Note that struct file, struct dir are per-thread.

[pintos-anon] / TODO

-*- text -*-

* In grading scripts, warn when a fault is caused by an attempt to
  write to the kernel text segment.  (Among other things we need to
  explain that "text" means "code".)

* Reconsider command line arg style--confuses everyone.

* Internal tests.

* Godmar: Introduce memory leak robustness tests - both for the
  well-behaved as well as the mis-behaved case - that tests that the
  kernel handles low-mem conditions well.

* Godmar: Another area is concurrency. I noticed that I had passed all
  tests with bochs 2.2.1 (in reproducibility mode). Then I ran them
  with qemu and hit two deadlocks (one of them in rox-*,
  incidentally). After fixing those deadlocks, I upgraded to bochs
  2.2.5 and hit yet another deadlock in reproducibility mode that
  didn't show up in 2.2.1. All in all, a standard grading run would
  have missed 3 deadlocks in my code.  I'm not sure how to exploit
  that for grading - either run with qemu n times (n=2 or 3), or run
  it with bochs and a set of -j parameters.  Some of which could be
  known to the students, some not, depending on preference.  (I ported
  the -j patch to bochs 2.2.5 -
  http://people.cs.vt.edu/~gback/pintos/bochs-2.2.5.jitter.patch but I
  have to admit I never tried it so I don't know if it would have
  uncovered the deadlocks that qemu and the switch to 2.2.5
  uncovered.)

* Godmar: There is also the option to require students to develop test
  workloads themselves, for instance, to demonstrate the effectiveness
  of a particular algorithm (page eviction & buffer cache replacement
  come to mind.) This could involve a problem of the form: develop a
  workload that you cover well, and develop a "worst-case" load where
  you algorithm performs poorly, and show the results of your
  quantitative evaluation in your report - this could then be part of
  their test score.

* Threads project:

  - Godmar:

    >> Describe a potential race in thread_set_priority() and explain how
    >> your implementation avoids it.  Can you use a lock to avoid this race?

    I'm not sure what you're getting at here:
    If changing the priority of a thread involves accessing the ready
    list, then of course there's a race with interrupt handlers and locks
    can't be used to resolve it.

    Changing the priority however also involves a race with respect to
    accessing a thread's "priority" field - this race is with respect to
    other threads that attempt to donate priority to the thread that's
    changing its priority. Since this is a thread-to-thread race, I would
    tend to believe that locks could be used, although I'm not certain.  [
    I should point out, though, that lock_acquire currently disables
    interrupts - the purpose of which I had doubted in an earlier email,
    since sema_down() sufficiently establishes mutual exclusion. Taking
    priority donation into account, disabling interrupts prevents the race
    for the priority field, assuming the priority field of each thread is
    always updated with interrupts disabled. ]

    What answer are you looking for for this design document question?

  - Godmar:

    >> Did any ambiguities in the scheduler specification make values in the
    >> table uncertain?  If so, what rule did you use to resolve them?  Does
    >> this match the behavior of your scheduler?

    My guess is that you're referring to the fact the scheduler
    specification does not prescribe any order in which the priorities of
    all threads are updated, so if multiple threads end up with the same
    priority, it doesn't say which one to pick.  ("round-robin" order
    would not apply here.)

    Is that correct?

  - Godmar:
    
    One of my groups implemented priority donation with these data
    structures in synch.cc:
    ---
    struct value
    {
      struct list_elem elem;      /* List element. */
      int value;                  /* Item value. */
    };

    static struct value values[10];
    static int start = 10;
    static int numNest = 0;
    ---
    In their implementation, the "elem" field in their "struct value" is
    not even used.

    The sad part is that they've passed all tests that are currently in
    the Pintos base with this implementation. (They do fail the additional
    tests I added priority-donate-sema & priority-donate-multiple2.)

    Another group managed to pass all tests with this construct:
    ---
    struct lock
      {
        struct thread *holder;      /* Thread holding lock (for debugging). */
        struct semaphore semaphore; /* Binary semaphore controlling access. */
        //*************************************
        int pri_prev;
        int pri_delta;              //Used for Priority Donation
        /**************************************************/
      };
    ---
    where "pri_delta" keeps track of "priority deltas." They even pass
    priority-donate-multiple2.

    I think we'll need a test where a larger number of threads & locks
    simultaneously exercise priority donation to weed out those
    implementations.

    It may also be a good idea to use non-constant deltas for the low,
    medium, and high priority threads in the tests - otherwise, adding a
    "priority delta" might give - by coincidence - the proper priority for
    a thread.

  - Godmar: Another thing: one group passed all tests even though they
    wake up all waiters on a lock_release(), rather than just
    one. Since there's never more than one waiter in our tests, they
    didn't fail anything. Another possible TODO item - this could be
    part a series of "regression tests" that check that they didn't
    break basic functionality in project 1. I don't think this would
    be insulting to the students.

* Userprog project:

  - Get rid of rox--causes more trouble than it's worth

  - Extra credit: specifics on how to implement sbrk, malloc.

  - Godmar: 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.

  - Godmar: 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.

  - Godmar: 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.

  - Godmar: is there a test that tests that they properly kill a process that
    attempts to access an invalid address in user code, e.g. *(void**)0 =
    42;?

    It seems all of the robustness tests deal with bad pointers passed to
    system calls (at least judging from test/userprog/Rubric.robustness),
    but none deals with bad accesses by user code, or I am missing
    something.

    ps: I found tests/vm/pt-bad-addr, which is in project 3 only, though.

    For completeness, we should probably check read/write/jump to unmapped
    user virtual address and to mapped kernel address, for a total of 6
    cases. I wrote up some tests, see
    http://people.cs.vt.edu/~gback/pintos/bad-pointers/

  - process_death test needs improvement

  - Godmar: In the wait() tests, there's currently no test that tests
    that a process can only wait for its own children. There's only
    one test that tests that wait() on an invalid pid returns -1 (or
    kills the process), but no test where a valid pid is used that is
    not a child of the current process.

    The current tests also do not ensure that both scenarios (parent waits
    first vs. child exits first) are exercised. In this context, I'm
    wondering if we should add a sleep() system call that would export
    timer_sleep() to user processes; this would allow the construction of
    such a test. It would also make it easier to construct a test for the
    valid-pid, but not-a-child scenario.

    As in Project 4, the baseline implementation of timer_sleep() should
    suffice, so this would not necessarily require basing Project 2 on
    Project 1. [ A related thought: IMO it would not be entirely
    unreasonable to require timer_sleep() and priority scheduling sans
    donation from Project 1 working for subsequent projects. ]

* VM project:

  - Godmar: Get rid of mmap syscall, add sbrk.

  - Godmar: page-linear, page-shuffle VM tests do not use enough
    memory to force eviction.  Should increase memory consumption.

  - Godmar: fix the page* tests to require swapping

  - Godmar: make sure the filesystem fails if not properly
    concurrency-protected in project 3.

  - Godmar: Another area in which tests could be created are for
    project 3: tests that combine mmap with a paging workload to see
    their kernel pages properly while mmapping pages - I don't think
    the current tests test that, do they?

* Filesys project:

  - 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.

    (Perhaps we should count disk reads and writes, not time.)

  - Need lots more tests.

  - Detect implementations that represent the cwd as a string, by
    removing a directory that is the cwd of another process, then
    creating a new directory of the same name and putting some files
    in it, then checking whether the process that had it as cwd sees
    them.

  - dir-rm-cwd should have a related test that uses a separate process
    to try to pin the directory as its cwd.

  - Godmar: I'm not sure if I mentioned that already, but I passed all
    tests for the filesys project without having implemented inode
    deallocation. A test is needed that checks that blocks are
    reclaimed when files are deleted.

  - Godmar: 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: I haven't analyzed the tests for project 4 yet, but I'm
    wondering if the fairness requirements your specification has for
    readers/writers are covered in the tests or not.


* Documentation:

  - 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.

* To add partition support:

  - Find four partition types that are more or less unused and choose
    to use them for Pintos.  (This is implemented.)

  - Bootloader reads partition tables of all BIOS devices to find the
    first that has the "Pintos kernel" partition type.  (This is
    implemented.)  Ideally the bootloader would make sure there is
    exactly one such partition, but I didn't implement that yet.

  - Bootloader reads kernel into memory at 1 MB using BIOS calls.
    (This is implemented.)

  - Kernel arguments have to go into a separate sector because the
    bootloader is otherwise too big to fit now?  (I don't recall if I
    did anything about this.)

  - Kernel at boot also scans partition tables of all the disks it can
    find to find the ones with the four Pintos partition types
    (perhaps not all exist).  After that, it makes them available to
    the rest of the kernel (and doesn't allow access to other devices,
    for safety).

  - "pintos" and "pintos-mkdisk" need to write a partition table to
    the disks that they create.  "pintos-mkdisk" will need to take a
    new parameter specifying the type.  (I might have partially
    implemented this, don't remember.)

  - "pintos" should insist on finding a partition header on disks
    handed to it, for safety.

  - Need some way for "pintos" to assemble multiple disks or
    partitions into a single image that can be copied directly to a
    USB block device.  (I don't know whether I came up with a good
    solution yet or not, or whether I implemented any of it.)

* To add USB support:

    - Needs to be able to scan PCI bus for UHCI controller.  (I
      implemented this partially.)

    - May want to be able to initialize USB controllers over CardBus
      bridges.  I don't know whether this requires additional work or
      if it's useful enough to warrant extra work.  (It's of special
      interest for me because I have a laptop that only has USB via
      CardBus.)

    - There are many protocol layers involved: SCSI over USB-Mass
      Storage over USB over UHCI over PCI.  (I may be forgetting one.)
      I don't know yet whether it's best to separate the layers or to
      merge (some of) them.  I think that a simple and clean
      organization should be a priority.

    - VMware can likely be used for testing because it can expose host
      USB devices as guest USB devices.  This is safer and more
      convenient than using real hardware for testing.

    - Should test with a variety of USB keychain devices because there
      seems to be wide variation among them, especially in the SCSI
      protocols they support.  Should try to use a "lowest-common
      denominator" SCSI protocol if any such thing really exists.

    - Might want to add a feature whereby kernel arguments can be
      given interactively, rather than passed on-disk.  Needs some
      though.