state of the currently running thread and restores the state of the
thread we're switching to.
-Using the @command{gdb} debugger, slowly trace through a context
-switch to see what happens (@pxref{gdb}). You can set a
+Using the GDB debugger, slowly trace through a context
+switch to see what happens (@pxref{GDB}). You can set a
breakpoint on @func{schedule} to start out, and then
-single-step from there.@footnote{@command{gdb} might tell you that
-@func{schedule} doesn't exist, which is arguably a @command{gdb} bug.
+single-step from there.@footnote{GDB might tell you that
+@func{schedule} doesn't exist, which is arguably a GDB bug.
You can work around this by setting the breakpoint by filename and
line number, e.g.@: @code{break thread.c:@var{ln}} where @var{ln} is
the line number of the first declaration in @func{schedule}.} Be sure
@item flags.h
Macros that define a few bits in the 80@var{x}86 ``flags'' register.
-Probably of no interest. See @bibref{IA32-v1}, section 3.4.3, for more
-information.
+Probably of no interest. See @bibref{IA32-v1}, section 3.4.3, ``EFLAGS
+Register,'' for more information.
@end table
@menu
interrupt handling latency, which can make a machine feel sluggish if
taken too far.
+You may need to add or modify code where interrupts are already
+disabled, such as in @func{sema_up} or @func{sema_down}. You should
+still try to keep this code as short as you can.
+
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.
+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 @func{thread_yield} is one form of busy waiting.
thread. You must also handle nested donation: if @var{H} is waiting on
a lock that @var{M} holds and @var{M} is waiting on a lock that @var{L}
holds, then both @var{M} and @var{L} should be boosted to @var{H}'s
-priority.
+priority. If necessary, you may impose a reasonable limit on depth of
+nested priority donation, such as 8 levels.
-You need not implement priority donation when a thread is waiting
-for a lock held by a lower-priority thread. You need not
-implement priority donation for semaphores or condition variables,
-but you are welcome to do so. You do need to implement
+You must implement priority donation for locks. You need not
+implement priority donation for semaphores or condition variables
+(but you are welcome to do so). You do need to implement
priority scheduling in all cases.
Finally, implement the following functions that allow a thread to
donation, returns the higher (donated) priority.
@end deftypefun
+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.
@node Advanced Scheduler
Implement a multilevel feedback queue scheduler similar to the
4.4@acronym{BSD} scheduler to
reduce the average response time for running jobs on your system.
-@xref{4.4BSD Scheduler}, for a detailed description of
-the MLFQS requirements.
+@xref{4.4BSD Scheduler}, for detailed requirements.
-The advanced scheduler builds on the priority scheduler. You should
-have the priority scheduler working, except possibly for priority
-donation, before you start work on the 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 so that we can choose a scheduling algorithm
-policy at Pintos startup time. By default, the round-robin 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 4.4@acronym{BSD}
scheduler
with the @option{-mlfqs} kernel option. Passing this
option sets @code{enable_mlfqs}, declared in @file{threads/init.h}, to
-true.
+true when the options are parsed by @func{parse_options}, which happens
+midway through @func{main}.
When the 4.4@acronym{BSD} scheduler is enabled, threads no longer
directly control their own priorities. The @var{priority} argument to
@command{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.
+
@verbatim
devices/timer.c | 42 +++++-
threads/fixed-point.h | 120 ++++++++++++++++++
5 files changed, 440 insertions(+), 29 deletions(-)
@end verbatim
+@file{fixed-point.h} is a new file added by the reference solution.
+
@item How do I update the @file{Makefile}s when I add a new source file?
@anchor{Adding Source Files}
There are @code{TIME_SLICE} ticks per time slice. This macro is
declared in @file{threads/thread.c}. The default is 4 ticks.
-We don't recommend changing this values, because any changes are likely
+We don't recommend changing this value, because any changes are likely
to cause many of the tests to fail.
+
+@item How do I run the tests?
+
+@xref{Testing}.
+
+@item Why do I get a test failure in @func{pass}?
+
+@anchor{The pass function fails}
+You are probably looking at a backtrace that looks something like this:
+
+@example
+0xc0108810: debug_panic (../../lib/kernel/debug.c:32)
+0xc010a99f: pass (../../tests/threads/tests.c:93)
+0xc010bdd3: test_mlfqs_load_1 (../../tests/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)
+@end example
+
+This is just confusing output from the @command{backtrace} program. It
+does not actually mean that @func{pass} called @func{debug_panic}. In
+fact, @func{fail} called @func{debug_panic} (via the @func{PANIC}
+macro). GCC knows that @func{debug_panic} does not return, because it
+is declared @code{NO_RETURN} (@pxref{Function and Parameter
+Attributes}), so it doesn't include any code in @func{pass} to take
+control when @func{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 @func{fail} in memory, which in this case happens
+to be @func{pass}.
+
+@xref{Backtraces}, for more information.
@end table
@menu
Yes. As long as there is a single highest-priority thread, it continues
running until it blocks or finishes, even if it calls
@func{thread_yield}.
-If there are multiple threads have the same highest priority,
+If multiple threads have the same highest priority,
@func{thread_yield} should switch among them in ``round robin'' order.
@item 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 @var{A} (with priority 5) donates
-to thread @var{B} (with priority 3), then @var{B}'s new priority is 5, not 8.
+Priority donation is not additive: if thread @var{A} (with priority 3) donates
+to thread @var{B} (with priority 5), then @var{B}'s new priority is 3, not 8.
@item Can a thread's priority change while it is on the ready queue?
The highest priority thread should run as soon as it is runnable,
preempting whatever thread is currently running.
+@item How does @func{thread_set_priority} affect a thread receiving donations?
+
+It should do something sensible, but no particular behavior is
+required. None of the test cases call @func{thread_set_priority} from a
+thread while it is receiving a priority donation.
+
@item Calling @func{printf} in @func{sema_up} or @func{sema_down} reboots!
@anchor{printf Reboots}
It doesn't have to. We won't test priority donation and the advanced
scheduler at the same time.
+
+@item Can I use one queue instead of 64 queues?
+
+Yes, that's fine. It's easiest to describe the algorithm in terms of 64
+separate queues, but that doesn't mean you have to implement it that
+way.
+
+If you use a single queue, it should probably be sorted.
@end table
projects / pintos-anon / blobdiff