Impact of large memory on the performance of optimistic concurrency control schemes

Under optimistic concurrency control (OCC) schemes, the buffer hit ratio and hence the abort probability of a rerun transaction can be affected by its previous runs, since the data items brought in from the previous runs may still be in memory. It is noted that this buffering effect on rerun transactions has been ignored in previous performance studies. In the present work the authors examine its effect on different OCC schemes. It is shown that, with sufficient buffer, a new approach to buffer management can be adopted so that data items referenced by aborted transactions continue to be retained in memory for access during rerun. By considering the I/O reduction during rerun, it is found that, at high contention levels, the broadcast OCC which attempts to abort conflicting transactions as early as possible can be inferior to the pure OCC which only aborts a transaction at its commit time. Second, combining the two schemes, with pure OCC during the first run of a transaction and broadcast OCC during any reruns, can typically lead to better performance, especially at high contention levels.