tions in TM systems. Recent software optimizations have managed to accelerate STM performance by 2%–15%. We believe such analysis is a good practice that should be extended to every piece of system software, especially open source. However, the gains are only a minor dent in the overheads we observed, indicating the challenge that lies before the community in making STM performance compelling.

conclusion

Based on our results, we believe that the road ahead for STM is quite challenging. Lowering the overheads of STM to a point where it is generally appealing is a difficult task and significantly better results have to be demonstrated. If we could stress a single direction for further research, it is the elimination of dynamically unnecessary read and write barriers—possibly the single most powerful lever toward further reduction of STM overheads. However, given the difficulty of similar problems explored by the research community such as alias analysis, escape analysis, and so on, this may be an uphill battle. And because the argument for TM hinges upon its simplicity and productivity benefits, we are deeply skeptical of any proposed solutions to performance problems that require extra work by the programmer.

We observed that the TM programming model itself, whether implemented in hardware or software, introduces complexities that limit the expected productivity gains, thus reducing the current incentive for migration to transactional programming, and the justification at present for anything more than a small amount of hardware support.

acknowledgments

We would like to thank Pratap Pattnaik for his continuous support, Christoph von Praun for numerous discussions, work on benchmarks and runtimes, and Rajesh Bordawekar for the B+tree code implementation.

 

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Călin Cas˛caval ( [e-mail]) is a research staff member and manager of programming models and tools for scalable systems at ibm tJ Watson research Center, yorktown heights, ny.

Colin Blundell is a member of the architecture and Compilers Group, department of Computer and information science, university of pennsylvania.

Maged Michael is a research staff research member at ibm tJ Watson research Center, yorktown heights, ny.

Trey Cain is a research staff member at ibm tJ Watson research Center, yorktown heights, ny.

Peng Wu is a research staff member at ibm tJ Watson research Center, yorktown heights, ny.

Stefanie Chiras is a manager in ibm's systems and technology Group.

Siddhartha Chatterjee is director of the austin research laboratory, ibm research, austin, tx.