Dr. Dobb's Journal - October 2008 - (Page 70)

D10Sutt_p5db 8/14/08 4:05 PM Page 70 Effective Concurrency • We always maintain one placeholder divider element between the producer and the consumer, so “publishing” the change to divider what would otherwise be one step too soon, so that refers to an unconsumed node rather than to a consumed node, happens to be innocuous as long as we’re only one step ahead. • There’s exactly one consumer thread, so multiple calls to Consume must run in sequence and can never get two steps ahead. You have one month. Think of how you would approach it, and we’ll take up the challenge when we return. Notes [1] H. Sutter. “Lock-Free Code: A False Sense of Security” (DDJ, September 2008). (www.ddj.com/cpp/209903274). [2] P. Marginean. “Lock-Free Queues” (DDJ, July 2008). (www .ddj.com/208801974). [3] This is just like a canonical exception safety pattern—do all the work off to the side, then commit to accept the new state using nonthrowing operations only. “Think in transactions” applies everywhere, and should be ubiquitous in the way we write our code. [4] Compare-and-swap (CAS) is the most widely available fundamental lock-free operation and so I’ll focus on it here. However, some systems instead provide the equivalently powerful loadlinked/store-conditional (LL/SC) instead. But it’s still a bad habit to get into. It’s not a good idea to cut corners by relying on “happy” accidents, especially because there’s not much to be gained here from breaking the correct pattern. Besides, even if we wrote D then C now, it might be just another thing we’d have to change anyway next month, because… Coming Up Next month, we will consider how to generalize the queue for multiple producer and consumer threads. Your homework: What new issues does this raise? What parts of the code we just considered would be broken in the presence of multiple consumers alone and why? What about multiple producers? What about both? Once you’ve discovered the problems, what would you need to change in the code and in the queue data structure itself to address them? Acknowledgment Thanks to Tim Harris for his comments on drafts of this article. 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. Download a proven, cross-platform database alternative • Unmatched performance in enterprise systems, desktop deployments, and embedded devices for over 25 years. • Simplified framework for rapid development of data-driven applications including graphical tools to speed your development and monitor critical metrics of the database engine. • Free development and free deployment in non-commercial environments. • Flexibility to use ISAM and SQL API’s within the same application. • Sophisticated feature set including 64-bit support, memory files, ADO.NET, LINQ, C#, VCL, and dbExpress. • Cross Platform support including Windows, UNIX, Linux, Mac OS X, and more. • OEM Database Engine simplifies your deployment with a single executable and flexible licensing options. Prove it to yourself in six minutes. 70 Download today! www.faircom.com Dr. Dobb’s Journal l www.ddj.com l October 2008 http://www.ddj.com/cpp/209903274 http://www.ddj.com/208801974 http://www.ddj.com/208801974 http://www.gotw.ca http://www.faircom.com http://www.faircom.com http://www.ddj.com

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

Dr. Dobb's Journal - October 2008 Contents Friday Night Fish Fry Alia Vox Developer Diaries Developer’s Notebook Is Your Next Language COBOL? Conversations Safe Coding Practices Code Signing in Adobe AIR OpenID Single Sign-On The Book Cipher Algorithm Indexing and Searching Image files Extending Continuous Integration Into ALM The Agile Edge Effective Concurrency Swaine’s Flames

Dr. Dobb's Journal - October 2008

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