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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.
You will be working primarily in the
threads directory for
this assignment, with some work in the
devices directory on the
side. Compilation should be done in the
Before you read the description of this project, you should read all of the following sections: 1. Introduction, C. Coding Standards, E. Debugging Tools, and F. Development Tools. You should at least skim the material from A.1 Loading through A.5 Memory Allocation, especially A.3 Synchronization. To complete this project you will also need to read B. 4.4BSD Scheduler.
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, condition variables, and optimization barriers).
Some of this code might seem slightly mysterious. If
you haven't already compiled and run the base system, as described in
the introduction (see section 1. Introduction), you should do so now. You
can read through parts of the source code to see what's going
on. If you like, you can add calls to
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
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
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
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
Synchronization primitives can force context switches when one
thread needs to wait for another thread to do something.
The mechanics of a context switch are
threads/switch.S, which is 80x86
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 GDB debugger, slowly trace through a context
switch to see what happens (see section E.5 GDB). You can set a
schedule() to start out, and then
single-step from there.(1) 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
another thread starts running, and the first thing the new thread does
is to return from
switch_threads(). You will understand the thread
system once you understand why and how the
gets called is different from the
switch_threads() that returns.
See section A.2.3 Thread Switching, for more information.
Warning: In Pintos, each thread is assigned a small,
fixed-size execution stack just under 4 kB in size. The kernel
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,
int buf;. Alternatives to stack allocation include
the page allocator and the block allocator (see section A.5 Memory Allocation).
Here is a brief overview of the files in the
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.
start.S. See section A.1.1 The Loader, for details. You should not need to look at this code or modify it.
start.Sto be near the beginning of the kernel image. See section A.1.1 The 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.
main(), the kernel's "main program." You should look over
main()at least to see what gets initialized. You might want to add your own initialization code here. See section A.1.3 High-Level Kernel Initialization, for details.
struct thread, which you are likely to modify in all four projects. See A.2.1
struct threadand A.2 Threads for more information.
free()for the kernel. See section A.5.2 Block Allocator, for more information.
devicesdirectory that you won't have to touch.
The basic threaded kernel also includes these files in the
printf()calls into the VGA display driver for you, so there's little reason to call this code yourself.
printf()calls this code for you, so you don't need to do so yourself. It handles serial input by passing it to the input layer (see below).
thread/init.cto choose an initial seed for the random number generator.
devices/speaker.cbecause each device uses one of the PIT's output channel.
lib/kernel contain useful library
lib/user will be used by user programs, starting in
project 2, but it is not part of the kernel.) Here's a few more
-rskernel command-line option on each run, or use a simulator other than Bochs, or specify the
printf()and a few other functions.
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 don't.
Instead, use semaphores, locks, and condition variables to solve the
bulk of your synchronization problems. Read the tour section on
synchronization (see section A.3 Synchronization) or the comments in
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
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
thread_yield() is one form of busy waiting.
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. 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.
Instead, we recommend integrating your team's changes early and often, using a source code control system such as Git (see section F.3 Git). This is less likely to produce surprises, because everyone can see everyone else's code as it is written, instead of just when it is finished. These systems also make it possible to review changes and, when a change introduces a bug, drop back to working versions of code.
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 (see section E. Debugging Tools). Be sure to read the section on backtraces (see section E.4 Backtraces), which will help you to get the most out of every kernel panic or assertion failure.
We suggest first implementing the following, which can happen in parallel:
timer_sleep()in exercise 1.1.
Then you can add full support for priority scheduling in exercise 1.2 by considering all other possible scenarios and the synchronization primitives.
Then you can tackle either exercise 1.3 first or exercise 1.2.2 first.
Before you turn in your project, you must copy the
project 1 design document template into your source tree under the name
pintos/src/threads/DESIGNDOC and fill it in. We recommend that
you read the design document template before you start working on the
project. See section D. Project Documentation, for a sample design document
that goes along with a fictitious project.
timer_sleep(), defined in
Although a working implementation is provided, it "busy waits," that
is, it spins in a loop checking the current time and calling
thread_yield() until enough time has gone by. Reimplement it to
avoid busy waiting.
timer_sleep() is useful for threads that operate in real-time,
e.g. for blinking the cursor once per second.
The argument to
timer_sleep() is expressed in timer ticks, not in
milliseconds or any another unit. There are
ticks per second, where
TIMER_FREQ is a macro defined in
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.
timer_nsleep() do exist for sleeping a specific number of
milliseconds, microseconds, or nanoseconds, respectively, but these will
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
-r option to
pintos (see section 1.1.4 Debugging versus Testing).
The alarm clock implementation is not needed for later projects, although it could be useful for project 4.
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
PRI_MIN (0) to
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
thread_create(). If there's no reason to choose another
PRI_DEFAULT (31). The
PRI_ macros are
threads/thread.h, and you should not change their
One issue with priority scheduling is "priority inversion". Consider high, medium, and low priority threads H, M, and L, respectively. If H needs to wait for L (for instance, for a lock held by L), and M is on the ready list, then 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 H to "donate" its priority to L while L is holding the lock, then recall the donation once L releases (and thus 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 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. 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.
Finally, implement the following functions that allow a thread to
examine and modify its own priority. Skeletons for these functions are
You need not provide any interface to allow a thread to directly modify other threads' priorities.
The priority scheduler is not used in any later project.
Implement a multilevel feedback queue scheduler similar to the 4.4BSD scheduler to reduce the average response time for running jobs on your system. See section B. 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 4.4BSD
-mlfqs kernel option. Passing this
thread_mlfqs, declared in
true when the options are parsed by
parse_options(), which happens
When the 4.4BSD scheduler is enabled, threads no longer
directly control their own priorities. The priority argument to
thread_create() should be ignored, as well as any calls to
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.
Here's a summary of our reference solution, produced by the
diffstat program. The final row gives total lines inserted
and deleted; a changed line counts as both an insertion and a deletion.
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.
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(-)
fixed-point.h is a new file added by the reference solution.
Makefiles when I add a new source file?
To add a
.c file, edit the top-level
Add the new file to variable
dir is the directory where you added the file. For this
project, that means you should add it to
devices_SRC. Then run
make. If your new file
make clean and then try again.
When you modify the top-level
Makefile.build and re-run
make, the modified
version should be automatically copied to
threads/build/Makefile. The converse is
not true, so any changes will be lost the next time you run
clean from the
threads directory. Unless your changes are
truly temporary, you should prefer to edit
.h file does not require editing the
warning: no previous prototype for `func'mean?
It means that you defined a non-
static function without
preceding it by a prototype. Because non-
static functions are
intended for use by other
.c files, for safety they should be
prototyped in a header file included before their definition. To fix
the problem, add a prototype in a header file that you include, or, if
the function isn't actually used by other
.c files, make it
Timer interrupts occur
TIMER_FREQ times per second. You can
adjust this value by editing
devices/timer.h. The default is
We don't recommend changing this value, because any changes are likely to cause many of the tests to fail.
TIME_SLICE ticks per time slice. This macro is
threads/thread.c. The default is 4 ticks.
We don't recommend changing this value, because any changes are likely to cause many of the tests to fail.
See section 1.2.1 Testing.
You are probably looking at a backtrace that looks something like this:
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)
This is just confusing output from the
backtrace program. It
does not actually mean that
debug_panic() (via the
macro). GCC knows that
debug_panic() does not return, because it
NO_RETURN (see section E.3 Function and Parameter Attributes), so it doesn't include any code in
fail() to take
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
fail() in memory, which in this case happens
See section E.4 Backtraces, for more information.
Every path into
schedule() disables interrupts. They eventually
get re-enabled by the next thread to be scheduled. Consider the
possibilities: the new thread is running in
see below), which is called by
schedule(), which is called by one
of a few possible functions:
thread_exit(), but we'll never switch back into such a thread, so it's uninteresting.
thread_yield(), which immediately restores the interrupt level upon return from
thread_block(), which is called from multiple places:
sema_down(), which restores the interrupt level before returning.
idle(), which enables interrupts with an explicit assembly STI instruction.
devices/intq.c, whose callers are responsible for re-enabling interrupts.
There is a special case when a newly created thread runs for the first
time. Such a thread calls
intr_enable() as the first action in
kernel_thread(), which is at the bottom of the call stack for every
kernel thread but the first.
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 Computer Science curriculum.
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.
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.
When a lock is released, the highest priority thread waiting for that 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.
Yes. If there is a single highest-priority thread, it continues
running until it blocks or finishes, even if it calls
If multiple threads have the same highest priority,
thread_yield() should switch among them in "round robin" order.
Priority donation only changes the priority of the donee thread. The donor thread's priority is unchanged. Priority donation is not 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 L that holds a lock. High-priority thread H attempts to acquire the lock and blocks, thereby donating its priority to ready thread L.
Yes. While a thread that has acquired lock L is blocked for any
reason, its priority can increase by priority donation if a
higher-priority thread attempts to acquire L. This case is
checked by the
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.
thread_set_priority()affect a thread receiving donations?
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
Suppose you are seeing output in which some test names are doubled, like this:
(alarm-priority) begin (alarm-priority) (alarm-priority) Thread priority 30 woke up. Thread priority 29 woke up. (alarm-priority) Thread priority 28 woke up.
What is happening is that output from two threads is being
interleaved. That is, one thread is printing
Thread priority 29 woke up.\n" and another thread is printing
"(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
printf() function in the
Pintos kernel attempts to prevent such interleaved output by acquiring
a console lock during the duration of the
printf call and
releasing it afterwards. However, the output of the test name,
(alarm-priority), and the message following it is output
using two calls to
printf, resulting in the console lock being
acquired and released twice.
It doesn't have to. We won't test priority donation and the advanced scheduler at the same time.
Yes. In general, your implementation may differ from the description, as long as its behavior is the same.
If your implementation mysteriously fails some of the advanced scheduler tests, try the following:
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