CompSci 143A: Principles of Operating System
Instructor: Ardalan Amiri Sani |
Pintos Project Lab 1: Threads
In this assignment, we give you a minimally functional thread system. Your job is to extend the functionality of this system to gain a better understanding of synchronization problems.
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 threads
directory.
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.
Background
Understanding Threads
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 printf()
almost
anywhere, then recompile and run to see what happens and in what
order. You can also run the kernel in a debugger and set breakpoints
at interesting spots, single-step through code and examine data, and
so on.
When a thread is created, you are creating a new context to be
scheduled. You provide a function to be run in this context as an
argument to 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 thread_create()
acting like main()
.
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
idle()
, runs.)
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
in 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
breakpoint on 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 switch_threads()
,
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 switch_threads()
that
gets called is different from the switch_threads()
that returns.
See section A.2.3
Thread Switching, for more information.
int buf[1000];. Alternatives to stack allocation include the page allocator and the block allocator (see section A.5 Memory Allocation).
Source Files
Refer to link
for a brief overview of the files in the 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.
Synchronization
Proper synchronization is an important part of the solutions to these
problems. Any synchronization problem can be easily solved by turning
interrupts off: while interrupts are off, there is no concurrency, so
there's no possibility for race conditions. Therefore, it's tempting to
solve all synchronization problems this way, but 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 synch.c
are
implemented by disabling interrupts. You may need to increase the
amount of code that runs with interrupts disabled here, but you should
still try to keep it to a minimum.
Disabling interrupts can be useful for debugging, if you want to make sure that a section of code is not interrupted. You should remove debugging code before turning in your project. (Don't just comment it out, because that can make the code difficult to read.)
There should be no busy waiting in your submission. A tight loop that
calls thread_yield()
is one form of busy waiting.
Development Suggestions
In the past, many groups divided the assignment into pieces, then each group member worked on his or her piece until just before the deadline, at which time the group reconvened to combine their code and submit. 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 project. 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.
Requirements
0. Design Document
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.
1. Alarm Clock
timer_sleep()
, defined in devices/timer.c. Although a working implementation is provided, it "busy waits," that is, it spins in a loop checking the current time and calling
thread_yield()
until enough time has gone by. Reimplement it to
avoid busy waiting.
- Function: void timer_sleep (int64_t ticks)
- Suspends execution of the calling thread until time has advanced by at
least x timer ticks. Unless the system is otherwise idle, the
thread need not wake up after exactly x ticks. Just put it on
the ready queue after they have waited for the right amount of time.
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 areTIMER_FREQ
timer ticks per second, whereTIMER_FREQ
is a macro defined indevices/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.
Separate functions timer_msleep()
, timer_usleep()
, and
timer_nsleep()
do exist for sleeping a specific number of
milliseconds, microseconds, or nanoseconds, respectively, but these will
call 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.
2. Priority Scheduling
Thread priorities range from PRI_MIN
(0) to PRI_MAX
(63).
Lower numbers correspond to lower priorities, so that priority 0
is the lowest priority and priority 63 is the highest.
The initial thread priority is passed as an argument to
thread_create()
. If there's no reason to choose another
priority, use PRI_DEFAULT
(31). The PRI_
macros are
defined in threads/thread.h
, and you should not change their
values.
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.
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.
threads/thread.c.
- Function: void thread_set_priority (int new_priority)
- Sets the current thread's priority to new_priority. If the current thread no longer has the highest priority, yields.
- Function: int thread_get_priority (void)
- Returns the current thread's priority. In the presence of priority donation, returns the higher (donated) priority.
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.
3. Advanced Scheduler
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
scheduler
with the -mlfqs
kernel option. Passing this
option sets thread_mlfqs
, declared in threads/thread.h
, to
true when the options are parsed by parse_options()
, which happens
early in main()
.
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_set_priority()
, and 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.
FAQ
- How much code will I need to write?
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.- How do I update the
Makefile
s when I add a new source file? To add a
.c
file, edit the top-levelMakefile.build
. Add the new file to variabledir_SRC
, where dir is the directory where you added the file. For this project, that means you should add it tothreads_SRC
ordevices_SRC
. Then runmake
. If your new file doesn't get compiled, runmake clean
and then try again.When you modify the top-level
Makefile.build
and re-runmake
, the modified version should be automatically copied tothreads/build/Makefile
. The converse is not true, so any changes will be lost the next time you runmake clean
from thethreads
directory. Unless your changes are truly temporary, you should prefer to editMakefile.build
.A new
.h
file does not require editing theMakefile
s.- What does
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 itstatic
.- What is the interval between timer interrupts?
Timer interrupts occur
TIMER_FREQ
times per second. You can adjust this value by editingdevices/timer.h
. The default is 100 Hz.We don't recommend changing this value, because any changes are likely to cause many of the tests to fail.
- How long is a time slice?
There are
TIME_SLICE
ticks per time slice. This macro is declared inthreads/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.
- How do I run the tests?
See section 1.2.1 Testing.
- Why do I get a test failure in
pass()
? 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 thatpass()
calleddebug_panic()
. In fact,fail()
calleddebug_panic()
(via thePANIC()
macro). GCC knows thatdebug_panic()
does not return, because it is declaredNO_RETURN
(see section E.3 Function and Parameter Attributes), so it doesn't include any code infail()
to take control whendebug_panic()
returns. This means that the return address on the stack looks like it is at the beginning of the function that happens to followfail()
in memory, which in this case happens to bepass()
.See section E.4 Backtraces, for more information.
- How do interrupts get re-enabled in the new thread following
schedule()
? 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 inswitch_thread()
(but see below), which is called byschedule()
, 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 fromschedule()
. -
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. -
wait()
indevices/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 inkernel_thread()
, which is at the bottom of the call stack for every kernel thread but the first.-
Alarm Clock FAQ
- Do I need to account for timer values overflowing?
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.
Priority Scheduling FAQ
- Doesn't priority scheduling lead to starvation?
Yes, strict priority scheduling can lead to starvation because a thread will not run if any higher-priority thread is runnable. The advanced scheduler introduces a mechanism for dynamically changing thread priorities.
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.
- What thread should run after a lock has been released?
When a lock is released, the highest priority thread waiting for that lock should be unblocked and put on the list of ready threads. The scheduler should then run the highest priority thread on the ready list.
- If the highest-priority thread yields, does it continue running?
Yes. If there is a single highest-priority thread, it continues running until it blocks or finishes, even if it calls
thread_yield()
. If multiple threads have the same highest priority,thread_yield()
should switch among them in "round robin" order.- What happens to the priority of a donating thread?
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.
- Can a thread's priority change while it is on the ready queue?
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.
- Can a thread's priority change while it is blocked?
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
priority-donate-sema
test.- Can a thread added to the ready list preempt the processor?
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.
- How does
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 by the
priority-donate-lower
test.- Doubled test names in output make them fail.
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
"(alarm-priority) 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 theprintf
call and releasing it afterwards. However, the output of the test name, e.g.,(alarm-priority)
, and the message following it is output using two calls toprintf
, resulting in the console lock being acquired and released twice.
Advanced Scheduler FAQ
- How does priority donation interact with the advanced scheduler?
It doesn't have to. We won't test priority donation and the advanced scheduler at the same time.
- Can I use one queue instead of 64 queues?
Yes. In general, your implementation may differ from the description, as long as its behavior is the same.
- Some scheduler tests fail and I don't understand why. Help!
If your implementation mysteriously fails some of the advanced scheduler tests, try the following:
-
Read the source files for the tests that you're failing, to make sure
that you understand what's going on. Each one has a comment at the
top that explains its purpose and expected results.
-
Double-check your fixed-point arithmetic routines and your use of them
in the scheduler routines.
- Consider how much work your implementation does in the timer interrupt. If the timer interrupt handler takes too long, then it will take away most of a timer tick from the thread that the timer interrupt preempted. When it returns control to that thread, it therefore won't get to do much work before the next timer interrupt arrives. That thread will therefore get blamed for a lot more CPU time than it actually got a chance to use. This raises the interrupted thread's recent CPU count, thereby lowering its priority. It can cause scheduling decisions to change. It also raises the load average.
-
Read the source files for the tests that you're failing, to make sure
that you understand what's going on. Each one has a comment at the
top that explains its purpose and expected results.