Dr. Dobb's Journal - May 2008 - (Page 51)

d05sutt_p4as 3/13/08 10:43 AM Page 51 cooperative model where the target thread can check to see whether someone has asked for it to interrupt work. This checking can be in between between interruption points (if you want to use both Options 3 and 4 together), or instead of interruption points (if you want to use Option 4 alone). We saw Option 4 in action in the previous two columns [1,2]: In a simple parallel search, once one worker finds an answer and records it in a shared location, the other workers can periodically check that shared location and stop their own work when they see that someone else has already found the answer. shoot. Incidentally, you may have noticed a recent trend: More recent operating systems are on the road to making all calls interruptible. For example, in Windows Vista, nearly all file and I/O APIs support interruption, so that you can stop them without just waiting for them to return. This shouldn’t be surprising, since we’ve been considering the importance of interruption in concurrent code. Summary Interrupt politely. Always use Options 3 and 4, which allow the thread or task to participate in the decision about whether and how it should clean up its work and/or continue on. Notify a thread of interruption requests only at well-defined predictable wait/join/sleep points, and make sure you write your code to be safe if interruption does happen at those points. Note that both Options 3 and 4 provide a strict superset of what is possible in Option 2: Anything you can code in Option 2, you can code in Option 3 or 4 as well. Avoid the peremptory Option 2 of not letting the thread participate in the decision. Even if you are running on Pthreads which does not support Option 3, you have the option of writing Option 4 yourself. Finally, never kill a thread or task as in Option 1, unless you can prove you’re truly in one of the rare cases where this questionable practice is safe and it’s okay to take down the whole process (or more) without any graceful cleanup at all. Most real-world attempts to kill a thread at arbitrary points are indefensible; every major threading library or environment started here, but now we know better—violence is not the answer. What About Library/OS Calls? What should you do about library calls that are not interruptible? If it doesn’t cooperate, it doesn’t cooperate. Don’t shoot! Violence is not the answer. What should you do if you need to call an OS (possibly kernel-mode) function that isn’t interruptible? The answer is the same: If it doesn’t cooperate, it doesn’t cooperate. Don’t Notes [1] H. Sutter. “Going Superlinear” (Dr. Dobb’s Journal, March 2008). [2] H. Sutter. “Super Linearity and the Bigger Machine” (Dr. Dobb’s Journal, March 2008). DDJ Herb is a software development consultant, a software architect at Microsoft, and chair of the ISO C++ standards committee. He can be contacted at www.gotw.ca. Get moving. Stay focused. Take control. Enterprise Architect from Sparx Systems redefines visual modeling with a huge set of built-in tools, technologies and capabilities, coupled with a lightweight footprint and great agility. With deep support for UML 2.1 and its related standards, Enterprise Architect 7.1 is the ideal tool to analyze, design, build and manage your next software project. Keep your project moving, keep your team focused and stay in total control. Visit sparxsystems.com for a free trial http://www.gotw.ca http://sparxsystems.com http://sparxsystems.com

Table of Contents Feed for the Digital Edition of Dr. Dobb's Journal - May 2008

Dr. Dobb's Journal - May 2008 Contents Friday Night Fish Fry Alia Vox Developer Diaries Software Development Goes to the Movies Cat: A Functional Stack-Based Little Language Mojax: Mobile Ajax Framework Kernel-Mode Databases Getting Better Search Results Effective Concurrency The Agile Edge

Dr. Dobb's Journal - May 2008

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