-*- text -*-
-* Clean up:
+* Bochs is not fully reproducible.
- - Replace AT&T assembly code by Intel assembly so that students can
- figure out the asm from the Intel manuals. Compare object files
- before and after to make sure nothing changed.
+Godmar says:
-* Grader:
+- 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.
- - Fix bug where failures are being treated as warnings.
+- 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.)
-* Userprog project:
+- 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
+
+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.
+
+Neither of these two cases is detected; do you currently check for
+these mistakes manually?
+
+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
+
+[...]
+
+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
+
+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
+
+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
+
+Greg Benson points out another potential TODO item for P1.
+
+----
+One thing I recall:
+
+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.
+
+---
+Way to test this: (from Godmar Back)
+
+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.
+
+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.)
+
+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.
+
+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:
- - Move `join' implementation here, from `threads' project, to help
- normalize the project difficulties.
+"You can store a pointer to inode data in struct inode, if you want,"
- - The semantics of the join system call should change so that it
- only returns the exit code once.
+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.)
+
+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).
+
+* 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.
-* Documentation:
+* Filesys project:
- - Finish writing tour.
+ - 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.
+
+* Documentation:
- Add "Digging Deeper" sections that describe the nitty-gritty x86
details for the benefit of those interested.
. Low-level x86 stuff, like paged page tables.
- . Other good ideas.
-
- - mmap/munmap should use segment IDs like Nachos. Too hard
- otherwise.
+ . Specifics on how to implement sbrk, malloc.
- - Add src/testcases/vm, src/testcases/filesys and make it clear to use
- them?
-
-* 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.
+ . Other good ideas.
- - 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.
+ . opendir/readdir/closedir
+ . everything needed for getcwd()
projects / pintos-anon / blobdiff