
402 finished. These systems also make it possible to review changes and,
403 when a change introduces a bug, drop back to working versions of code.
406 @node Problem 1-1 Alarm Clock
407 @section Problem 1-1: Alarm Clock
409 Improve the implementation of the timer device defined in
410 @file{devices/timer.c} by reimplementing @func{timer_sleep}.
411 Threads call @code{timer_sleep(@var{x})} to suspend execution until
412 time has advanced by at least @w{@var{x} timer ticks}. This is
413 useful for threads that operate in real-time, for example, for
414 blinking the cursor once per second. There is no requirement that
415 threads start running immediately after waking up; just put them on
416 the ready queue after they have waited for approximately the right
419 A working implementation of this function is provided. However, the
420 version provided is poor, because it ``busy waits,'' that is, it spins
421 in a tight loop checking the current time until the current time has
422 advanced far enough. This is undesirable because it wastes time that
423 could potentially be used more profitably by another thread. Your
424 solution should not busy wait.
426 The argument to @func{timer_sleep} is expressed in timer ticks, not in
427 milliseconds or any another unit. There are @code{TIMER_FREQ} timer
428 ticks per second, where @code{TIMER_FREQ} is a macro defined in
429 @code{devices/timer.h}.
431 Separate functions @func{timer_msleep}, @func{timer_usleep}, and
432 @func{timer_nsleep} do exist for sleeping a specific number of
433 milliseconds, microseconds, or nanoseconds, respectively, but these will
434 call @func{timer_sleep} automatically when necessary. You do not need
437 If your delays seem too short or too long, reread the explanation of the
438 @option{-r} option to @command{pintos} (@pxref{Debugging versus
441 @node Problem 1-2 Join
442 @section Problem 1-2: Join
444 Implement @code{thread_join(tid_t)} in @file{threads/thread.c}. There
445 is already a prototype for it in @file{threads/thread.h}, which you
446 should not change. This function causes the currently running thread
447 to block until the thread whose thread id is passed as an argument
448 exits. If @var{A} is the running thread and @var{B} is the argument,
449 then we say that ``@var{A} joins @var{B}.''
451 Incidentally, we don't use @code{struct thread *} as
452 @func{thread_join}'s parameter type because a thread pointer is not
453 unique over time. That is, when a thread dies, its memory may be,
454 whether immediately or much later, reused for another thread. If
455 thread @var{A} over time had two children @var{B} and @var{C} that
456 were stored at the same address, then @code{thread_join(@var{B})} and
457 @code{thread_join(@var{C})} would be ambiguous. Introducing a thread
458 id or @dfn{tid}, represented by type @code{tid_t}, that is
459 intentionally unique over time solves the problem. The provided code
460 uses an @code{int} for @code{tid_t}, but you may decide you prefer to
463 The model for @func{thread_join} is the @command{wait} system call
464 in Unix-like systems. (Try reading the manpages.) That system call
465 can only be used by a parent process to wait for a child's death. You
466 should implement @func{thread_join} to have the same restriction.
467 That is, a thread may only join its immediate children.
469 A thread need not ever be joined. Your solution should properly free
470 all of a thread's resources, including its @struct{thread},
471 whether it is ever joined or not, and regardless of whether the child
472 exits before or after its parent. That is, a thread should be freed
473 exactly once in all cases.
475 Joining a given thread is idempotent. That is, joining a thread T
476 multiple times is equivalent to joining it once, because T has already
477 exited at the time of the later joins. Thus, joins on T after the
478 first should return immediately.
480 Calling @func{thread_join} on an thread that is not the caller's
481 child should cause the caller to return immediately.
483 Consider all the ways a join can occur: nested joins (@var{A} joins
484 @var{B}, then @var{B} joins @var{C}), multiple joins (@var{A} joins
485 @var{B}, then @var{A} joins @var{C}), and so on. Does your join work
486 if @func{thread_join} is called on a thread that has not yet been
487 scheduled for the first time? You should handle all of these cases.
488 Write test code that demonstrates the cases your join works for.
489 Don't overdo the output volume, please!
491 Be careful to program this function correctly. You will need its
492 functionality for project 2.
494 Once you've implemented @func{thread_join}, define
495 @code{THREAD_JOIN_IMPLEMENTED} in @file{constants.h}.
496 @xref{Conditional Compilation}, for more information.
498 @node Problem 1-3 Priority Scheduling
499 @section Problem 1-3: Priority Scheduling
501 Implement priority scheduling in Pintos. Priority scheduling is a key
502 building block for real-time systems. Implement functions
503 @func{thread_set_priority} to set the priority of the running thread
504 and @func{thread_get_priority} to get the running thread's priority.
505 (This API only allows a thread to examine and modify its own
506 priority.) There are already prototypes for these functions in
507 @file{threads/thread.h}, which you should not change.
509 Thread priority ranges from @code{PRI_MIN} (0) to @code{PRI_MAX} (59).
510 The initial thread priority is passed as an argument to
511 @func{thread_create}. If there's no reason to choose another
512 priority, use @code{PRI_DEFAULT} (29). The @code{PRI_} macros are
513 defined in @file{threads/thread.h}, and you should not change their
516 When a thread is added to the ready list that has a higher priority
517 than the currently running thread, the current thread should
518 immediately yield the processor to the new thread. Similarly, when
519 threads are waiting for a lock, semaphore or condition variable, the
520 highest priority waiting thread should be woken up first. A thread
521 may set its priority at any time.
523 One issue with priority scheduling is ``priority inversion'': if a
524 high priority thread needs to wait for a low priority thread (for
525 instance, for a lock held by a low priority thread, or in
526 @func{thread_join} for a thread to complete), and a middle priority
527 thread is on the ready list, then the high priority thread will never
528 get the CPU because the low priority thread will not get any CPU time.
529 A partial fix for this problem is to have the waiting thread
530 ``donate'' its priority to the low priority thread while it is holding
531 the lock, then recall the donation once it has acquired the lock.
534 You will need to account for all different orders in which priority
535 donation and inversion can occur. Be sure to handle multiple
536 donations, in which multiple priorities are donated to a thread. You
537 must also handle nested donation: given high, medium, and low priority
538 threads @var{H}, @var{M}, and @var{L}, respectively, if @var{H} is
539 waiting on a lock that @var{M} holds and @var{M} is waiting on a lock
540 that @var{L} holds, then both @var{M} and @var{L} should be boosted to
543 You only need to implement priority donation when a thread is waiting
544 for a lock held by a lower-priority thread. You do not need to
545 implement this fix for semaphores, condition variables, or joins,
546 although you are welcome to do so. However, you do need to implement
547 priority scheduling in all cases.
549 You may assume a static priority for priority donation, that is, it is
550 not necessary to ``re-donate'' a thread's priority if it changes
551 (although you are free to do so).
553 @node Problem 1-4 Advanced Scheduler
554 @section Problem 1-4: Advanced Scheduler
556 Implement Solaris's multilevel feedback queue scheduler (MLFQS) to
557 reduce the average response time for running jobs on your system.
558 @xref{Multilevel Feedback Scheduling}, for a detailed description of
559 the MLFQS requirements.
561 Demonstrate that your scheduling algorithm reduces response time
562 relative to the original Pintos scheduling algorithm (round robin) for
563 at least one workload of your own design (i.e.@: in addition to the
566 You must write your code so that we can turn the MLFQS on and off at
567 compile time. By default, it must be off, but we must be able to turn
568 it on by inserting the line @code{#define MLFQS 1} in
569 @file{constants.h}. @xref{Conditional Compilation}, for details.
576 @b{I am adding a new @file{.h} or @file{.c} file. How do I fix the
577 @file{Makefile}s?}@anchor{Adding c or h Files}
579 To add a @file{.c} file, edit the top-level @file{Makefile.build}.
580 You'll want to add your file to variable @samp{@var{dir}_SRC}, where
581 @var{dir} is the directory where you added the file. For this
582 project, that means you should add it to @code{threads_SRC}, or
583 possibly @code{devices_SRC} if you put in the @file{devices}
584 directory. Then run @code{make}. If your new file doesn't get
585 compiled, run @code{make clean} and then try again.
587 When you modify the top-level @file{Makefile.build}, the modified
588 version should be automatically copied to
589 @file{threads/build/Makefile} when you re-run make. The opposite is
590 not true, so any changes will be lost the next time you run @code{make
591 clean} from the @file{threads} directory. Therefore, you should
592 prefer to edit @file{Makefile.build} (unless your changes are meant to
595 There is no need to edit the @file{Makefile}s to add a @file{.h} file.
598 @b{How do I write my test cases?}
600 Test cases should be replacements for the existing @file{test.c}
601 file. Put them in a @file{threads/testcases} directory.
602 @xref{TESTCASE}, for more information.
605 @b{Why can't I disable interrupts?}
607 Turning off interrupts should only be done for short amounts of time,
608 or else you end up losing important things such as disk or input
609 events. Turning off interrupts also increases the interrupt handling
610 latency, which can make a machine feel sluggish if taken too far.
611 Therefore, in general, setting the interrupt level should be used
612 sparingly. Also, any synchronization problem can be easily solved by
613 turning interrupts off, since while interrupts are off, there is no
614 concurrency, so there's no possibility for race conditions.
616 To make sure you understand concurrency well, we are discouraging you
617 from taking this shortcut at all in your solution. If you are unable
618 to solve a particular synchronization problem with semaphores, locks,
619 or conditions, or think that they are inadequate for a particular
620 reason, you may turn to disabling interrupts. If you want to do this,
621 we require in your design document a complete justification and
622 scenario (i.e.@: exact sequence of events) to show why interrupt
623 manipulation is the best solution. If you are unsure, the TAs can
624 help you determine if you are using interrupts too haphazardly. We
625 want to emphasize that there are only limited cases where this is
628 You might find @file{devices/intq.h} and its users to be an
629 inspiration or source of rationale.
632 @b{Where might interrupt-level manipulation be appropriate?}
634 You might find it necessary in some solutions to the Alarm problem.
636 You might want it at one small point for the priority scheduling
637 problem. Note that it is not required to use interrupts for these
638 problems. There are other, equally correct solutions that do not
639 require interrupt manipulation. However, if you do manipulate
640 interrupts and @strong{correctly and fully document it} in your design
641 document, we will allow limited use of interrupt disabling.
644 @b{What does ``warning: no previous prototype for `@var{function}''
647 It means that you defined a non-@code{static} function without
648 preceding it by a prototype. Because non-@code{static} functions are
649 intended for use by other @file{.c} files, for safety they should be
650 prototyped in a header file included before their definition. To fix
651 the problem, add a prototype in a header file that you include, or, if
652 the function isn't actually used by other @file{.c} files, make it
657 * Problem 1-1 Alarm Clock FAQ::
658 * Problem 1-2 Join FAQ::
659 * Problem 1-3 Priority Scheduling FAQ::
660 * Problem 1-4 Advanced Scheduler FAQ::
663 @node Problem 1-1 Alarm Clock FAQ
664 @subsection Problem 1-1: Alarm Clock FAQ
668 @b{Why can't I use most synchronization primitives in an interrupt
671 As you've discovered, you cannot sleep in an external interrupt
672 handler. Since many lock, semaphore, and condition variable functions
673 attempt to sleep, you won't be able to call those in
674 @func{timer_interrupt}. You may still use those that never sleep.
676 Having said that, you need to make sure that global data does not get
677 updated by multiple threads simultaneously executing
678 @func{timer_sleep}. Here are some pieces of information to think
683 Interrupts are turned off while @func{timer_interrupt} runs. This
684 means that @func{timer_interrupt} will not be interrupted by a
685 thread running in @func{timer_sleep}.
688 A thread in @func{timer_sleep}, however, can be interrupted by a
689 call to @func{timer_interrupt}, except when that thread has turned
693 Examples of synchronization mechanisms have been presented in lecture.
694 Going over these examples should help you understand when each type is
695 useful or needed. @xref{Synchronization}, for specific information
696 about synchronization in Pintos.
700 @b{What about timer overflow due to the fact that times are defined as
701 integers? Do I need to check for that?}
703 Don't worry about the possibility of timer values overflowing. Timer
704 values are expressed as signed 63-bit numbers, which at 100 ticks per
705 second should be good for almost 2,924,712,087 years.
708 @b{The test program mostly works but reports a few out-of-order
709 wake ups. I think it's a problem in the test program. What gives?}
710 @anchor{Out of Order 1-1}
712 This test is inherently full of race conditions. On a real system it
713 wouldn't work perfectly all the time either. There are a few ways you
714 can help it work more reliably:
718 Make time slices longer by increasing @code{TIME_SLICE} in
719 @file{timer.c} to a large value, such as 100.
722 Make the timer tick more slowly by decreasing @code{TIMER_FREQ} in
723 @file{timer.h} to its minimum value of 19.
726 The former two changes are only desirable for testing problem 1-1 and
727 possibly 1-3. You should revert them before working on other parts
728 of the project or turn in the project.
731 @b{Should @file{p1-1.c} be expected to work with the MLFQS turned on?}
733 No. The MLFQS will adjust priorities, changing thread ordering.
736 @node Problem 1-2 Join FAQ
737 @subsection Problem 1-2: Join FAQ
741 @b{Am I correct to assume that once a thread is deleted, it is no
742 longer accessible by the parent (i.e.@: the parent can't call
743 @code{thread_join(child)})?}
745 A parent joining a child that has completed should be handled
746 gracefully and should act as a no-op.
749 @node Problem 1-3 Priority Scheduling FAQ
750 @subsection Problem 1-3: Priority Scheduling FAQ
754 @b{Doesn't the priority scheduling lead to starvation? Or do I have to
755 implement some sort of aging?}
757 It is true that strict priority scheduling can lead to starvation
758 because thread may not run if a higher-priority thread is runnable.
759 In this problem, don't worry about starvation or any sort of aging
760 technique. Problem 4 will introduce a mechanism for dynamically
761 changing thread priorities.
763 This sort of scheduling is valuable in real-time systems because it
764 offers the programmer more control over which jobs get processing
765 time. High priorities are generally reserved for time-critical
766 tasks. It's not ``fair,'' but it addresses other concerns not
767 applicable to a general-purpose operating system.
770 @b{After a lock has been released, does the program need to switch to
771 the highest priority thread that needs the lock (assuming that its
772 priority is higher than that of the current thread)?}
774 When a lock is released, the highest priority thread waiting for that
775 lock should be unblocked and put on the ready to run list. The
776 scheduler should then run the highest priority thread on the ready
780 @b{If a thread calls @func{thread_yield} and then it turns out that
781 it has higher priority than any other threads, does the high-priority
782 thread continue running?}
784 Yes. If there is a single highest-priority thread, it continues
785 running until it blocks or finishes, even if it calls
789 @b{If the highest priority thread is added to the ready to run list it
790 should start execution immediately. Is it immediate enough if I
791 wait until next timer interrupt occurs?}
793 The highest priority thread should run as soon as it is runnable,
794 preempting whatever thread is currently running.
797 @b{What happens to the priority of the donating thread? Do the priorities
800 No. Priority donation only changes the priority of the low-priority
801 thread. The donating thread's priority stays unchanged. Also note
802 that priorities aren't additive: if thread A (with priority 5) donates
803 to thread B (with priority 3), then B's new priority is 5, not 8.
806 @b{Can a thread's priority be changed while it is sitting on the ready
809 Yes. Consider this case: low-priority thread L currently has a lock
810 that high-priority thread H wants. H donates its priority to L (the
811 lock holder). L finishes with the lock, and then loses the CPU and is
812 moved to the ready queue. Now L's old priority is restored while it
813 is in the ready queue.
816 @b{Can a thread's priority change while it is sitting on the queue of a
819 Yes. Same scenario as above except L gets blocked waiting on a new
820 lock when H restores its priority.
823 @b{Why is @file{p1-3.c}'s FIFO test skipping some threads? I know my
824 scheduler is round-robin'ing them like it's supposed to. Our output
825 starts out okay, but toward the end it starts getting out of order.}
827 The usual problem is that the serial output buffer fills up. This is
828 causing serial_putc() to block in thread @var{A}, so that thread
829 @var{B} is scheduled. Thread @var{B} immediately tries to do output
830 of its own and blocks on the serial lock (which is held by thread
831 @var{A}). Now that we've wasted some time in scheduling and locking,
832 typically some characters have been drained out of the serial buffer
833 by the interrupt handler, so thread @var{A} can continue its output.
834 After it finishes, though, some other thread (not @var{B}) is
835 scheduled, because thread @var{B} was already scheduled while we
836 waited for the buffer to drain.
838 There's at least one other possibility. Context switches are being
839 invoked by the test when it explicitly calls @func{thread_yield}.
840 However, the time slice timer is still alive and so, every tick (by
841 default), a thread gets switched out (caused by @func{timer_interrupt}
842 calling @func{intr_yield_on_return}) before it gets a chance to run
843 @func{printf}, effectively skipping it. If we use a different jitter
844 value, the same behavior is seen where a thread gets started and
845 switched out completely.
847 Normally you can fix these problems using the same techniques
848 suggested on problem 1-1 (@pxref{Out of Order 1-1}).
851 @b{What happens when a thread is added to the ready list which has
852 higher priority than the currently running thread?}
854 The correct behavior is to immediately yield the processor. Your
855 solution must act this way.
858 @b{What should @func{thread_get_priority} return in a thread while
859 its priority has been increased by a donation?}
861 The higher (donated) priority.
864 @b{Should @file{p1-3.c} be expected to work with the MLFQS turned on?}
866 No. The MLFQS will adjust priorities, changing thread ordering.
869 @b{@func{printf} in @func{sema_up} or @func{sema_down} makes the
872 Yes. These functions are called before @func{printf} is ready to go.
873 You could add a global flag initialized to false and set it to true
874 just before the first @func{printf} in @func{main}. Then modify
875 @func{printf} itself to return immediately if the flag isn't set.
878 @node Problem 1-4 Advanced Scheduler FAQ
879 @subsection Problem 1-4: Advanced Scheduler FAQ
883 @b{What is the interval between timer interrupts?}
885 Timer interrupts occur @code{TIMER_FREQ} times per second. You can
886 adjust this value by editing @file{devices/timer.h}. The default is
889 You can also adjust the number of timer ticks per time slice by
890 modifying @code{TIME_SLICE} in @file{devices/timer.c}.
893 @b{Do I have to modify the dispatch table?}
895 No, although you are allowed to. It is possible to complete
896 this problem (i.e.@: demonstrate response time improvement)
900 @b{When the scheduler changes the priority of a thread, how does this
901 affect priority donation?}
903 Short (official) answer: Don't worry about it. Your priority donation
904 code may assume static priority assignment.
906 Longer (unofficial) opinion: If you wish to take this into account,
907 however, your design may end up being ``cleaner.'' You have
908 considerable freedom in what actually takes place. I believe what
909 makes the most sense is for scheduler changes to affect the
910 ``original'' (non-donated) priority. This change may actually be
911 masked by the donated priority. Priority changes should only
912 propagate with donations, not ``backwards'' from donees to donors.
915 @b{What is meant by ``static priority''?}
917 Once thread A has donated its priority to thread B, if thread A's
918 priority changes (due to the scheduler) while the donation still
919 exists, you do not have to change thread B's donated priority.
920 However, you are free to do so.
923 @b{Do I have to make my dispatch table user-configurable?}
925 No. Hard-coding the dispatch table values is fine.