1 @node Project 1--Threads, Project 2--User Programs, Introduction, 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.
15 * Understanding Threads::
17 * Debugging versus Testing::
19 * Problem 1-1 Alarm Clock::
21 * Problem 1-3 Priority Scheduling::
22 * Problem 1-4 Advanced Scheduler::
26 @node Understanding Threads
27 @section Understanding Threads
29 The first step is to read and understand the initial thread system.
30 Pintos, by default, implements thread creation and thread completion,
31 a simple scheduler to switch between threads, and synchronization
32 primitives (semaphores, locks, and condition variables).
34 However, there's a lot of magic going on in some of this code, so if
35 you haven't already compiled and run the base system, as described in
36 the introduction (@pxref{Introduction}), you should do so now. You
37 can read through parts of the source code by hand to see what's going
38 on. If you like, you can add calls to @code{printf()} almost
39 anywhere, then recompile and run to see what happens and in what
40 order. You can also run the kernel in a debugger and set breakpoints
41 at interesting spots, single-step through code and examine data, and
42 so on. @xref{i386-elf-gdb}, for more information.
44 When a thread is created, you are creating a new context to be
45 scheduled. You provide a function to be run in this context as an
46 argument to @code{thread_create()}. The first time the thread is
47 scheduled and runs, it will start from the beginning of that function
48 and execute it in the context. When that function returns, that thread
49 completes. Each thread, therefore, acts like a mini-program running
50 inside Pintos, with the function passed to @code{thread_create()}
51 acting like @code{main()}.
53 At any given time, Pintos is running exactly one thread, with the
54 others switched out. The scheduler decides which thread to run next
55 when it needs to switch between them. (If no thread is ready to run
56 at any given time, then the special ``idle'' thread runs.) The
57 synchronization primitives are used to force context switches when one
58 thread needs to wait for another thread to do something.
60 The exact mechanics of a context switch are pretty gruesome and have
61 been provided for you in @file{threads/switch.S} (this is 80@var{x}86
62 assembly; don't worry about understanding it). It involves saving the
63 state of the currently running thread and restoring the state of the
64 thread we're switching to.
66 Using the @command{gdb} debugger, slowly trace through a context
67 switch to see what happens (@pxref{i386-elf-gdb}). You can set a
68 breakpoint on the @code{schedule()} function to start out, and then
69 single-step from there. Be sure to keep track of each thread's
70 address and state, and what procedures are on the call stack for each
71 thread. You will notice that when one thread calls
72 @code{switch_threads()}, another thread starts running, and the first
73 thing the new thread does is to return from
74 @code{switch_threads()}. We realize this comment will seem cryptic to
75 you at this point, but you will understand threads once you understand
76 why the @code{switch_threads()} that gets called is different from the
77 @code{switch_threads()} that returns.
79 @strong{Warning}: In Pintos, each thread is assigned a small,
80 fixed-size execution stack just under @w{4 kB} in size. The kernel
81 does try to detect stack overflow, but it cannot always succeed. You
82 may cause bizarre problems, such as mysterious kernel panics, if you
83 declare large data structures as non-static local variables,
84 e.g. @samp{int buf[1000];}. Alternatives to stack allocation include
85 the page allocator in @file{threads/palloc.c} and the block allocator
86 in @file{threads/malloc.c}. Note that the page allocator doles out
87 @w{4 kB} chunks and that @code{malloc()} has a @w{2 kB} block size
88 limit. If you need larger chunks, consider using a linked structure
94 Here is a brief overview of the files in the @file{threads}
95 directory. You will not need to modify most of this code, but the
96 hope is that presenting this overview will give you a start on what
102 The kernel loader. Assembles to 512 bytes of code and data that the
103 PC BIOS loads into memory and which in turn loads the kernel into
104 memory, does basic processor initialization, and jumps to the
105 beginning of the kernel. You should not need to look at this code or
109 The linker script used to link the kernel. Sets the load address of
110 the kernel and arranges for @file{start.S} to be at the very beginning
111 of the kernel image. Again, you should not need to look at this code
112 or modify it, but it's here in case you're curious.
115 Jumps to @code{main()}.
119 Kernel initialization, including @code{main()}, the kernel's ``main
120 program.'' You should look over @code{main()} at least to see what
125 Basic thread support. Much of your work will take place in these
126 files. @file{thread.h} defines @code{struct thread}, which you will
127 modify in the first three projects.
131 Assembly language routine for switching threads. Already discussed
136 Page allocator, which hands out system memory in multiples of 4 kB
141 A very simple implementation of @code{malloc()} and @code{free()} for
146 Basic interrupt handling and functions for turning interrupts on and
151 A Perl program that outputs assembly for low-level interrupt handling.
155 Basic synchronization primitives: semaphores, locks, and condition
156 variables. You will need to use these for synchronization through all
161 Test code. For project 1, you will replace this file with your test
165 Functions for I/O port access. This is mostly used by source code in
166 the @file{devices} directory that you won't have to touch.
169 Functions and macros related to memory management, including page
170 directories and page tables. This will be more important to you in
171 project 3. For now, you can ignore it.
180 @subsection @file{devices} code
182 The basic threaded kernel also includes these files in the
183 @file{devices} directory:
188 System timer that ticks, by default, 100 times per second. You will
189 modify this code in Problem 1-1.
193 VGA display driver. Responsible for writing text to the screen.
194 You should have no need to look at this code. @code{printf()} will
195 call into the VGA display driver for you, so there's little reason to
196 call this code yourself.
200 Serial port driver. Again, @code{printf()} calls this code for you,
201 so you don't need to do so yourself. Feel free to look through it if
206 Supports reading and writing sectors on up to 4 IDE disks. This won't
207 actually be used until project 2.
211 Interrupt queue, for managing a circular queue that both kernel
212 threads and interrupt handlers want to access. Used by the keyboard
217 @subsection @file{lib} files
219 Finally, @file{lib} and @file{lib/kernel} contain useful library
220 routines. (@file{lib/user} will be used by user programs, starting in
221 project 2, but it is not part of the kernel.) Here's a few more
238 Implementation of the standard C library. @xref{C99}, for information
239 on a few recently introduced pieces of the C library that you might
240 not have encountered before. @xref{Unsafe String Functions}, for
241 information on what's been intentionally left out for safety.
245 Functions and macros to aid debugging. @xref{Debugging Tools}, for
250 Pseudo-random number generator.
256 System call numbers. Not used until project 2.
260 Doubly linked list implementation. Used all over the Pintos code, and
261 you'll probably want to use it a few places yourself in project 1.
263 @item kernel/bitmap.c
264 @itemx kernel/bitmap.h
265 Bitmap implementation. You can use this in your code if you like, but
266 you probably won't have any need for project 1.
270 Hash table implementation. Likely to come in handy for project 3.
272 @item kernel/console.c
273 @itemx kernel/console.h
274 Implements @code{printf()} and a few other functions.
277 @node Debugging versus Testing
278 @section Debugging versus Testing
280 When you're debugging code, it's useful to be able to be able to run a
281 program twice and have it do exactly the same thing. On second and
282 later runs, you can make new observations without having to discard or
283 verify your old observations. This property is called
284 ``reproducibility.'' The simulator we use, Bochs, can be set up for
285 reproducibility, and that's the way that @command{pintos} invokes it.
287 Of course, a simulation can only be reproducible from one run to the
288 next if its input is the same each time. For simulating an entire
289 computer, as we do, this means that every part of the computer must be
290 the same. For example, you must use the same disks, the same version
291 of Bochs, and you must not hit any keys on the keyboard (because you
292 could not be sure to hit them at exactly the same point each time)
295 While reproducibility is useful for debugging, it is a problem for
296 testing thread synchronization, an important part of this project. In
297 particular, when Bochs is set up for reproducibility, timer interrupts
298 will come at perfectly reproducible points, and therefore so will
299 thread switches. That means that running the same test several times
300 doesn't give you any greater confidence in your code's correctness
301 than does running it only once.
303 So, to make your code easier to test, we've added a feature to Bochs
304 that makes timer interrupts come at random intervals, but in a
305 perfectly predictable way. In particular, if you invoke
306 @command{pintos} with the option @option{-j @var{seed}}, timer
307 interrupts will come at irregularly spaced intervals. Within a single
308 @var{seed} value, execution will still be reproducible, but timer
309 behavior will change as @var{seed} is varied. Thus, for the highest
310 degree of confidence you should test your code with many seed values.
315 There should be no busy-waiting in any of your solutions to this
316 assignment. Furthermore, resist the temptation to directly disable
317 interrupts in your solution by calling @code{intr_disable()} or
318 @code{intr_set_level()}, although you may find doing so to be useful
319 while debugging. Instead, use semaphores, locks and condition
320 variables to solve synchronization problems. Hint: read the comments
321 in @file{threads/synch.h} if you're unsure what synchronization
322 primitives may be used in what situations.
324 Given some designs of some problems, there may be one or two instances
325 in which it is appropriate to directly change the interrupt levels
326 instead of relying on the given synchroniztion primitives. This must
327 be justified in your @file{DESIGNDOC} file. If you're not sure you're
330 While all parts of this assignment are required if you intend to earn
331 full credit on this project, keep in mind that Problem 1-2 (Join) will
332 be needed for future assignments, so you'll want to get this one
333 right. We don't give out solutions, so you're stuck with your Join
334 code for the whole quarter. Problem 1-1 (Alarm Clock) could be very
335 handy, but not strictly required in the future. The upshot of all
336 this is that you should focus heavily on making sure that your
337 implementation of @code{thread_join()} works correctly, since if it's
338 broken, you will need to fix it for future assignments. The other
339 parts can be turned off in the future if you find you can't make them
342 Also keep in mind that Problem 1-4 (the MLFQS) builds on the features you
343 implement in Problem 1-3, so to avoid unnecessary code duplication, it
344 would be a good idea to divide up the work among your team members
345 such that you have Problem 1-3 fully working before you begin to tackle
348 @node Problem 1-1 Alarm Clock
349 @section Problem 1-1: Alarm Clock
351 Improve the implementation of the timer device defined in
352 @file{devices/timer.c} by reimplementing @code{timer_sleep()}.
353 Threads call @code{timer_sleep(@var{x})} to suspend execution until
354 time has advanced by at least @w{@var{x} timer ticks}. This is
355 useful for threads that operate in real-time, for example, for
356 blinking the cursor once per second. There is no requirement that
357 threads start running immediately after waking up; just put them on
358 the ready queue after they have waited for approximately the right
361 A working implementation of this function is provided. However, the
362 version provided is poor, because it ``busy waits,'' that is, it spins
363 in a tight loop checking the current time until the current time has
364 advanced far enough. This is undesirable because it wastes time that
365 could potentially be used more profitably by another thread. Your
366 solution should not busy wait.
368 The argument to @code{timer_sleep()} is expressed in timer ticks, not
369 in milliseconds or some other unit.
371 @node Problem 1-2 Join
372 @section Problem 1-2: Join
374 Implement @code{thread_join(tid_t)} in @file{threads/thread.c}. There
375 is already a prototype for it in @file{threads/thread.h}, which you
376 should not change. This function causes the currently running thread
377 to block until the thread whose thread id is passed as an argument
378 exits. If A is the running thread and B is the argument, then we say
379 that ``A joins B'' in this case.
381 Incidentally, we don't use @code{struct thread *} as
382 @file{thread_join()}'s parameter type because a thread pointer is not
383 unique over time. That is, when a thread dies, its memory may be,
384 whether immediately or much later, reused for another thread. If
385 thread A over time had two children B and C that were stored at the
386 same address, then @code{thread_join(@r{B})} and
387 @code{thread_join(@r{C})} would be ambiguous. Introducing a thread id
388 or @dfn{tid}, represented by type @code{tid_t}, that is intentionally
389 unique over time solves the problem. The provided code uses an
390 @code{int} for @code{tid_t}, but you may decide you prefer to use some
393 The model for @code{thread_join()} is the @command{wait} system call
394 in Unix-like systems. (Try reading the manpages.) That system call
395 can only be used by a parent process to wait for a child's death. You
396 should implement @code{thread_join()} to have the same restriction.
397 That is, a thread may only join its immediate children.
399 A thread need not ever be joined. Your solution should properly free
400 all of a thread's resources, including its @code{struct thread},
401 whether it is ever joined or not, and regardless of whether the child
402 exits before or after its parent. That is, a thread should be freed
403 exactly once in all cases.
405 Joining a given thread is idempotent. That is, joining a thread T
406 multiple times is equivalent to joining it once, because T has already
407 exited at the time of the later joins. Thus, joins on T after the
408 first should return immediately.
410 Calling @code{thread_join()} on an thread that is not the caller's
411 child should cause the caller to return immediately.
413 Consider all the ways a join can occur: nested joins (A joins B when B
414 is joined on C), multiple joins (A joins B, then A joins C), and so
415 on. Does your join work if @code{thread_join()} is called on a thread
416 that has not yet been scheduled for the first time? You should handle
417 all of these cases. Write test code that demonstrates the cases your
418 join works for. Don't overdo the output volume, please!
420 Be careful to program this function correctly. You will need its
421 functionality for project 2.
423 Once you've implemented @code{thread_join()}, define
424 @code{THREAD_JOIN_IMPLEMENTED} in @file{constants.h}.
425 @xref{Conditional Compilation}, for more information.
427 @node Problem 1-3 Priority Scheduling
428 @section Problem 1-3: Priority Scheduling
430 Implement priority scheduling in Pintos. Priority scheduling is a key
431 building block for real-time systems. Implement functions
432 @code{thread_set_priority()} to set the priority of the running thread
433 and @code{thread_get_priority()} to get the running thread's priority.
434 (A thread can examine and modify only its own priority.) There are
435 already prototypes for these functions in @file{threads/thread.h},
436 which you should not change.
438 Thread priority ranges from @code{PRI_MIN} (0) to @code{PRI_MAX} (59).
439 The initial thread priority is passed as an argument to
440 @code{thread_create()}. If there's no reason to choose another
441 priority, use @code{PRI_DEFAULT} (29). The @code{PRI_} macros are
442 defined in @file{threads/thread.h}, and you should not change their
445 When a thread is added to the ready list that has a higher priority
446 than the currently running thread, the current thread should
447 immediately yield the processor to the new thread. Similarly, when
448 threads are waiting for a lock, semaphore or condition variable, the
449 highest priority waiting thread should be woken up first. A thread's
450 priority may be set at any time, including while the thread is waiting
451 on a lock, semaphore, or condition variable.
453 One issue with priority scheduling is ``priority inversion'': if a
454 high priority thread needs to wait for a low priority thread (for
455 instance, for a lock held by a low priority thread, or in
456 @code{thread_join()} for a thread to complete), and a middle priority
457 thread is on the ready list, then the high priority thread will never
458 get the CPU because the low priority thread will not get any CPU time.
459 A partial fix for this problem is to have the waiting thread
460 ``donate'' its priority to the low priority thread while it is holding
461 the lock, then recall the donation once it has acquired the lock.
464 You will need to account for all different orders that priority
465 donation and inversion can occur. Be sure to handle multiple
466 donations, in which multiple priorities are donated to a thread. You
467 must also handle nested donation: given high, medium, and low priority
468 threads H, M, and L, respectively, if H is waiting on a lock that M
469 holds and M is waiting on a lock that L holds, then both M and L
470 should be boosted to H's priority.
472 You only need to implement priority donation when a thread is waiting
473 for a lock held by a lower-priority thread. You do not need to
474 implement this fix for semaphores, condition variables or joins.
475 However, you do need to implement priority scheduling in all cases.
477 @node Problem 1-4 Advanced Scheduler
478 @section Problem 1-4: Advanced Scheduler
480 Implement Solaris's multilevel feedback queue scheduler (MLFQS) to
481 reduce the average response time for running jobs on your system.
482 @xref{Multilevel Feedback Scheduling}, for a detailed description of
483 the MLFQS requirements.
485 Demonstrate that your scheduling algorithm reduces response time
486 relative to the original Pintos scheduling algorithm (round robin) for
487 at least one workload of your own design (i.e.@: in addition to the
490 You may assume a static priority for this problem. It is not necessary
491 to ``re-donate'' a thread's priority if it changes (although you are
494 You must write your code so that we can turn the MLFQS on and off at
495 compile time. By default, it must be off, but we must be able to turn
496 it on by inserting the line @code{#define MLFQS 1} in
497 @file{constants.h}. @xref{Conditional Compilation}, for details.
504 @b{I am adding a new @file{.h} or @file{.c} file. How do I fix the
505 @file{Makefile}s?}@anchor{Adding c or h Files}
507 To add a @file{.c} file, edit the top-level @file{Makefile.build}.
508 You'll want to add your file to variable @samp{@var{dir}_SRC}, where
509 @var{dir} is the directory where you added the file. For this
510 project, that means you should add it to @code{threads_SRC}, or
511 possibly @code{devices_SRC} if you put in the @file{devices}
512 directory. Then run @code{make}. If your new file doesn't get
513 compiled, run @code{make clean} and then try again.
515 When you modify the top-level @file{Makefile.build}, the modified
516 version should be automatically copied to
517 @file{threads/build/Makefile} when you re-run make. The opposite is
518 not true, so any changes will be lost the next time you run @code{make
519 clean} from the @file{threads} directory. Therefore, you should
520 prefer to edit @file{Makefile.build} (unless your changes are meant to
523 There is no need to edit the @file{Makefile}s to add a @file{.h} file.
526 @b{How do I write my test cases?}
528 Test cases should be replacements for the existing @file{test.c}
529 file. Put them in a @file{threads/testcases} directory.
530 @xref{TESTCASE}, for more information.
533 @b{Why can't I disable interrupts?}
535 Turning off interrupts should only be done for short amounts of time,
536 or else you end up losing important things such as disk or input
537 events. Turning off interrupts also increases the interrupt handling
538 latency, which can make a machine feel sluggish if taken too far.
539 Therefore, in general, setting the interrupt level should be used
540 sparingly. Also, any synchronization problem can be easily solved by
541 turning interrupts off, since while interrupts are off, there is no
542 concurrency, so there's no possibility for race condition.
544 To make sure you understand concurrency well, we are discouraging you
545 from taking this shortcut at all in your solution. If you are unable
546 to solve a particular synchronization problem with semaphores, locks,
547 or conditions, or think that they are inadequate for a particular
548 reason, you may turn to disabling interrupts. If you want to do this,
549 we require in your design document a complete justification and
550 scenario (i.e.@: exact sequence of events) to show why interrupt
551 manipulation is the best solution. If you are unsure, the TAs can
552 help you determine if you are using interrupts too haphazardly. We
553 want to emphasize that there are only limited cases where this is
556 You might find @file{devices/intq.h} and its users to be an
557 inspiration or source of rationale.
560 @b{Where might interrupt-level manipulation be appropriate?}
562 You might find it necessary in some solutions to the Alarm problem.
564 You might want it at one small point for the priority scheduling
565 problem. Note that it is not required to use interrupts for these
566 problems. There are other, equally correct solutions that do not
567 require interrupt manipulation. However, if you do manipulate
568 interrupts and @strong{correctly and fully document it} in your design
569 document, we will allow limited use of interrupt disabling.
572 @b{What does ``warning: no previous prototype for `@var{function}''
575 It means that you defined a non-@code{static} function without
576 preceding it by a prototype. Because non-@code{static} functions are
577 intended for use by other @file{.c} files, for safety they should be
578 prototyped in a header file included before their definition. To fix
579 the problem, add a prototype in a header file that you include, or, if
580 the function isn't actually used by other @file{.c} files, make it
585 * Problem 1-1 Alarm Clock FAQ::
586 * Problem 1-2 Join FAQ::
587 * Problem 1-3 Priority Scheduling FAQ::
588 * Problem 1-4 Advanced Scheduler FAQ::
591 @node Problem 1-1 Alarm Clock FAQ
592 @subsection Problem 1-1: Alarm Clock FAQ
596 @b{Why can't I use most synchronization primitives in an interrupt
599 As you've discovered, you cannot sleep in an external interrupt
600 handler. Since many lock, semaphore, and condition variable functions
601 attempt to sleep, you won't be able to call those in
602 @code{timer_interrupt()}. You may still use those that never sleep.
604 Having said that, you need to make sure that global data does not get
605 updated by multiple threads simultaneously executing
606 @code{timer_sleep()}. Here are some pieces of information to think
611 Interrupts are turned off while @code{timer_interrupt()} runs. This
612 means that @code{timer_interrupt()} will not be interrupted by a
613 thread running in @code{timer_sleep()}.
616 A thread in @code{timer_sleep()}, however, can be interrupted by a
617 call to @code{timer_interrupt()}, except when that thread has turned
621 Examples of synchronization mechanisms have been presented in lecture.
622 Going over these examples should help you understand when each type is
627 @b{What about timer overflow due to the fact that times are defined as
628 integers? Do I need to check for that?}
630 Don't worry about the possibility of timer values overflowing. Timer
631 values are expressed as signed 63-bit numbers, which at 100 ticks per
632 second should be good for almost 2,924,712,087 years.
635 @node Problem 1-2 Join FAQ
636 @subsection Problem 1-2: Join FAQ
640 @b{Am I correct to assume that once a thread is deleted, it is no
641 longer accessible by the parent (i.e.@: the parent can't call
642 @code{thread_join(child)})?}
644 A parent joining a child that has completed should be handled
645 gracefully and should act as a no-op.
648 @node Problem 1-3 Priority Scheduling FAQ
649 @subsection Problem 1-3: Priority Scheduling FAQ
653 @b{Doesn't the priority scheduling lead to starvation? Or do I have to
654 implement some sort of aging?}
656 It is true that strict priority scheduling can lead to starvation
657 because thread may not run if a higher-priority thread is runnable.
658 In this problem, don't worry about starvation or any sort of aging
659 technique. Problem 4 will introduce a mechanism for dynamically
660 changing thread priorities.
662 This sort of scheduling is valuable in real-time systems because it
663 offers the programmer more control over which jobs get processing
664 time. High priorities are generally reserved for time-critical
665 tasks. It's not ``fair,'' but it addresses other concerns not
666 applicable to a general-purpose operating system.
669 @b{After a lock has been released, does the program need to switch to
670 the highest priority thread that needs the lock (assuming that its
671 priority is higher than that of the current thread)?}
673 When a lock is released, the highest priority thread waiting for that
674 lock should be unblocked and put on the ready to run list. The
675 scheduler should then run the highest priority thread on the ready
679 @b{If a thread calls @code{thread_yield()} and then it turns out that
680 it has higher priority than any other threads, does the high-priority
681 thread continue running?}
683 Yes. If there is a single highest-priority thread, it continues
684 running until it blocks or finishes, even if it calls
685 @code{thread_yield()}.
688 @b{If the highest priority thread is added to the ready to run list it
689 should start execution immediately. Is it immediate enough if I
690 wait until next timer interrupt occurs?}
692 The highest priority thread should run as soon as it is runnable,
693 preempting whatever thread is currently running.
696 @b{What happens to the priority of the donating thread? Do the priorities
699 No. Priority donation only changes the priority of the low-priority
700 thread. The donating thread's priority stays unchanged. Also note
701 that priorities aren't additive: if thread A (with priority 5) donates
702 to thread B (with priority 3), then B's new priority is 5, not 8.
705 @b{Can a thread's priority be changed while it is sitting on the ready
708 Yes. Consider this case: low-priority thread L currently has a lock
709 that high-priority thread H wants. H donates its priority to L (the
710 lock holder). L finishes with the lock, and then loses the CPU and is
711 moved to the ready queue. Now L's old priority is restored while it
712 is in the ready queue.
715 @b{Can a thread's priority change while it is sitting on the queue of a
718 Yes. Same scenario as above except L gets blocked waiting on a new
719 lock when H restores its priority.
722 @b{Why is pubtest3's FIFO test skipping some threads! I know my scheduler
723 is round-robin'ing them like it's supposed to! Our output is like this:}
737 @noindent @b{which repeats 5 times and then}
747 This happens because context switches are being invoked by the test
748 when it explicitly calls @code{thread_yield()}. However, the time
749 slice timer is still alive and so, every tick (by default), thread 1
750 gets switched out (caused by @code{timer_interrupt()} calling
751 @code{intr_yield_on_return()}) before it gets a chance to run its
752 mainline. It is by coincidence that Thread 1 is the one that gets
753 skipped in our example. If we use a different jitter value, the same
754 behavior is seen where a thread gets started and switched out
757 Solution: Increase the value of @code{TIME_SLICE} in
758 @file{devices/timer.c} to a very high value, such as 10000, to see
759 that the threads will round-robin if they aren't interrupted.
762 @b{What happens when a thread is added to the ready list which has
763 higher priority than the currently running thread?}
765 The correct behavior is to immediately yield the processor. Your
766 solution must act this way.
769 @b{What should @code{thread_get_priority()} return in a thread while
770 its priority has been increased by a donation?}
772 The higher (donated) priority.
775 @node Problem 1-4 Advanced Scheduler FAQ
776 @subsection Problem 1-4: Advanced Scheduler FAQ
780 @b{What is the interval between timer interrupts?}
782 Timer interrupts occur @code{TIMER_FREQ} times per second. You can
783 adjust this value by editing @file{devices/timer.h}. The default is
786 You can also adjust the number of timer ticks per time slice by
787 modifying @code{TIME_SLICE} in @file{devices/timer.c}.
790 @b{Do I have to modify the dispatch table?}
792 No, although you are allowed to. It is possible to complete
793 this problem (i.e.@: demonstrate response time improvement)
797 @b{When the scheduler changes the priority of a thread, how does this
798 affect priority donation?}
800 Short (official) answer: Don't worry about it. Your priority donation
801 code may assume static priority assignment.
803 Longer (unofficial) opinion: If you wish to take this into account,
804 however, your design may end up being ``cleaner.'' You have
805 considerable freedom in what actually takes place. I believe what
806 makes the most sense is for scheduler changes to affect the
807 ``original'' (non-donated) priority. This change may actually be
808 masked by the donated priority. Priority changes should only
809 propagate with donations, not ``backwards'' from donees to donors.
812 @b{What is meant by ``static priority''?}
814 Once thread A has donated its priority to thread B, if thread A's
815 priority changes (due to the scheduler) while the donation still
816 exists, you do not have to change thread B's donated priority.
817 However, you are free to do so.
820 @b{Do I have to make my dispatch table user-configurable?}
822 No. Hard-coding the dispatch table values is fine.