-*- text -*-
-* Test pintos with GSX.
+* Bochs is not fully reproducible.
-* Get rid of warnings for NDEBUG build.
+Godmar says:
-* Need targets for checking single tests or single subdirs
+- In Project 2, we're missing tests that pass arguments to system calls
+that span multiple pages, where some are mapped and some are not.
+An implementation that only checks the first page, rather than all pages
+that can be touched during a call to read()/write() passes all tests.
- - `make check' should say how to get more information
+- In Project 2, we're missing a test that would fail if they assumed
+that contiguous user-virtual addresses are laid out contiguously
+in memory. The loading code should ensure that non-contiguous
+physical pages are allocated for the data segment (at least.)
+
+- Need some tests that test that illegal accesses lead to process
+termination. I have written some, will add them. In P2, obviously,
+this would require that the students break this functionality since
+the page directory is initialized for them, still it would be good
+to have.
+
+- There does not appear to be a test that checks that they close all
+fd's on exit. Idea: add statistics & self-diagnostics code to palloc.c
+and malloc.c. Self-diagnostics code could be used for debugging.
+The statistics code would report how much kernel memory is free.
+Add a system call "get_kernel_memory_information". User programs
+could engage in a variety of activities and notice leaks by checking
+the kernel memory statistics.
+
+---
+
+From: "Godmar Back" <godmar@gmail.com>
+Subject: priority donation tests
+To: "Ben Pfaff" <blp@cs.stanford.edu>
+Date: Fri, 3 Mar 2006 11:02:08 -0500
- - `make recheck', `make clean-check'
+Ben,
+
+it seems the priority donation tests are somewhat incomplete and allow
+incorrect implementations to pass with a perfect score.
+
+We are seeing the following wrong implementations pass all tests:
+
+- Implementations that assume locks are released in the opposite order
+in which they're acquired. The students implement this by
+popping/pushing on the donation list.
+
+- Implementations that assume that the priority of a thread waiting on
+a semaphore or condition variable cannot change between when the
+thread was blocked and when it is unblocked. The students implement
+this by doing an insert into an ordered list on block, rather than
+picking the maximum thread on unblock.
-* Need to put time limits on tests.
+Neither of these two cases is detected; do you currently check for
+these mistakes manually?
-* Add a way for students to prints messages ignored by the autograder.
+I wrote a test that checks for the first case; it is here:
+http://people.cs.vt.edu/~gback/pintos/priority-donate-multiple-2.patch
- - Ignore text between /* and */.
+[...]
+
+I also wrote a test case for the second scenario:
+http://people.cs.vt.edu/~gback/pintos/priority-donate-sema.c
+http://people.cs.vt.edu/~gback/pintos/priority-donate-sema.ck
- - Allow for a comment on `exit' lines?
+I put the other tests up here:
+http://people.cs.vt.edu/~gback/pintos/priority-donate-multiple2.c
+http://people.cs.vt.edu/~gback/pintos/priority-donate-multiple2.ck
-* Improve automatic interpretation of exception messages.
+From: "Godmar Back" <godmar@gmail.com>
+Subject: multiple threads waking up at same clock tick
+To: "Ben Pfaff" <blp@cs.stanford.edu>
+Date: Wed, 1 Mar 2006 08:14:47 -0500
-* Remove names from synchronization primitives? They don't seem
- useful in practice.
+Greg Benson points out another potential TODO item for P1.
-* debug_backtrace() in kernel should only dereference valid pointers.
+----
+One thing I recall:
-* Mention how to use add-symbol-file in gdb.
+The alarm tests do not test to see if multiple threads are woken up if
+their timers have expired. That is, students can write a solution
+that just wakes up the first thread on the sleep queue rather than
+check for additional threads. Of course, the next thread will be
+woken up on the next tick. Also, this might be hard to test.
-* We need better and more example programs.
+---
+Way to test this: (from Godmar Back)
- - Need an mmap example program as a replacement for the crappy mmap FAQ
- question.
+Thread A with high priority spins until 'ticks' changes, then calls to
+timer_sleep(X), Thread B with lower priority is then resumed, calls
+set_priority to make its priority equal to that of thread A, then
+calls timer_sleep(X), all of that before the next clock interrupt
+arrives.
- - How about `diff' and `cmp' programs?
+On wakeup, each thread records wake-up time and calls yield
+immediately, forcing the scheduler to switch to the other
+equal-priority thread. Both wake-up times must be the same (and match
+the planned wake-up time.)
-* Threads:
+PS:
+I actually tested it and it's hard to pass with the current ips setting.
+The bounds on how quickly a thread would need to be able to return after
+sleep appear too tight. Need another idea.
- - mlfqs tests suck. They aren't even correct, e.g. the amarv
- submission from win0405 is graded incorrectly.
+---
+From: "Waqar Mohsin" <wmohsin@gmail.com>
+Subject: 3 questions about switch_threads() in switch.S
+To: blp@cs.stanford.edu, joshwise@stanford.edu
+Date: Fri, 3 Mar 2006 17:09:21 -0800
-* Userprog project:
+QUESTION 1
+
+In the section
+
+ # Save current stack pointer to old thread's stack, if any.
+ movl SWITCH_CUR(%esp), %eax
+ test %eax, %eax
+ jz 1f
+ movl %esp, (%eax,%edx,1)
+1:
+
+ # Restore stack pointer from new thread's stack.
+ movl SWITCH_NEXT(%esp), %ecx
+ movl (%ecx,%edx,1), %esp
+
+why are we saving the current stack pointer only if the "cur" thread pointer
+is non-NULL ? Isn't it gauranteed to be non-NULL because switch_threads() is
+only called form schedule(), where we have
+
+ struct thread *cur = running_thread ();
+
+which should always be non-NULL (given the way kernel pool is laid out).
+
+QUESTION 2
+
+ # This stack frame must match the one set up by thread_create().
+ pushl %ebx
+ pushl %ebp
+ pushl %esi
+ pushl %edi
+
+I find the comment confusing. thread_create() is a special case: the set of
+registers popped from switch_threads stack frame for a newly created thread
+are all zero, so their order shouldn't dictate the order above.
+
+I think all that matters is that the order of pops at the end of
+switch_threads() is the opposite of the pushes at the beginning (as shown
+above).
+
+QUESTION 3
+
+Is it true that struct switch_threads_frame does NOT strictly require
+
+ struct thread *cur; /* 20: switch_threads()'s CUR argument. */
+ struct thread *next; /* 24: switch_threads()'s NEXT argument. */
+at the end ?
+
+When a newly created thread's stack pointer is installed in switch_threads(),
+all we do is pop the saved registers and return to switch_entry() which pops
+off and discards the above two simulated (and not used) arguments to
+switch_threads().
+
+If we remove these two from struct switch_threads_frame and don't do a
+
+ # Discard switch_threads() arguments.
+ addl $8, %esp
+in switch_entry(), things should still work. Am I right ?
+
+Thanks
+Waqar
+
+From: "Godmar Back" <godmar@gmail.com>
+Subject: thread_yield in irq handler
+To: "Ben Pfaff" <blp@cs.stanford.edu>
+Date: Wed, 22 Feb 2006 22:18:50 -0500
+
+Ben,
+
+you write in your Tour of Pintos:
+
+"Second, an interrupt handler must not call any function that can
+sleep, which rules out thread_yield(), lock_acquire(), and many
+others. This is because external interrupts use space on the stack of
+the kernel thread that was running at the time the interrupt occurred.
+If the interrupt handler tried to sleep and that thread resumed, then
+the two uses of the single stack would interfere, which cannot be
+allowed."
+
+Is the last sentence really true?
+
+I thought the reason that you couldn't sleep is that you would put
+effectively a random thread/process to sleep, but I don't think it
+would cause problems with the kernel stack. After all, it doesn't
+cause this problem if you call thread_yield at the end of
+intr_handler(), so why would it cause this problem earlier.
+
+As for thread_yield(), my understanding is that the reason it's called
+at the end is to ensure it's done after the interrupt is acknowledged,
+which you can't do until the end because Pintos doesn't handle nested
+interrupts.
+
+ - Godmar
+
+From: "Godmar Back" <godmar@gmail.com>
+
+For reasons I don't currently understand, some of our students seem
+hesitant to include each thread in a second "all-threads" list and are
+looking for ways to implement the advanced scheduler without one.
+
+Currently, I believe, all tests for the mlfqs are such that all
+threads are either ready or sleeping in timer_sleep(). This allows for
+an incorrect implementation in which recent-cpu and priorities are
+updated only for those threads that are on the alarm list or the ready
+list.
+
+The todo item would be a test where a thread is blocked on a
+semaphore, lock or condition variable and have its recent_cpu decay to
+zero, and check that it's scheduled right after the unlock/up/signal.
+
+From: "Godmar Back" <godmar@gmail.com>
+Subject: set_priority & donation - a TODO item
+To: "Ben Pfaff" <blp@cs.stanford.edu>
+Date: Mon, 20 Feb 2006 22:20:26 -0500
+
+Ben,
+
+it seems that there are currently no tests that check the proper
+behavior of thread_set_priority() when called by a thread that is
+running under priority donation. The proper behavior, I assume, is to
+temporarily drop the donation if the set priority is higher, and to
+reassume the donation should the thread subsequently set its own
+priority again to a level that's lower than a still active donation.
+
+ - Godmar
+
+From: Godmar Back <godmar@gmail.com>
+Subject: project 4 question/comment regarding caching inode data
+To: Ben Pfaff <blp@cs.stanford.edu>
+Date: Sat, 14 Jan 2006 15:59:33 -0500
+
+Ben,
+
+in section 6.3.3 in the P4 FAQ, you write:
+
+"You can store a pointer to inode data in struct inode, if you want,"
+
+Should you point out that if they indeed do that, they likely wouldn't
+be able to support more than 64 open inodes systemwide at any given
+point in time.
+
+(This seems like a rather strong limitation; do your current tests
+open more than 64 files?
+It would also point to an obvious way to make the projects harder by
+specifically disallowing that inode data be locked in memory during
+the entire time an inode is kept open.)
+
+ - Godmar
+
+From: Godmar Back <godmar@gmail.com>
+Subject: on caching in project 4
+To: Ben Pfaff <blp@cs.stanford.edu>
+Date: Mon, 9 Jan 2006 20:58:01 -0500
+
+here's an idea for future semesters.
+
+I'm in the middle of project 4, I've started by implementing a buffer
+cache and plugging it into the existing filesystem. Along the way I
+was wondering how we could test the cache.
+
+Maybe one could adopt a similar testing strategy as in project 1 for
+the MLQFS scheduler: add a function that reads "get_cache_accesses()"
+and a function "get_cache_hits()". Then create a version of pintos
+that creates access traces for a to-be-determined workload. Run an
+off-line analysis that would determine how many hits a perfect cache
+would have (MAX), and how much say an LRU strategy would give (MIN).
+Then add a fudge factor to account for different index strategies and
+test that the reported number of cache hits/accesses is within (MIN,
+MAX) +/- fudge factor.
+
+(As an aside - I am curious why you chose to use a clock-style
+algorithm rather than the more straightforward LRU for your buffer
+cache implementation in your sample solution. Is there a reason for
+that? I was curious to see if it made a difference, so I implemented
+LRU for your cache implementation and ran the test workload of project
+4 and printed cache hits/accesses.
+I found that for that workload, the clock-based algorithm performs
+almost identical to LRU (within about 1%, but I ran nondeterministally
+with QEMU). I then reduced the cache size to 32 blocks and found again
+the same performance, which raises the suspicion that the test
+workload might not force any cache replacement, so the eviction
+strategy doesn't matter.)
- - Don't emphasize that stuff needs to be copied from user space to
- kernel space. Instead, emphasize validation and suggest that
- copying is a common solution and that it will be necessary in
- project 3 and in real OSes. Also revise the grading criteria to
- match.
+Godmar Back <godmar@gmail.com> writes:
+
+> in your sample solution to P4, dir_reopen does not take any locks when
+> changing a directory's open_cnt. This looks like a race condition to
+> me, considering that dir_reopen is called from execute_process without
+> any filesystem locks held.
+
+* Get rid of rox--causes more trouble than it's worth
+
+* Reconsider command line arg style--confuses everyone.
+
+* Finish writing tour.
+
+* Introduce a "yield" system call to speed up the syn-* tests.
+
+via Godmar Back:
+
+* Project 3 solution needs FS lock.
+
+* Get rid of mmap syscall, add sbrk.
+
+* Make backtrace program accept multiple object file arguments,
+ e.g. add -u option to allow backtracing user program also.
+
+* page-linear, page-shuffle VM tests do not use enough memory to force
+ eviction. Should increase memory consumption.
+
+* Add FS persistence test(s).
+
+* lock_acquire(), lock_release() don't need additional intr_dis/enable
+ calls, because the semaphore protects lock->holder.
+ [ Think this over: is this really true when priority donation is
+ implemented? intr_dis/enable prevents the race with thread_set_priority.
+ Leaving it there could help the students getting the correct synchronization
+ right.
+ ]
+
+
+
+* process_death test needs improvement
+
+* Internal tests.
+
+* Improve automatic interpretation of exception messages.
+
+* Userprog project:
- Mark read-only pages as actually read-only in the page table. Or,
since this was consistently rated as the easiest project by the
Alternately we could just remove the synchronization on pid
selection and check that students fix it.
-* VM project:
-
- - Discuss the perils of mixing dirty bits between kernel and user virtual
- memory.
-
- - Sample solution.
-
- - Update grading/vm to reflect new mmap, munmap forms.
-
* Filesys project:
- - Increase maximum disk size from 8 MB to something that actually
- requires doubly indirect nodes. There is a negative pressure here
- from the bitmap object--perhaps we need a specialized bitmap that
- doesn't have to be all in-memory at once.
-
- Alternatively, shrink the inode size.
-
- - Add option to disable buffer cache.
-
- - Sample solution.
-
- Need a better way to measure performance improvement of buffer
cache. Some students reported that their system was slower with
cache--likely, Bochs doesn't simulate a disk with a realistic
speed.
- - Clarify effect of remove(cwd).
-
* Documentation:
- - Finish writing tour.
-
- Add "Digging Deeper" sections that describe the nitty-gritty x86
details for the benefit of those interested.
- Add explanations of what "real" OSes do to give students some
perspective.
- - GNU make 3.80 or later is required.
-
* Assignments:
- Add extra credit:
. opendir/readdir/closedir
. everything needed for getcwd()
-
-* Tests:
-
- - Release some of them.
-
- - The threads, userprog, vm test source files could use
- factorization and cleanup along the lines of fslib in the filesys
- tests.
-
- - The p1-4.c testcase needs significant tuning. Currently it takes
- too long (especially when SHOW_PROGRESS is turned on) and doesn't
- show significant improvement.
-
-* Code:
-
- - Need an optimization barrier and an explanation of it in the
- documentation.
-
- - Need to check the wait system call more thoroughly.
projects / pintos-anon / blobdiff