Computing in a Parallel Universe
Multicore chips could bring about the biggest change in computing since the microprocessor
Losing My Minds
If our universe is a peculiarly friendly place for builders of digital computers, it is not so benign for creators of programs that run concurrently on parallel hardware. Or maybe the difficulty lies in the human mind rather than in the nature of the universe.
Think of a program that reserves airline seats. Travelers access the program through a Web site that shows a diagram of the aircraft interior, with each seat marked as either vacant or occupied. When I click on seat 3A, the program first checks its database to make sure 3A is still available; if it is, I get a confirming message, and the database is updated to show that seat 3A has been taken. All's well, at least in a sequential world. But you too may be booking a seat on the same flight, and you may want 3A. If my transaction completes before your request arrives, then I'm afraid you're out of luck. On the other hand, if you are quicker with the mouse, I'm the one who will be disappointed. But what happens if the two requests are essentially simultaneous and are handled in parallel by a multiprocessing computer? Suppose the program has just assigned the seat to me but has not yet revised the database record when your request reaches the Web server. At that instant a check of the database indicates 3A is still vacant, and so we both get confirming messages. It's going to be a cozy flight!
Of course there are remedies for this problem. Programming techniques for ensuring exclusive access to resources have been known for 50 years; they are key assets in the intellectual heritage of computer science, and the airline's programmer should certainly know all about them. Many of the same issues arise even in uniprocessor systems where "time slicing" creates the illusion that multiple programs are running at the same time.
Writing correct concurrent programs is not impossible or beyond human abilities, but parallelism does seem to make extreme demands on mental discipline. The root of the difficulty is nondeterminism: Running the same set of programs on the same set of inputs can yield different results depending on the exact timing of events. This is disconcerting if your approach to programming is to try to think like a computer.
Even though the brain is a highly parallel neural network, the mind seems to be single-threaded. You may be able to walk and chew gum at the same time, but it's hard to think two thoughts at once. Consciousness is singular. In trying to understand a computer program, I often imagine myself standing at a certain place in the program text or in a flow chart. As the instructions are executed, I follow along, tracing out the program's path. I may have to jump from place to place to follow branches and loops, but at any given moment there is always one location that I can call here. Furthermore, I am the only actor on the stage. Nothing ever happens behind my back or out of sight. Those airline seats can't be assigned unless I assign them.
That's how it works in a sequential program. With parallel processing, the sense of single-mindedness is lost. If I try to trace the path of execution, I have to stand in many places at once. I don't know who "I" am anymore, and there are things happening all around me that I don't remember doing. "I contain multitudes," declared Walt Whitman, but for a computer programmer this is not a healthy state of mind.
Edward A. Lee, of the University of California, Berkeley, recently described the mental challenge of writing nondeterministic programs:
A folk definition of insanity is to do the same thing over and over again and expect the results to be different. By this definition, we in fact require that programmers of multithreaded systems be insane. Were they sane, they could not understand their programs.
Lee also writes:
I conjecture that most multithreaded general-purpose applications are so full of concurrency bugs that—as multicore architectures become commonplace—these bugs will begin to show up as system failures. This scenario is bleak for computer vendors: Their next-generation machines will become widely known as the ones on which many programs crash.
Cynics, of course, will reply that computers of every generation have exactly that reputation.
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