Dr. Dobb's Journal - November 2008 - (Page 38)

Core Technology GREEN TELNET Listing Two 1. void send_data(int s_client, int send_data_fd, struct replay_buffer *replay_buff) { 2. struct gqnode dequeued_data; 3. struct stat queue_stats; 4. struct flock fl; 5. char gt_packet[BUF_SIZE + 2]; 6. char data_notifier; 7. char *p; 8. char c; 9. long queue_file_size; 10. int temp = 0; 11. int bytes_written = 0; 12. int queue_fd; 13. int status; 14. int messages_written; 15. int bytes_read = 0; 16. int i = 0; 17. 18. while (!client_wants_sleep && 19. read(send_data_fd, &data_notifier, sizeof(data_notifier)) > 0) { 20. queue_fd = open (queue_file_path, O_RDWR); 21. fl = acquire_queue_lock (queue_fd); 22. while (read(queue_fd, &dequeued_data, sizeof(dequeued_data)) > 0) { 23. if (!client_wants_sleep && 24. (bytes_written = write (s_client, dequeued_data.buf, dequeued_data.nbytes)) > 0); 25. replay_buff->byte_count = replay_buff->byte_count + bytes_written; 26. for (i = 0; i buffer[replay_buff->index] = dequeued_data.buf[i]; 28. replay_buff->index = (replay_buff->index + 1) % RB_MAX; 29. } 30. if (client_wants_sleep) { 31. status = fstat (queue_fd, &queue_stats); 32. queue_file_size = queue_stats.st_size; 33. temp_queue_fd = open (queue_file_path, O_RDWR); 34. messages_written = 0; 35. while (read(queue_fd, &dequeued_data, sizeof (dequeued_data)) > 0) { 36. write(temp_queue_fd, &dequeued_data, sizeof (dequeued_data)); 37. messages_written++; 38. } 39. close(temp_queue_fd); 40. ftruncate(queue_fd, messages_written*sizeof(dequeued_data)); 41. release_queue_lock(queue_fd, fl); 42. close(queue_fd); 43. return; 44. } 45. } 46. ftruncate(queue_fd, 0); 47. release_queue_lock(queue_fd, fl); 48. close(queue_fd); 49. } 50. return; 51. } Charlie, however, reconnects at 05:46PM and has not received any of the chatting that took place between 05:35PM and 05:46PM. Next Steps: Spreading the Meme The developed gtelnetd is a useable application. All source code is available at sourceforge.net/projects/gtelnet and contributions are welcome. The techniques applied in the creation of gtelnet can be used in other client/server applications. Any interactive program in which data might be generated and delivery desirable, even though a user may not wish to respond in real time, is ripe for making power-state aware. While telnet is not used often anymore, SSH is popular and implementing power-state aware functionality should be very similar to our green telnet modifications. Instant messaging applications are another set of software in which clients may want to receive messages sent while the machine sleeps. A Lawrence Berkeley National Laboratory technical report (enduse.lbl .gov/Info/LBNL-45917b.pdf ) stated that, “proper functioning of power management would achieve additional savings of 17 TWh/year.” At $0.10 per kWh, this is $1.7 billion per year—a very large savings potential! Many, if not most, desktop computers have power management completely disabled due to the inconvenience it causes to users. By removing the application state’s dependency on a network connection as we have done with gtelnetd, users will be less inclined to disable power management. This can result in large savings of money and also reduce the CO2 footprint of PCs. continued from page 36 data for any reason it is simply resent in the normal manner after the client wakes up. gtelnetd in Action Figure 4 shows gtelnetd in action. Three clients are connected to an IRC server and chatting in the same room. Chatter “Alice” stays connected to the server the entire time, chatter “Bob” is using the gtelnet client, and chatter “Charlie” is using a regular telnet client. Both Bob and Charlie are logged into a shell account from which they run their respective IRC clients. At 05:33PM, Alice sees 38 both Bob and Charlie enter the chat room. After some initial conversation, both Bob and Charlie send their machines to sleep at 05:35PM. Alice receives a message indicating that Charlie has disconnected from the IRC server due to a dropped TCP connection to the server he is running his IRC client on. Bob, however, effectively never leaves the chat room due to his usage of the gtelnet client. When his machine goes to sleep, the gtelnetd server buffers all data that is transmitted. When Bob wakes-up his machine at 05:44PM, he sees the messages that Alice transmitted when his machine was asleep. Acknowledgments The development of this material is based on work supported by the National Science Foundation under grant CNS-0520081. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author and do not necessarily reflects the views of the National Science Foundation (NSF). The authors thank Axel Vigo from the University of Puerto Rico, Mayaguez for his early prototypes of green client/server applications. DDJ Dr. Dobb’s Journal l www.ddj.com l November 2008 http://www.sourceforge.net/projects/gtelnet http://enduse.lbl.gov/Info/LBNL-45917b.pdf http://enduse.lbl.gov/Info/LBNL-45917b.pdf http://www.ddj.com

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

Dr. Dobb's Journal - November 2008 Contents Friday Night Fish Fry Alia Vox Developer Diaries Developer's Notebook Saving Open Source Conversations iPhone Building Your Own Web Server Green Telnet What's New In Boost Threads? Testing Service Oriented Architectures Test Case Generation, UML, and Eclipse Unit Testing Web Services C3 Programming The Agile Edge Swaine's Flames Effective Concurrency

Dr. Dobb's Journal - November 2008

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