With the advent of chip multiprocessors, exploiting intratransaction parallelism in database systems is an attractive way of improving transaction performance. However, exploiting intratransaction parallelism is difficult for two reasons: first, significant changes are required to avoid races or conflicts within the DBMS; and second, adding threads to transactions requires a high level of sophistication from transaction programmers. In this article we show how dividing a transaction into speculative threads solves both problems-it minimizes the changes required to the DBMS, and the details of parallelization are hidden from the transaction programmer. Our technique requires a limited number of small, localized changes to a subset of the low-level data structures in the DBMS. Through this method of incrementally parallelizing transactions, we can dramatically improve performance: on a simulated four-processor chip-multiprocessor, we improve the response time by 44-66% for three of the five TPC-C transactions, assuming the availability of idle processors.
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