Week 6.b
CS5600
2/23 2022
https://naizhengtan.github.io/22spring/

1. Lab2
2. Condition variables
3. Semaphores
4. Monitors and standards
5. Advice for concurrent programming
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Admin

  --next class is virtual

  --midterm in <2weeks (03/07)
    --closed book
    --three topics: processes, scheduling, and concurrency

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1. Lab2

  - pipe
    -- draw how it looks
    -- double close?


2. Condition variables

    A. Motivation

        [last time]

        --producer/consumer queue

        --very common paradigm. also called "bounded buffer":

          --producer puts things into a shared buffer
          --consumer takes them out
          --producer must wait if buffer is full; consumer must
            wait if buffer is empty
          --shows up everywhere
          --Soda machine: producer is delivery person, consumer
              is soda drinkers, shared buffer is the machine
          --OS implementation of pipe()
          --DMA buffers

    --producer/consumer queue using mutexes (see handoutw5b)

        --what's the problem with that?

        --answer: a form of *busy waiting* 

          --analogy: the next person keeps knocking the door

    --It is convenient to break synchronization into two types:

        --*mutual exclusion*: allow only one thread to access a given
        set of shared state at a time

        --*scheduling constraints*: wait for some other thread to do
        something (finish a job, produce work, consume work, accept
        a connection, get bytes off the disk, etc.)

    B. Usage

    --API

      --void cond_init (Cond *, ...); 
      --Initialize

      --void cond_wait(Cond *c, Mutex* m);
      --Atomically unlock m and sleep until c signaled 
      --Then re-acquire m and resume executing 

      --void cond_signal(Cond* c);
      --Wake one thread waiting on c

      [in some pthreads implementations, the analogous
      call wakes *at least* one thread waiting on c. Check the
      the documentation (or source code) to be sure of the
      semantics. But, actually, your implementation shouldn't
      change since you need to be prepared to be "woken" at
      any time, not just when another thread calls signal().
      More on this below.]

      --void cond_broadcast(Cond* c);
      --Wake all threads waiting on c

    C. Important points

       (1) We MUST use "while", not "if". Why?

        --Because we can get an interleaving like this:

            --The signal() puts the waiting thread on the ready list
            but doesn't run it

            --That now-ready thread is ready to acquire() the mutex
            (inside cond_wait()).

            --But a *different* thread (a third thread: not the
            signaler, not the now-ready thread) could acquire() the
            mutex, work in the critical section, and now invalidates
            whatever condition was being checked

            --Our now-ready thread eventually acquire()s the
            mutex...

            --...with no guarantees that the condition it was
            waiting for is still true

        --Solution is to use "while" when waiting on a condition
        variable

        --DO NOT VIOLATE THIS RULE; doing so will (almost always)
        lead to incorrect code

            --NOTE: NOTE: NOTE: There are two ways to understand
            while-versus-if:
            (a) It's the 'while' condition that actually guards the program.
            (b) There's simply no guarantee when the thread comes out of
            wait that the condition holds. 

        (2) cond_wait releases the mutexes and goes into the waiting
        state in one function call (see panel 2b of the handoutw5b). 

            --QUESTION: Why?

        --Answer: can get stuck waiting.

            Producer: while (count == BUFFER_SIZE)
            Producer: release()
            Consumer: acquire()
            Consumer: .....
            Consumer: cond_signal(&nonfull)
            Producer: cond_wait(&nonfull)

        --Producer will never hear the signal!


    [skipped in class]
    Discussion: thread_cond_wait implementation

    --pthread implementation: 
    https://code.woboq.org/userspace/glibc/nptl/pthread_cond_wait.c.html

        thread_cond_wait(...) {

            release lock;

            do {
                wait-for-signal;
            } while (!signal)

            acquire lock;
        }


[skipped in class]
3. Semaphores

    --(In this class, we require you to use condition variables and
    mutexes; semaphores will be considered incorrect.)

    --Advice: don't use these. We're mentioning them only for
    completeness and for historical reasons: they were the first
    general-purpose synchronization primitive, and they were the first
    synchronization primitive that Unix supported.

    --Introduced by Edsger Dijkstra in late 1960s

    --Semaphore is initialized with an integer, N

    --Two functions:
    --Down() and Up() [also known as P() and V()]
    --The guarantee is that Down() will return only N more times
    than Up() is called
    --Basically a counter that, when it reaches 0, causes a thread
    to sleep()

    --Another way to say the same thing:
    --Semaphore holds a count
    --Down() is an atomic operation that waits for the count to
    become positive; it then decrements the count by 1
    --Up() is an atomic operation that increments the count by 1 and
    then wakes up a thread waiting on Down(), if any

    --Reasons we recommend against their use:
    --semaphores are dual-purpose (for mutual exclusion and
    scheduling constraints), so hard to read code and hard to get
    code right
    --semaphores have hidden internal state
    --getting a program right requires careful interleaving of
    "synchronization" and "mutex" semaphores


4. Monitors and standards

    Monitors = mutex + condition variables

    --High-level idea: an object (as in object-oriented systems)

    --in which methods do not execute concurrently; and

    --that has one or more condition variables

    --More detail

    --Every method call starts with acquire(&mutex), and ends with
    release(&mutex)

    --Technically, these acquire()/release() are invisible to the
    programmer because it is the programming language (i.e., the
    compiler+run-time) that is implementing the monitor

        --So, technically, a monitor is a programming language
        concept

        --But technical definition isn't hugely useful because no
        programming languages in widespread usage have true monitors

        --Java has something close: a class in which every method is
        set by the programmer to be "synchronized" (i.e., implicitly
        protected by a mutex)

        --Not exactly a monitor because there's nothing forcing
        every method to be synchronized

        --And we can *use* mutexes and condition variables to
        implement our own manual versions of monitors, though we
        have to be careful
    
    --Given the above, we are going to use the term "monitor" more
    loosely to refer to both the technical definition and also a
    "manually constructed" monitor, wherein:
    
        --all method calls are protected by a mutex (that is, the
        programmer inserts those acquire()/release() on entry and
        exit from every procedure *inside* the object)
        
        --synchronization happens with condition variables whose
        associated mutex is the mutex that protects the method calls

    --In other words, we will use the term "monitor" to refer to the
    programming conventions that you should follow when building
    multithreaded applications

    --Example: see handout week6.b, panel 1

    B. Standards & why?

      - RULES:
       --acquire/release at beginning/end of methods
       --hold lock when doing condition variable operations
       --always use "while" to check invariants, not "if"

      - why?

        --Mike Dahlin stands on the desk when proclaiming the standards

        --see Mike D's "Programming With Threads"

        --You are required to follow this document

        --You will lose points (potentially many!) on
        the exam if you stray from these standards

        --Note that in his example in section 4, there needs to be
        another line:

        --right before mutex->release(), he should have:
            assert(invariants hold)

       --the primitives may seem strange, and the rules may seem
    arbitrary: why one thing and not another?

        --there is no absolute answer here

        --**However**, history has tested the approach that we're
        using. If you use the recommended primitives and follow
        their suggested use, you will find it easier to write
        correct code

    --For now, just take the recommended approaches as a given,
    and use them for a while. If you can come up with something
    better after that, by all means do so!

    --But please remember three things:

        a. lots of really smart people have thought really hard
        about the right abstractions, so a day or two of
        thinking about a new one or a new use is unlikely to
        yield an advance over the best practices.

        b. the consequences of getting code wrong can be
        atrocious. see for example:

        http://www.nytimes.com/2010/01/24/health/24radiation.html
        http://sunnyday.mit.edu/papers/therac.pdf
        http://en.wikipedia.org/wiki/Therac-25

        c. people who tend to be confident about their abilities
        tend to perform *worse*, so if you are confident you are
        a Threading and Concurrency Ninja and/or you think you
        truly understand how these things work, then you may
        wish to reevaluate.....

        --Dunning-Kruger effect
          --https://en.wikipedia.org/wiki/Dunning%E2%80%93Kruger_effect

    C. The Commandments

        --RULE:

        --acquire/release at beginning/end of methods

        --RULE:

        --hold lock when doing condition variable operations

        --Some people
            [for example, Andrew Birrell in this paper:
                http://www.hpl.hp.com/techreports/Compaq-DEC/SRC-RR-35.pdf ]
        will say: "for experts only, no need to
        hold the lock when signaling". IGNORE THIS. Putting the signal
        outside the lock is only a small performance optimization, and
        it is likely to lead you to write incorrect code.

        --RULE:

        --a thread that is in wait() must be prepared to be restarted at
        any time, not just when another thread calls "signal()".

        --why? because the implementor of the threads and condition
        variables package *assumes* that the user of the threads package
        is doing while(){wait()}.

5. Advice for concurrent programming

    [We will use handout week6.b as a case study.....]

    --Here's a four-step design approach:

        1. Getting started:

         1a. Identify units of concurrency. Make each a thread with
         a go() method or main loop. Write down the actions a thread
         takes at a high level.  

         1b. Identify shared chunks of state. Make each shared
         *thing* an object. Identify the methods on those objects,
         which should be the high-level actions made *by* threads
         *on* these objects. Plan to have these objects be protected by mutexes.

         1c. Write down the high-level main loop of each thread. 

        Advice: stay high level here. Don't worry about synchronization 
        yet. Let the objects do the work for you. 
         
        Separate threads from objects. The code associated with a
        thread should not access shared state directly (and so there
        should be no access to locks/condition variables in the
        "main" procedure for the thread). Shared state and
        synchronization should be encapsulated in shared objects. 

        --QUESTION: how does this apply to the example on the
        handout?
        --separate loops for producer(), consumer(), and
        synchronization happens inside MyBuffer.
         
        Now, for each object: 
         
        2. Write down the synchronization constraints on the
        solution. Identify the type of each constraint: mutual
        exclusion or scheduling. For scheduling constraints, ask,
        "when does a thread wait"?

        --NOTE: usually, the mutual exclusion constraint is
        satisfied by the fact that we're programming with
        monitors.

        --QUESTION: how does this apply to the example on the
        handout?
            --Only one thread can manipulate the buffer at a time
            (mutual exclusion constraint)
            --Producer must wait for consumer to empty slots if all
            full (scheduling constraint)
            --Consumer must wait for producer to fill slots if all
            empty (scheduling constraint)

        3. Create a lock or condition variable corresponding to each 
        constraint 

        --QUESTION: how does this apply to the example on the
        handout?
            --Answer: need a lock and two condition variables.
            (lock was sort of a given from the fact of a monitor).
         
        4. Write the methods, using locks and condition variables for 
        coordination  


[thanks to David Mazieres, Mike Dahlin, and Mike Walfish
for content in portions of this lecture.]