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Notes on video lecture:
Concurrency Challenges
Notes taken by Edward Tanguay on June 24, 2014 (go to class or lectures)
Choose from these words to fill the blanks below:
operations, concurrency, actual, response, queue, background, simultaneously, core, SQLite, middleware, POSIX, low, communication, software, vision, quad, RenderScript
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prime reason for concurrency
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leveraging advances in hardware and software
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it's hard to buy a computing device that has just one         
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today there are even         -core Android phones
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historically
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event driven architecture
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one thread
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for blocking operations, had to post on message           , then handle it later on an event loop
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awkward to program
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hard to optimize
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used the reactor pattern
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now operating system and                      have better concurrency support
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Android provides several concurrency frameworks which enable multiple threads to perform long-running computations in the                     
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this maps efficiently onto multiple cores
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these background operations can block independently of the UI thread
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using these frameworks effectively requires knowledge of concurrency patterns
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e.g. worker thread pattern:
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increasing performance
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via parallelism
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overlaps computation and                            mechanisms
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background threads that download images can run in parallel with each other, and with a thread that interacts with the user
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use the                          framework
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targeted at image processing, computational photography, or computer             
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improve responsiveness
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even with one core, concurrency can be used to improve perceived                  time
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e.g. ensures that the user interface doesn't ignore input while other                      are running
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challenges
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accidental complexities
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written in programs like C
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pthread
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low-level and error-prone data types
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casting disable compile-time type checking, causes errors
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some types are not compatible on other platforms, e.g. non-           platforms
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most operating systems today have these kinds of problems when       -level programming languages like C are used
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developers use tools that aren't set up to handle                       
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you need tools that enable you to drill down and look inside your                  to figure out what is happening
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the behavior you see in the debugger doesn't necessarily reflect the behavior in the              software
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inherent complexities
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scheduling and synchronization
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the order and time that operations are performed
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ensure that multiple concurrent threads don't                              execute in criticial sections of a program at the same time
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analogy:
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air traffic controllers share runway resource for mulitple planes
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likewise, Android needs to synchronize access to the              contact database
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ensures that threads are given proper access to system resources
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deadlock
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two or more competing actions are waiting for the other to finish and so neither ever do
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need to be identified and removed
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heisenbug
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bugs that change their behavior when observed
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you may use a debugger to step through one thread while other threads are running

Vocabulary:

heisenbug, n. a software bug that seems to disappear or alter its behavior when one attempts to study it ⇒ "Heisenbugs occur because common attempts to debug a program, such as inserting output statements or running it in a debugger, usually modify the code, change the memory addresses of variables and the timing of its execution."
Concurrency Challenges