  Multi-versioned database systems have the potential to significantly increase
the amount of concurrency in transaction processing because they can avoid
read-write conflicts. Unfortunately, the increase in concurrency usually comes
at the cost of transaction serializability. If a database user requests full
serializability, modern multi-versioned systems significantly constrain
read-write concurrency among conflicting transactions and employ expensive
synchronization patterns in their design. In main-memory multi-core settings,
these additional constraints are so burdensome that multi-versioned systems are
often significantly outperformed by single-version systems.
  We propose Bohm, a new concurrency control protocol for main-memory
multi-versioned database systems. Bohm guarantees serializable execution while
ensuring that reads never block writes. In addition, Bohm does not require
reads to perform any book-keeping whatsoever, thereby avoiding the overhead of
tracking reads via contended writes to shared memory. This leads to excellent
scalability and performance in multi-core settings. Bohm has all the above
characteristics without performing validation based concurrency control.
Instead, it is pessimistic, and is therefore not prone to excessive aborts in
the presence of contention. An experimental evaluation shows that Bohm performs
well in both high contention and low contention settings, and is able to
dramatically outperform state-of-the-art multi-versioned systems despite
maintaining the full set of serializability guarantees.
