Taking the heat off transactions: Dynamic selection of pessimistic concurrency control

In this paper we investigate feedback-directed dynamic selection between different implementations of atomic blocks. We initially execute atomic blocks using STM with optimistic concurrency control. At runtime, we identify ldquohotrdquo variables that cause large numbers of transactions to abort. For these variables we selectively switch to using pessimistic concurrency control, in the hope of deferring transactions until they will be able to run to completion. This trades off a reduction in single-threaded speed (since pessimistic concurrency control is not as streamlined as our optimistic implementation), against a reduced amount of wasted work in aborted transactions. We describe our implementation in the Haskell programming language, and examine its performance with a range of micro-benchmarks and larger programs. We show that our technique is effective at reducing the amount of wasted work, but that for current workloads there is often not enough wasted work for an overall improvement to be possible. As we demonstrate, our technique is not appropriate for some workloads: the extra work introduced by lock-induced deadlock is greater than the wasted work saved from aborted transactions. For other workloads, we show that using mutual exclusion locks for ldquohotrdquo variables could be preferable to multi-reader locks because mutual exclusion avoids deadlocks caused by concurrent attempts to upgrade to write access.