Class 5
CS 439
31 January 2013

On the board
------------

1. Last time
2. Condition variables
3. Semaphores
4. Monitors
5. Standards for concurrent programming
6. Practice

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1. Last time

    --what is concurrency?

    --strategies for managing:

        --provide atomicity

        --protect critical sections. how?

        Mutexes!

    --another synchronization primitive:

        condition variables!

        conceptually is a queue

        confusing because it's hard to talk about everything at once,
        but:

            --a waiter is waiting for something to happen

            --a signaler expresses that that something has happened

            --unfortunately for programmability, the waiter has to be
            prepared to wake up even if nothing happened (or if
            something happened, but the core condition is no longer
            true)

2. Condition variables

    A. Motivation (last time)
    B. Usage (last time)
    C. Two ways to understand if-vs-while

        (1) It's the 'while' condition that actually guards the program.

        (2) There's simply no guarantee when the thread proceeds that
        the condition hods. 

	    --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

	--QUESTION: Why must cond_wait both release the mutex and sleep?
	(see handout, 1b)

	    --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!


3. Semaphores

    --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
    
	--Dijkstra was a highly notable figure in computer science who
	spent the latter part of his career here at UT

    --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

    --Don't use these!

    --Problems:
	--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

    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

	    --Book follows this technical definition
	    
	    --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

	    --you must follow these conventions on lab 2

    --Example: see handout, item 2

    --RULE:
    
	--acquire/release at beginning/end of functions

    --RULE:

	--hold lock when doing condition variable operations

	--Some (e.g., Birrell) 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.
	
	--to get credit in Lab 2, you must hold the associated mutex
	when doing a condition variable operation

    --Different styles of monitors:

	--Hoare-style: signal() immediately wakes the waiter

	--Hansen-style and what we will use:
	signal() eventually wakes the waiter. Not an immediate transfer

    --Can we replace SIGNAL with BROADCAST, given our monitor semantics?
     (Answer: yes, always.) Why?

	--while() condition tests the needed invariant. program
	doesn't progress pass while() unless the needed invariant is
	true.

	--result: spurious wake-ups are acceptable....

	--...which implies you can always wakeup a thread at any
	moment with no loss of correctness....

	--....which implies you can replace SIGNAL with BROADCAST
	[though it may hurt performance to have a bunch of
	needlessly awake threads contending for a mutex that they
	will then acquire() and release().]

    --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()}.

    --Can we replace BROADCAST with SIGNAL?

	--Answer: not always. 

	--Example:
	    --memory allocator
	    --threads allocate and free memory in variable-sized chunks
	    --if no memory free, wait on a condition variable
	    --now posit:
		--two threads waiting to allocate chunks of memory
		--no memory free at all
		--then, a third thread frees 10,000 bytes
	    --SIGNAL alone does the wrong thing: we need to awaken both
	    threads

---------------------------------------------------------------------------

admin notes:

    --lab partnerships

    --Therac-25

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5. Standards [and advice] for concurrent programming

    A. Standards

	--see Mike D's "Programming With Threads", linked from lab 2

	    --You are required to follow this document

	    --You will lose points (potentially many!) on the lab and 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
		--http://www.nytimes.com/2000/01/23/weekinreview/january-16-22-i-m-no-doofus-i-m-a-genius.html

	--MikeD stands on the desk when proclaiming the standards

    B. Top-level piece of advice: SAFETY FIRST.

	--Locking at coarse grain is easiest to get right, so do
	that (one big lock for each big object or collection of
	them)
	
	--Don't worry about performance at first

	--In fact, don't even worry about liveness at first
	
	    --In other words don't view deadlock as a disaster

	--Key invariant: make sure your program never does the wrong thing

    C. More detailed advice: design approach

	[We will use item #2 on today's handout 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 monitors.
		 
		 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 buffers 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.
		    But lock was sort of a given from the monitor.
	     
	    4. Write the methods, using locks and condition variables for 
	    coordination  
	

    D. More advice

	1. Don't manipulate synchronization variables or shared state
	variables in the code associated with a thread; do it with the
	code associated with a shared object.  
      
	    --Threads tend to have "main" loops. These loops tend to
	    access shared objects. *However*, the "thread" piece of it
	    should not include locks or condition variables. Instead,
	    locks and CVs should be encapsulated in the shared objects.

	    --Why?

		(a) Locks are for synchronizing across multiple threads.  Doesn't 
		make sense for one thread to "own" a lock.

		(b) Encapsulation -- details of synchronization are
		internal details of a shared object. Caller should not
		know about these details.  "Let the shared objects do
		the work."

	    --Common confusion: trying to acquire and release locks
	    inside the threads' code (i.e., not following this advice).
	    Bad idea! Synchronization should happen within the shared
	    objects. Mantra: "let the shared objects do the work".
	
	    --Note: our first example of condition variables -- 1c on
	    today's handout -- doesn't actually follow the advice, but
	    that is in part so you can see all of the parts working
	    together.

	2. Different way to state what's above:
	
	    --You want to decompose your problem into objects, as in
	    object-oriented style of programming.

	    --Thus:

	       (1) Shared object encapsulates code, synchronization 
		   variables, and state variables 

		(2) Shared objects are separate from threads 

	
6. Practice with concurrent programming

    --sleeping barber question from prior midterm posted (as today's
    reading). use it as practice

    --we guarantee to test concurrent programming in this course

    --today, we work a different example:

	--workers interact with a database
	    --motivation: banking, airlines, etc.

	--readers never modify database

	--writers read and modify data

	--using only a single mutex lock would be overly restrictive.
	Instead, want
	    --many readers at the same time
	    --only one writer at a time

    --let's follow the concurrency advice from last time (and above).....

	    1. Getting started
		a. what are units of concurrency? [readers/writers]
		b. what are shared chunks of state? [database]
		c. what does the main function look like?
		    read() 
			check in -- wait until no writers
			access DB
			check out -- wake up waiting writer, if appropriate

		    write()
			check in -- wait until no readers or writers
			access DB
			check out -- wake up waiting readers or writers

	    2. and 3. Synchronization constraints and objects

		--reader can access DB when no writers (condition:
		okToRead)

		--writer can access DB when no other readers or writers
		(condition: okToWrite)

		--only one thread manipulates shared variables at a
		time. NOTE: **this does not mean only one thread in the
		DB at a time** (mutex)

    
	    4. write the methods

		--inspiration required:
		    int AR = 0; // active readers
		    int AW = 0; // # active writers
		    int WR = 0; // waiting readers
		    int WW = 0; // waiting writers
	
		--see handout for the code

    --QUESTION: why not just hold the lock all the way through "Execute
    req"? (Answer: the whole point was to provide more concurrency,
    i.e., to move away from exclusive access.)

    --QUESTION: what if we had shared locks? The implementation of
    shared locks is given on the handout

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