By Anandhi Sundaram

1. Paper Review of Why Events Are A Bad Idea(for high-concurrency servers)Rob von Behren, Jeremy Condit and Eric Brewer By Anandhi Sundaram

2. THE STAGE • Highly Concurrent Applications • Internet Servers • Transaction Processing Databases • Challenges Posed • Large numbers of Concurrent tasks, require scalable Data Structures • Operate near max capacity, Resource Contention, Scheduling Sensitivity • Inevitable Overload, must be handled to avoid thrashing, well-conditioned • Race Conditions, Code Complexity Ideal Peak: some resource at max Performance Overload: someresource thrashing Load (concurrent tasks) CS533 - Concepts of Operating Systems

3. Events are Better • Recent Arguments for Events • Inexpensive synchronization due to co-operative multitasking • Lower Overhead for managing state (no stacks) • Better scheduling and locality • More Flexible Control Flow • True But • All above properties can be implemented in Threads • Problem is with Thread Implementations, and not Threads CS533 - Concepts of Operating Systems

4. Overcoming the “Problems” with Threads • Performance • Criticism: Threads don’t perform well for high concurrency • Reasons : • Presence of O(n) thread operations • High Context Switch overhead • Response • Avoid O(n) operations • Co-operative Multi-tasking CS533 - Concepts of Operating Systems

5. Simple Scalability Test • Slightly modified GNU Pth • Removed O(n) operations from scheduler • Thread-per-task Vs Single Thread • Same Performance CS533 - Concepts of Operating Systems

6. Overcoming the “Problems” with Threads • Control Flow • Criticism: Threads have restrictive Control flow • Response: Code Structure of Web Server and applications fall under three simple categories - call/return - Parallel Calls - Pipeline Threads express the above more naturally Complex Patterns are unnatural, hard to understand, Error-prone CS533 - Concepts of Operating Systems

7. Overcoming the “Problems” with Threads • Synchronization • Criticism: Thread Synchronization mechanisms are too heavyweight • Response : • Co-operative Multitasking works for threads too • Compiler Support in MultiProcessor Systems CS533 - Concepts of Operating Systems

8. Overcoming the “Problems” with Threads • State Management • Criticism: Thread Stacks are an ineffective way to manage live state • Response : • Compiler support for Dynamic Stack Growth • Compiler support for better Live State Management • Entire frame could be popped in case of tail call • Variables with overlapping lifetimes could be moved off stack • Compiler warns about inefficiency CS533 - Concepts of Operating Systems

9. Overcoming the “Problems” with Threads • Scheduling • Criticism: Thread schedulers are too generic • Can’t use application-specific information • Response • 2D scheduling: task & program location • Threads schedule based on task only • Events schedule by location (e.g. SEDA) • Allows batching, prediction for SRCT • Resource Aware Scheduling in Capriccio • Runtime system tracks current location • Block graph allows prediction Task Program Location Events Threads CS533 - Concepts of Operating Systems

10. The Case For Threads • Observations About Modern Servers • Concurrent Requests are Largely Independent • Code handling each request is Sequential • Threads – A more Natural Programming Model • Express Better Control Flow • Automatic State Management • Exception Handling Easier in Threads CS533 - Concepts of Operating Systems

11. The Case for Threads • Lots of Infrastructure for Threads • Debuggers, Languages, Compilers • Consequences • More amenable to Analysis • Less effort to get working systems CS533 - Concepts of Operating Systems

12. The Duality Argument • Two Models are Duals of Each other • With Good Practices both approaches should yield equivalent performance • No inherent problem with threads! • Equivalent Good Thread implementations possible • Thread-based approach is preferrable • More natural Programming Model • Co-operative multitasking threads • Compiler Support CS533 - Concepts of Operating Systems

13. Evaluation • User-level cooperative threads package • Minimal context switching • Converts Blocking I/O to Asynchronous I/O • No O(n) operations • Support > 100K threads • Simple web server: Knot • Knot-A : Favors accepting new clients over processing • Knot-C : Favors processing existing clients over accepting new clients CS533 - Concepts of Operating Systems

14. 900 800 KnotC (Favor Connections) KnotA (Favor Accept) 700 Haboob Mbits / second 600 500 400 300 200 100 0 1 4 16 64 256 1024 4096 16384 Concurrent Clients Evaluation • Compared performance of Haboob with Knot-A, Knot-C • The Steady-state bandwidth of • Knot-C higher than Haboob • Performance degradation of Knot-A, Haboob is due to poor scalability of poll() • Haboob ran out of memory with 16384 clients CS533 - Concepts of Operating Systems

15. Evaluation • Reasons for Haboob’s Poor Performance • Haboob’s context switches 6X times as Knot • Large number of queueing operations • Relies heavily on garbage collection • Ambiguous control flow, reduces compiler optimization, increases CPU-stalls Conclusion : A well-designed thread package can achieve the same scaling behaviour as a well-designed event system. CS533 - Concepts of Operating Systems

16. References • 1. Why Events are a Bad Idea (for high-concurrency servers), Rob von Behren, Jeremy Condit, and Eric Brewer. In Proceedings of the 10th Workshop on Hot Topics in Operating Systems (HotOS IX), Lihue, Hawaii, May 2003. [pdf] http://warthog.cs.berkeley.edu/pubs/threads_hotos_2003_slides.ppt • 2. On the Duality of Operating System Structures, Hugh C.Lauer, Roger M.Needham • 3. Event Driven Programming for Robust Software, Frank Dabek, Nickolai Zeldovich, Frans Kaashoek, David Mazieres, Robert Morris • 4. SEDA: An Architecture for Well-Conditioned, Scalable Internet Services, Matt Welsh, David Culler, and Eric Brewer • 5. Capriccio: Scalable Threads for Internet Services, Rob von Behren, Jeremy Condit, Feng Zhou, George C. Necula, and Eric Brewer. In Proceedings of the Ninteenth Symposium on Operating System Principles (SOSP-19), Lake George, New York. October 2003 http://warthog.cs.berkeley.edu/pubs/capriccio-sosp-2003.pdf CS533 - Concepts of Operating Systems

17. References • GNU Pth – The GNU Portable Threads http://www.gnu.org/software/pth CS533 - Concepts of Operating Systems