1. Copyright, 1996 © Dale Carnegie & Associates, Inc. Life with Hardware Threads to Burn Todd C. Mowry Intel Research Pittsburgh & Carnegie Mellon University ISCA Panel June 7, 2005

2. Todd C. Mowry ISCA Panel, June 7, 2005- 2 - Functional Correctness Trumps Performance oIf your software doesn’t behave correctly, it doesn’t matter how fast it runs. oWhat does matter (see “Software Engineering”):  code development and testing time  how quickly/easily a new developer can modify existing code  achieving acceptable performance oMost programmers who choose not to write parallel code today are making a rational choice  the cost/benefit ratio is unfavorable in too many cases

3. Todd C. Mowry ISCA Panel, June 7, 2005- 3 - Making Parallel Programming More Attractive oMinimizing programming complexity is the key:  Consistency models: SC -> RC -> SC  Machine models: PRAM -> logP -> ??? oBenchmarks for a successful parallel programming model:  does it work well when you’re modifying someone else’s code?  can we teach it to freshmen in an Algs & Data Structures course? oA “semi-automatic” approach seems like the sweet spot  the programmer thinks about sources of parallelism at a high level  the system does the heavy lifting and always preserves correctness  programmer can tweak performance through localized changes

4. Todd C. Mowry ISCA Panel, June 7, 2005- 4 - Use CMPs to Improve Functional Correctness oImpressive tools already exist to help identify/fix problems  bugs, data races, memory leaks, faults, security violations, etc. oHowever, sophisticated tools can add significant runtime overhead oBy offloading these tools onto other threads, we can:  run existing tools continuously  enable new tools that are even more sophisticated